Introduction

The immune system is a complex, diverse and diffuse collection of cells, tissues, organs and molecules (e.g. interferons [IFNs]). It is programmed to respond to the many challenges presented to it by foreign particles and cells, microorganisms such as bacteria, viruses, fungi and protozoa, and cancer cells. Various defence mechanisms function to protect the body from harm, and in order to carry out this function it must be able to recognise ‘self’ from ‘non-self’.

The defence mechanisms providing immunity include: physical, chemical and microbiological barriers, second-line non-specific defences and specific immune responses (Storey & Jordan 2008). These can be categorised as first-, second- and third-line defences (Figure 6.1):

Figure 6.1 Overview – first-, second- and third-line defence mechanisms.

(Reproduced from Rennie-Meyer 2007.)

Although immunity is generally described as being either natural non-specific immunity or adaptive specific immunity, the functions are inextricably linked. Moreover, the interdependence and interactions between humoral and cell-mediated immunity are increasingly recognised.

For the most part the immune system functions to protect individuals from microbes and infection (see Ch. 16), and natural killer (NK) cells (a type of lymphocyte) undertake immunological surveillance in order to detect and destroy foreign, infected, damaged or cancerous cells. However, abnormalities may occur and immune responses may be excessive (hypersensitivity), may fail to function properly (immunodeficiency) or may fail to recognise ‘self’ leading to an immune attack on body cells (autoimmune disorders).

This chapter outlines immune responses, concentrating on adaptive immunity, and describes the nursing management of some immune response disorders: hypersensitivity, immunodeficiency and autoimmune conditions. It will come as no surprise that problems of the immune system impact on all body systems. Nurses working in all areas of practice may encounter people with immune disorders. Readers are provided with cross references to relevant chapters (e.g. Chs 5, 11, 16, 31), and suggestions for further reading.

Currently, there are many developments in the management of immunological disorders, transplantation and the use of targeted immunological-based treatments for conditions such as cancer. The use of monoclonal antibodies, cytokines and cancer vaccines, for example, is of increasing importance and will impact on nursing practice.

Components and features of the immune system – an overview

The diverse and widespread cells, tissues, organs and molecules that form the immune system include the lymphatic system – lymph capillaries and vessels that convey lymph, lymph nodes, the spleen, the thymus gland, the mucosa-associated lymphoid tissue (MALT) such as the tonsils and that found in the gastrointestinal tract, the bone marrow and B and T lymphocytes.

Types of immunity

This part of the chapter outlines natural non-specific immunity and considers both types of adaptive specific immunity. Table 6.1 contrasts the features of natural and adaptive immunity. As mentioned earlier there is considerable interaction between natural and adaptive immunity, and between both parts of adaptive immune responses. For example, tissue macrophages engulf antigens and act as antigen-presenting cells (APCs), which stimulate T-lymphocyte responses.

Table 6.1 Contrasting the features of adaptive and natural immunity

(adapted from Staines et al 1993)

Information about the numerous cells and chemicals involved in immune responses is provided in Box 6.1.

Natural non-specific immunity

Natural non-specific immunity represents first- and second-line body defences (see Figure 6.1) and includes: intact skin/mucosae, specialised epithelial surfaces, body secretions containing antibacterial substances such as lysozyme and immunoglobulins (Storey & Jordan 2008) (Box 6.2) and the microorganisms that comprise the normal flora. The processes of inflammation (see Ch. 23) and phagocytosis (engulfing and destroying foreign particles by white blood cells) are also included (see Further reading, e.g. Waugh & Grant 2010). These defences can be breached in a number of situations, for example when skin is inflamed or damaged (see Ch. 12).

Additionally, natural killer (NK) cells, a type of large granular lymphocyte (LGL) that detects and destroys damaged or malignant cells and virus-infected cells, undertake non-specific immunosurveillance. NK cells are activated by various chemicals including interferons (IFNs), which have a role in increasing cell resistance to viral infection.

Adaptive specific immunity

Adaptive immune responses involve specially programmed cells (B and T lymphocytes, also called B and T cells) that respond to recognised antigens. There is specificity of response as each lymphocyte is equipped to recognise only one antigen. Once contact has been made with that antigen and it has been destroyed, some ‘memory’ cells remain in the body. If the same antigen is encountered again (perhaps many years later), the remaining progeny of that memory cell are stimulated to replicate (clonal expansion), and this second response is both faster and more intense.

The adaptive specific immune response comprises the humoral (antibody-mediated) immune response, initiated by B lymphocytes, and the cell-mediated response, initiated by T lymphocytes. The humoral (antibody-mediated) response deals mainly with extracellular organisms through the production of antibodies (immunoglobulins), whereas cell-mediated responses are important for dealing with intracellular organisms such as viruses.

Humoral (antibody-mediated) immune response

B lymphocytes produce antibodies (immunoglobulins). The antigen receptor on their surface is specific for one antigen only. B lymphocytes are capable of memory and are specialised to deal with microorganisms which do not, of their own accord, enter host cells, e.g. circulating bacteria. Following contact with an antigen the B cell either differentiates into a plasma cell which produces an antibody, or becomes a memory cell.

Antibodies (immunoglobulins)

Antibodies are globular proteins with the ability to recognise and bind to a specific antigen, usually a microorganism. They are produced by plasma cells and, once formed, circulate in the blood and other body fluids. Antibodies are characteristically Y-shaped, with two identical heavy chains, each connected to two identical light chains (Figure 6.2).

Figure 6.2 Antibody (immunoglobulin) structure – the heavy chain determines the class of antibody, e.g. IgG, IgA, etc.

(Reproduced from Boon et al 2006 with permission.)

They have many roles that include:

• binding to a specific antigen, which causes antigen agglutination (clumping together), thus improving conditions for subsequent phagocytosis

• acting as an opsonin to tag the antigen for destruction by phagocytes

• activating the complement pathway (interaction between natural and adaptive immunity)

• enhancing the ability of other cells, e.g. eosinophils and NK cells, to destroy foreign or abnormal cells (interaction between natural and adaptive immunity).

Although antibodies function in many different ways, all involve joining with the antