Bacteria

Bacteria are single-celled microscopic organisms without a nucleus that have the capacity to reproduce rapidly. Bacteria may be classified by reactions to laboratory techniques and by their appearance. Gram-staining, growing bacteria by culture and sensitivity testing are some of the laboratory techniques that identify bacteria and determine which antibiotic will be effective against them. Gram-staining involves placing live bacteria onto a microscope slide then applying methyl violet followed by iodine solution. The cells are stained blue at this stage. They are then treated briefly with acetone. Bacteria that retain the original blue stain are Gram-positive. Gram-negative cells are de-colourised by the acetone and appear red (Wilson 2000). Gram-negative bacteria are the more dangerous because they produce an endotoxin that can cause haemorrhagic shock and severe gastrointestinal disturbance, such as acute diarrhoea. They also lower the client’s resistance to other pathogens, making them susceptible to other different infections.

To culture (grow a sample of) bacteria, a sample from the client is transferred to growth medium, such as nutrient agar, that promotes multiplication and growth. When bacteria are present in sufficient quantity, sensitivity tests are carried out to determine the appropriate antibiotic to prescribe (deWit 2005).

Some species of bacteria have the ability to develop highly resistant round or oval structures, called spores, when they are exposed to adverse conditions such as lack of nutrients and/or water. Spores are resistant to disinfectants and to high or low temperatures, so they are difficult to destroy. They are resistant to sunlight and even freezing conditions so may remain viable in adverse environments for many years. They germinate when the environmental conditions become favourable, and this allows the bacterial cell within to begin multiplying (Wilson 2000).

Bacteria appear in a variety of sizes, shapes and arrangements. There are three main shapes — rod shaped (bacilli), round (cocci) and spiral shaped (spirilla, vibrios and spirochaetes). Figure 25.1 provides examples of bacterial shapes.

Figure 25.1 Bacterial shapes

Diseases caused by cocci include meningitis, gonorrhoea, pneumonia and skin infections such as boils and impetigo; diseases caused by bacilli include tetanus, diphtheria, tuberculosis (TB) and Legionnaires’ disease; the disease most commonly caused by spirochaete infection is syphilis. Different kinds of bacteria tend to affect different organs and systems of the body, producing a range of infectious diseases, each with its own group of symptoms. Bacteria make up the largest group of pathogens and are usually treated with antibiotics. The advent of antibiotics in the 1940s provided a powerful weapon to treat bacterial infections; however, because bacteria continually mutate as they divide, some had natural resistance to the effects of antibiotics and have developed into resistant strains. These pathogens include methicillin-resistant Staphylococcus aureus (MRSA), vancomycin-resistant Enterococcus (VRE) and penicillin-resistant Pneumococcus. These multi-drug-resistant organisms are dangerous threats, and emphasise the need for nurses and all health care workers to practise effective infection prevention and control measures (Wilson 2005).

Rickettsiae and Chlamydiae are microorganisms classified as specialised bacteria but they are smaller than most other bacteria and can only reproduce within the living cells of a host. Organisms that can only multiply within living cells are called parasites. Rickettsiae are often carried by arthropod vectors such as fleas, ticks and lice. A vector is the term used for a carrier that transfers an infective agent from one host to another. Body lice, for example, are arthropod vectors that carry the Rickettsiae responsible for epidemic typhus. Arthropods are creatures with jointed legs and include insects and ticks. Arthropods that live on the body’s surface (skin and mucous membranes) are called ectoparasites. Ectoparasites are responsible for conditions such as scabies and head or pubic lice (see Chapters 27 and 37 concerning nursing care for these conditions).

Chlamydial organisms are the cause of several significant human illnesses, including trachoma, the leading cause of blindness in the world today. Chlamydiae trachomatis is the cause of a prevalent sexually transmitted disease that can result in problems ranging from urethritis to serious inflammation of the reproductive system. Infection with Chlamydiae trachomatis can lead to pelvic inflammatory disease in women, with consequences such as infertility, ectopic pregnancy or miscarriage.

Mycoplasmas are a type of ultramicroscopic bacteria that lack a cell wall. They are responsible for some respiratory and genital-tract infections in humans. Mycoplasma pneumoniae causes a virulent type of pneumonia that spreads via respiratory secretions and is transmitted slowly but easily between humans (Wilson 2005; Wilson 2000).

Viruses

Viruses are specialised microorganisms with a number of distinctive characteristics:

The major viral diseases of concern within the health care setting are infectious mononucleosis (glandular fever [Epstein–Barr virus infection]), cytomegalovirus infection, herpes simplex virus infection, viral hepatitis (hepatitis A, B and C viruses), influenza, measles, human immunodeficiency virus (HIV) infection, respiratory syncytial virus infection, rotaviral enteritis, chickenpox (varicella virus), shingles (herpes zoster virus), rubella (German measles), viral haemorrhagic fevers and, most recently, severe acute respiratory syndrome (SARS, caused by a novel coronavirus) (Lashley & Durham 2007; Wilson 2005; Wilson 2000).

Fungi

Fungi are tiny organisms such as moulds and yeasts that belong to the world of plants but contain no chlorophyll. Fungi are present in the soil, air and water and they multiply by producing various kinds of spores. Most species of fungi are non-pathogenic, but a few cause disease. There are three types of fungal (mycotic) infections: superficial, which affect the skin, mucous membranes, hair and nails; intermediate, which affect subcutaneous tissues; and systemic, which infect deep tissues and organs. Tinea (ringworm) is one genus of the fungal group of organisms called dermatophytes. Tinea species cause superficial infections of the keratin found in skin, scalp and nails. Sometimes antibiotic medications alter the body’s normal flora, providing an environment that allows fungal infections such as vaginal candidiasis (vaginal thrush) and tinea pedis (athlete’s foot) to develop (Wilson 2000).

Protozoa

Protozoa are single-cell organisms of the animal kingdom. All protozoa require large amounts of water and are abundant in soil and water, and in and on plants and animals. They are classified according to their ability to move. Amoebic dysentery and giardiasis are serious conditions caused by protozoan parasites. The route of entry is usually by ingestion of contaminated water or food, which commonly results in severe diarrhoea.

Some protozoans are transmitted by insects (vectors) to man. Pathogenic protozoa include the plasmodium species, which is carried by mosquitoes and causes malaria. Malaria still causes over three million deaths each year in the more tropical regions of the world (Herlihy & Maebius 2000). Certain strains of protozoa are a major cause of opportunistic infections, meaning that they take the opportunity to establish themselves in a host whose resistance is lowered. For example, Pneumocystis carinii is commonly the cause of pneumonia in people with acquired immune deficiency syndrome (AIDS), whose immune systems are weakened by the presence of the AIDS virus (HIV) (Lashley & Durham 2007).

Helminths

Helminths are parasitic worms, or flukes — multicellular animals that are pathogenic to humans. They are classified according to external appearance: flatworms (platyhelminths) or roundworms (nematodes). Parasitic worms often live in relative balance with human hosts, taking only enough nutrients to survive without destroying the health of the host. Most worm infections are transmitted from person to person via faeces that contaminate food and water. Flatworms include tapeworms and flukes that reside in the intestine of the host. Tapeworms commonly enter the human body via contaminated or insufficiently cooked pork and may grow 1–12 metres in length. Flukes are not common in Australia or New Zealand but are a serious problem in many Asian, tropical and subtropical countries. They enter the body via improperly cooked fish and invade the blood and organs such as the liver, lungs and intestines. Infestation by large flatworms causes anaemia, fatigue, abdominal pain and weight loss.

Roundworms include ascarides, hookworms, trichinae, pinworms and a minute worm that causes filariasis (elephantitis). Pinworm is one of the most common parasitic worms. It affects children more often than adults. This nematode is less than a centimetre long and inhabits the upper part of the large intestine, but lays its eggs on the outer perianal area of the host. The deposition of the eggs causes irritation, the child scratches, hands are contaminated and eggs are transferred from hand to mouth and sometimes to friends. This circular pattern of reinfestation makes pinworm particularly hard to control (King, Belman & Kramer 2001).

The diagnosis of worm infestation is made by microscopic examination of the client’s faeces, which reveals the presence of adult worms or their larvae. Infestation by parasitic worms is treated with medications called anthelmintics (Bryant, Knights & Salerno 2006). Education of clients regarding prevention of spread and reinfestation is essential. Table 25.1 provides a list of common infection-causing organisms.

TABLE 25.1 COMMON PATHOGENS AND THE INFECTIONS/CONDITIONS THEY CAUSE

Human sources

Most organisms that infect humans are acquired from human sources. A variety of organisms, normal flora, reside on the surface of the skin and within body cavities, fluids and discharges. Some areas of the body contain larger populations of normal flora than others, for example, the skin, respiratory tract, colon, mouth and vagina. Areas of the body that are usually considered sterile (without normal flora) include the spinal fluid and blood, the urinary system and the peritoneal cavity. The entrance of a foreign object or organism into a sterile site leads to a high risk of infection. Auto-infection occurs when normal flora cause infection by being transferred from their normal place of residence to a different site in the same host. Crossinfection may occur when organisms from one person are transferred to another person, for example, when organisms on a nurse’s hands are transferred to a client’s wound.

A person incubating a disease is another source of infection. An incubation period is the time between the entrance of the pathogen into the host and the appearance of the clinical symptoms of the disease. During the incubation period the organisms multiply and can be transmitted and infect others before the host or anyone else knows the disease is present. Nurses therefore should treat every client as if they were a potential source of infection.

A person with an infection can liberate large numbers of pathogens into the environment. In some diseases the risk of infecting others may last only a short time; in others, pathogens continue to be released by the infected person over a long period; for example, the tuberculosis Mycobacterium may be present in the host for many years (Wilson 2000; Wilson 2005).

A carrier is an individual who is the host for pathogenic microorganisms. An individual may be a carrier during the incubation period of a disease, after an attack of the disease, or without ever experiencing symptoms of the disease that the organism causes.

Animal sources

Animals, birds and insects (vectors) can also be reservoirs for infectious organisms; for example, Q fever is caused by a parasite found in cattle, psittacosis is transmitted from birds and malaria from mosquitoes.

Inanimate sources

Soil, seawater, food, water and milk are additional reservoirs for pathogens. Many organisms live in the soil and obtain their nourishment from decaying vegetable and animal matter. Some anaerobic spore-forming bacteria cause disease if they gain entry to human tissue; for example, Clostridium tetani lives in the soil and, if it enters the body, can cause the illness known as tetanus, which results in serious damage to the nervous system. Route of entry to a human host is via puncture wounds, cuts or other lesions. Because anaerobic bacteria can only survive and multiply in the absence of oxygen, they can survive if they are present in canned or vacuum-packed foods. If the toxin they produce is ingested the result may be extremely serious or even lethal.

To grow and reproduce, organisms require a suitable environment. Environment relates to the availability of food and water, the level of light, oxygen and heat and the pH range. Microorganisms thrive most effectively in warm dark environments such as those within body cavities and under wound dressings. The need for oxygen varies with different groups of bacteria. Some can survive with or without oxygen (facultative anaerobes), some need oxygen (obligate aerobes), while others survive only in the absence of oxygen (anaerobes). Large numbers of anaerobes are present in the human intestine. Most microorganisms grow only in certain temperature ranges. The ideal temperature for most human pathogens is about 37°C but it is possible for some to multiply in temperatures ranging from10–60°C. Most microorganisms survive most efficiently in an environment within a pH range of 5–8.

Skin and mucous membranes

Any break in the skin or mucous membranes can lead to infection. A surgical wound, for example, can be an entry point, a portal of exit and a reservoir for a pathogen.

Gastrointestinal tract

Although most organisms of the mouth are normal flora, the mouth is one of the most bacterially contaminated sites of the body; therefore, saliva (including kissing) is a portal of exit for pathogens. Other portals of exit include expectorated sputum, faeces, vomitus, bile or discharge from wounds. Any drainage tube or opening from the gastrointestinal tract is a potential portal of exit.

Urinary tract

When there is an infection of the genitourinary tract, microorganisms can exit via the urine.

Respiratory tract

Pathogens that infect the respiratory tract can be released through sneezing, coughing, talking or even breathing. The nose or mouth should be covered when sneezing or coughing, and disposable tissues should be used to control the exit of microorganisms. In some health care institutions, routine investigations are performed, such as nose and throat swabbing, to ensure that team members are not harbouring pathogenic microorganisms that can be transmitted to others via respiratory secretions. Such precautions are particularly important in promoting the safety of more vulnerable clients in areas such as operating rooms, burns units or neonatal units. For example, nurses whose swabs test positive for staphylococcal organisms would not be allocated to work with vulnerable clients.

Blood

Blood is normally sterile but, when a client has a blood-borne infectious disease such as hepatitis B or C or AIDS, it becomes a reservoir for the causative pathogens. Any break in the skin that allows blood to escape, and menstrual blood from the vagina, are portals of exit for blood-borne pathogens.

Reproductive tract

A male’s urethral meatus or a female’s vaginal canal can provide the means of exit for pathogens that are transferred from the body via semen or vaginal discharge. Chlamydia, herpes simplex, gonorrhoea and syphilis are examples of infectious diseases transmitted in this way.

Airborne transmission

Airborne dissemination may occur via either airborne droplet (droplet nuclei) or dust particle. Tiny pathogens can be carried on airborne particles such as dust, water and respiratory droplets and, if inhaled by a susceptible host, cause infection. Pathogens expelled from the respiratory tract during coughing, sneezing or talking can be transmitted to a susceptible host by this route. The distance and extent of distribution depends on the force of the expulsion and gravity. Droplet transmission can also occur during medical procedures such as suctioning and bronchoscopy. Examples of illnesses spread by droplet nuclei are measles, rubella, influenza, pneumonia, meningitis, meningococcal disease and TB.

Droplets of moisture that contain organisms do not have to be inhaled to spread infection. They contaminate all surfaces on which they fall, so transmission can occur via indirect contact. Dust consists of environmental dust particles, dead skin, flakes, fluff from clothing, dried secretions and microorganisms. It is liberated from humans by means of normal body movements and from clothing during normal activity and dispersed by sweeping, dusting, bed making and other physical activities. It then settles on all surfaces in the environment. Microorganisms can therefore settle on and contaminate items such as clothing and bedclothes, books, papers, crockery, cutlery, toys, toilet utensils, furniture and fittings, dressing materials, needles, tubing, instruments and stethoscopes. Non-living, inanimate objects that can transmit microorganisms are called fomites. Indirect contact occurs when there is personal contact of the host with fomites in the environment.

Airborne transmission includes the spread of pathogens that may happen during procedures such as bed-making or manual washing of instruments or equipment. Microorganisms that are displaced into the atmosphere are carried and distributed by air currents. Ventilation and air-conditioning systems can rapidly distribute microorganisms.

Contact transmission

This is the most significant route of transmission in health care settings (Wilson 2000). Microorganisms can be transferred directly from one individual to another by physical contact between an infected person and a new host. Contact transmission can occur by touching the skin or body fluids of a client with an infection, or through the use of contaminated nursing or medical equipment. The hands can be the means of transfer if, after contact with an infectious client, they are not washed adequately before tending another client. Effective hand washing is the single most important way to limit cross-contamination and the spread of infection. Cross-contamination is said to have occurred when pathogens have been transmitted from one person or object to another. Contact transmission can also occur as a result of sexual activity or from a mother to an unborn baby via the placenta (transplacentally).

Many diseases are spread by direct contact, including glandular fever, caused by the Epstein–Barr virus. Some disorders can be transmitted by airborne droplet or by direct contact. The common cold or influenza, for example, can be transmitted as a result of pathogens released when sneezing, coughing or talking being carried on airborne droplets and inhaled by another person, while directly touching nasal or oral secretions can result in direct contact transmission.

Transmission by ingestion

Microorganisms can enter the gastrointestinal tract in a variety of ways, including via infected food or water, contaminated eating or drinking utensils or hands or, more often in children than adults, via contaminated objects being placed in the mouth. A person with a gastrointestinal infection can transfer infection via their hands if they are not washed effectively after defecation.

Contaminated food and water are responsible for the spread of diseases such as Creutzfeldt-Jakob disease (CJD) and cholera. Food poisoning results from ingesting food contaminated with toxic substances or bacteria that contain toxins; for example, Salmonella enteritidis is a bacteria that produces toxins resulting in food poisoning. Food or water may become contaminated:

Vector transmission

Vector-borne transmission (via mosquitoes, flies, rats and other animals) as a health concern is not as significant in Australia and New Zealand as it is in other parts of the world. Examples of microorganisms spread by this mode are Plasmodium falciparum (malaria) and Yersinia pestis (plague).

Box 25.1 summarises the modes of transmission of microorganisms.

External mechanical and chemical barriers

These are the natural physical barriers that prevent microorganisms from entering the body or expel them before they proliferate. Physical barriers include the skin, mucous membranes, normal body flora and some body secretions (Figure 25.3).

Figure 25.3 Natural external non-specific defences against infection

(adapted from Wilson 2000)

The body as a whole is protected by the skin, which forms an intact waterproof barrier against microorganisms. Sweat and sebum secreted through the skin are bactericidal for some organisms.

The gastrointestinal tract is protected by mucous membrane and the normal flora of the bowel. In addition, saliva produced by glands in the mouth contains the enzyme lysozyme, which inhibits the growth of bacteria. Acids secreted in the stomach create a low pH environment unfavourable to the survival of most pathogens. The high pH of alkaline bile secreted into the duodenum is also unfavourable to survival and is antibacterial.

The respiratory tract is protected by mucous membrane. The membrane of the upper airway is lined with hair-like structures called cilia, that waft inhaled microorganisms away from the lungs and into the oropharynx to be expectorated or swallowed. Nasal secretions, like saliva, also contain lysozyme.

The eyes are protected by tears that, like saliva and nasal secretions, also contain lysozyme. The fluid secreted protects the surface of the eyes by washing them.

The vagina is protected by normal flora called lactobacilli, which cause vaginal secretions to have a local pH of 4–5, which is low enough to inhibit invasion by other microorganisms.

The urinary tract is protected by the flushing action of urine as it flows from the bladder, and the natural acidity of urine is not conducive to the survival of microorganisms.

Inflammation

Microorganisms that manage to foil the body’s mechanical and chemical barriers and so gain entry to the tissues trigger an internal inflammatory response. The inflammatory response is the body’s attempt to destroy as many invading organisms as possible and to confine them to the point of entry. Although often associated with infection, inflammation is the body’s physical response to any form of injury or damage. It is usually characterised by four basic physical symptoms: redness, pain, heat and swelling. Histamine and other substances are released from tissues irritated by invading organisms, causing blood vessels to dilate. This increases blood flow through the damaged tissues, resulting in redness and heat. The histamine and other substances also increase the permeability of the blood vessels, resulting in fluid seeping from the blood into the tissues, which accounts for the swelling. The pressure caused by this accumulation of extracellular fluid in the tissues can also cause pain. Pain can also be caused by damage to nerve fibres or by the release of toxic chemicals from microorganisms.

Phagocytic cells

White blood cells called neutrophils and macrophages are attracted to an infection site by chemicals released from damaged cells. These white cells are phagocytes, which means that they attack the pathogens by enveloping, ingesting and then digesting them with enzymes, a process called phagocytosis.

Neutrophils have a shorter life span than macrophages. During a period of about 6–8 hours they circulate and attach onto microorganisms and digest them. Macrophages are able to engulf and ingest microorganisms over a much longer period of time. As these phagocytes carry out the destruction of microorganisms, many of them die in the process. The debris from the white cells, pathogens, damaged tissue cells and fluid in the area of inflammation accumulates to become a thick yellowish substance called pus. If the pus cannot drain to the outside of the body, cells build a wall of tissue to surround this infected debris. This contained walled-off area is an abscess. An abscess can be protective because it restricts the spread of the infection to other areas of the body. If antibiotic therapy does not effectively destroy the pathogens causing the infection and the accumulation of pus, an abscess may require surgical intervention to lance the wall to release and drain the pus from the area.

As phagocytes act they release pyrogens. Pyrogens stimulate the hypothalamus to reset the body’s temperature at a higher level. This is what causes the fever associated with infection. Fever is also a defence in that many microorganisms do not thrive above certain temperatures. The higher body temperature therefore reduces the ability of certain pathogens to multiply. For this reason it is best not to reduce a fever unless it is of such intensity that there is a risk of seizures or damage to the brain.

Natural killer cells

Natural killer (NK) cells are a specialised type of lymphocyte capable of binding to and killing pathogens by breaking holes in cell membrane and releasing destructive enzymes. They are particularly effective against parasites, some virus-infected cells and some malignant tumour cells. NK cells are manufactured predominantly in the bone marrow and in the spleen.

Protective proteins

There are two groups of protective proteins — complement proteins and interferons. Complement comprises a set of about 20 proteins that help phagocytic cells identify pathogens. They do this by a series of reactions that eventuate in a protein substance, called C3b, coating the surface of the pathogens. Phagocytic cell receptors are able to recognise this substance, bind to it then ingest the pathogen. They are also involved in destroying foreign bacteria by a process called cytolysis. The complement achieves this by first changing the shape of the bacteria to a doughnut shape, then making a hole through the cell membrane. This permits ions and water to enter and swell the cell until it ruptures (Crisp & Taylor 2005).

Interferons are a group of proteins secreted by cells infected with a virus. They are capable of binding onto other surrounding cells and protecting them from viral replication. They are also able to enhance the destructive abilities of NK cells against malignant tumour cells. All cells can produce alpha and beta interferons, which interfere with the replication of viruses. T lymphocytes and NK cells can produce gamma-interferon, which coordinates and enhances the activities of other parts of the immune system (Wilson 2000).

If invading pathogens survive the external barrier defences and the other non-specific responses and continue to proliferate without effective treatment, infection may spread to other parts of the body, primarily via the circulatory and lymphatic systems.

The lymphatic system

The body’s immune responses are under the control of the lymphatic system. It is a highly efficient system that responds very specifically to the threat of infection. It is made up of lymph, lymphatic vessels, lymph nodes, red bone marrow, the thymus gland, spleen and tonsils. The immune system develops when lymphoid stem cells are manufactured in the red bone marrow and travel to the thymus gland, spleen and other lymphoid tissues and organs in the body. These organs and tissues are coordinated to produce, mature and activate cells effective in providing the body with its immune response (Bryant et al 2006). Lymphocytes are the most significant cells in the body’s immune responses.

Lymphocytes

There are two categories of lymphocytes involved in the specific immune response — T lymphocytes (T cells) and B lymphocytes (B cells). They reside in areas of lymphoid tissue around the body (Figure 25.4) and circulate in the blood. It is the job of T cells and B cells to attack antigens. Fetal stem cells in the bone marrow initiate production of T cells and B cells. T cells travel and mature in the thymus gland. B cells do not travel to the thymus gland and mature and differentiate in fetal blood or bone marrow. After maturing, both types reside in various lymphoid tissue around the body and also circulate in the blood. T cells comprise 70–80% of circulating lymphocytes, B cells comprise the other 20–30%.

Figure 25.4 Location of organs and tissues of the immune system

(adapted from Wilson 2000)

Both T and B cells attack antigens. T cells are predominant in cell-mediated (cellular) immunity, which occurs through direct cell-to-cell contact of T cells with foreign cells. B cells are predominant in antibody-mediated (humoral) immunity, which involves indirect interaction with antigens through the secretion of antibodies.

Recognising ‘self’ cells

To function effectively at defending the body from invading organisms, these defence responses must be able to distinguish between the body’s own cells and foreign agents. During the period between life as a fetus and several years after birth, the thymus gland is busy setting up the antibody response system. During this process T cells are maturing. To enable the process of identifying cells that belong to the ‘self’ and should not be attacked, markers are placed on the outer surfaces of the body’s own cells. The marker is a group of molecules called the major histocompatibility complex (MHC). Each person has their own unique MHC. This accounts for why the body recognises and attacks tissue or organs donated from others. Any T cells that recognise and respond to ‘self’ cells are discarded.

Malfunction of a person’s immune system can cause a failure to recognise cells that belong to the self. This results in damage or destruction of ‘self’ cells. An attack on the body’s own cells is the basis of autoimmune disorders such as rheumatoid arthritis, juvenile-onset (type 1) diabetes mellitus, pernicious anaemia, multiple sclerosis and systemic lupus erythematosus. In juvenile-onset diabetes the body attacks the islet cells of the pancreas, interfering with the production and secretion of the insulin they normally produce. It is thought that the mechanisms that cause autoimmune disorders are of genetic origin.

Cell-mediated immunity

Two types of T cells are involved in cell-mediated immunity: T-helper (CD4) cells, and cytotoxic T cells. Cell-mediated immunity is particularly effective against fungi, protozoan parasites, malignant cells and foreign tissue that is alien to the body, for example, transplanted organs (Herlihy & Maebius 2000). The following steps are involved in cellular immunity:

Antibody-mediated (humoral) immunity

It is mostly B lymphocytes that are involved in the antibody-mediated response. Each B cell is programmed to produce a unique antibody. Each B cell displays its own specific antibody on its outer surface. This acts as a receptor for passing antigens. Antibodies only bind with antigens that specifically fit their unique receptor — the antigen must have a shape that precisely fits the antibody, rather like the way two pieces of a jigsaw puzzle fit together. B cells produce vast quantities of specific antibodies to lock with, then generate a reaction against, specific antigens. Antibodies are gamma globulins, one subtype of a class of proteins called immunoglobulins. Immunoglobulins are classified into five identifiable groups: IgG, IgM, IgA, IgE and IgD. Table 25.2 describes some specific properties of each class of immunoglobulin. Between them they perform these general tasks:

TABLE 25.2 FUNCTIONS OF THE FIVE CLASSES OF IMMUNOGLOBULINS

The following steps are involved in antibody-mediated immunity:

On first exposure to an antigen, B cells produce quantities of plasma cells that secrete antibodies. The B cells also produce quantities of memory cells. This first exposure, the first time B cells are activated by a specific antigen, is termed the primary response. This would occur on the first exposure to the measles virus, for example. During the primary response the antibodies develop relatively slowly, with a relatively low amount in the blood plasma. This allows symptoms of the illness, such as the measles, to manifest. When the immune system is confronted with the same antigen on subsequent occasions, the body mounts a much speedier response and is able to produce antibodies in much larger numbers because the memory cells already reside in the plasma. This is termed the secondary response and is usually quick enough and strong enough to prevent the associated pathogen from becoming established and symptoms of the illness developing. This demonstrates that the body has developed immunity to the specific infectious microorganism, such as the measles virus. The antibodies to a specific disorder can be measured to determine the level of immunity a person has to a specific disorder. This measurement is called an antibody titre.

Innate immunity

Innate immunity is sometimes referred to as genetic or inborn immunity. Effective defences against potentially harmful microorganisms are present from birth and do not depend upon having previous experience with any particular microorganism. This is known as the innate immune mechanism, which is effective against a range of potentially infective agents and is, therefore, non-specific. Humans are born with inborn immunity to certain diseases that affect animals or plants, and animals and plants are not usually susceptible to the infectious diseases that afflict humans. For example, humans cannot get Dutch Elm disease and dogs and cats do not get measles or chicken pox.

Naturally acquired immunity

Acquired immunity is not present at birth but is developed during life; it may be naturally or artificially acquired. Naturally acquired immunity is gained in two ways, either passively or actively. Passive natural immunity is acquired before birth when immunoglobulins in the mother’s blood are transferred across the placenta and enter the fetal blood. It is acquired after birth when a breastfed baby receives small amounts of immunoglobulins from its mother’s breast milk. These are most abundant in colostrum, the milk produced very soon after delivery of the baby. Passively acquired immunity is not permanent and does not last as long as actively acquired immunity — the immunoglobulins passed from mother to baby may remain in circulation and provide immunity for only up to a few months.

Active natural immunity occurs after being exposed to an infectious organism and developing specific antibodies and many memory cells to ward off future encounters with the same pathogen. Active immunity is acquired as a result of the stimulation of an individual’s immune system, with active involvement of the body in the production of its own immunoglobulins. For example, if measles, mumps or chicken pox is experienced as a child, active natural immunity to these illnesses is established.

Artificially acquired immunity

Artificial immunity is acquired in two ways: either by means of a vaccine or by injection of immunoglobulin. Vaccines are antigenic materials that induce a specific active artificial immunity to infection by a specific microorganism. Vaccines are suspensions of live, dead or attenuated (weakened) pathogens introduced into the body to stimulate the production of specific immunoglobulins. For example, the measles vaccine is developed by weakening the live measles virus that causes the illness. When injected into the recipient, the weakened virus stimulates the immune system to produce antibodies to protect against the illness. Vaccine can also be made using the toxin secreted by a pathogen. The toxin is modified to limit its ability to cause harm, while keeping it strong enough to induce immunity. The modified toxin is called a toxoid.

Specific vaccines provide protection against some infectious diseases for months or years. It is recommended that vaccination against many diseases be provided at a very young age; for example, Sabin vaccine, which is ingested orally, provides protection against poliomyelitis, and the first dose should be given when a child is only 2 months old. (For further information about vaccination, refer to the Immunise Australia website www.immunise.health.gov.au.)

Bacille Calmette–Guérin (BCG) is a freeze-dried, live attenuated vaccine available for people who are not immune to TB. A ‘Mantoux test’ (named after the French physician Charles Mantoux) is performed first by injecting tuberculin (a protein derivative from the tuberculosis bacillus) into the skin on the forearm. Immunity to TB is demonstrated by a positive skin reaction at 72 hours, indicated by an area of oedema 5 mm or more in diameter. (A positive reaction may be due to a current infection, a previous infection, or previous vaccination with BCG vaccine.) If the reaction to the Mantoux test is negative, BCG vaccine may be given by intradermal injection (inoculation). Other vaccines are given to provide immunity against cholera, meningococcus, diphtheria, whooping cough (pertussis vaccine), typhoid fever, typhus and influenza.

Immunoglobulin is different to vaccine. It provides artificial passive immunity because it contains antibodies (immunoglobulins) which have already been developed within a human or animal donor. The antibodies have developed in response to a specific infection that the donor has been exposed to. These ‘ready prepared’ antibodies are injected into a recipient with the aim of providing them with immediate antibody protection. For example, a nurse who has accidentally suffered exposure to the hepatitis B virus via a needle-stick injury and who does not have immunity may receive hepatitis B immunoglobulin in an attempt to provide instant protection from the virus. As this is passive immunity, protection is short lived.

Immunoglobulins are also available for diseases such as rubella, hepatitis A, rabies and tetanus. Tetanus immunoglobulin may be injected to treat a client with tetanus but may also be used in the management of a tetanus-prone wound. Tetanus immunoglobulin contains antibodies that counteract toxins secreted by the anaerobic Clostridium tetanus bacillus; it is therefore called tetanus antitoxin (TAT). Antitoxins contain antibodies that neutralise the toxins secreted by pathogens, although they do not affect the pathogens themselves. Botulism and diphtheria are other examples of conditions treated with antitoxins. Antivenoms contain antibodies that combat the ill effects of bites or stings sustained from venomous creatures such as snakes, spiders, box jellyfish and stone fish.

Worldwide immunisation programs have eradicated or substantially reduced the incidence of smallpox, diphtheria, tetanus, whooping cough and poliomyelitis. Nurses can promote the health of the community by providing education about immunisation programs. It is strongly recommended that nurses access all available immunisation protection from diseases they may themselves be exposed to, in particular TB and hepatitis A and B. Figure 25.5 indicates the different types of immunity.

Figure 25.5 Types of immunity

(adapted from Herlihy 2007: 221)

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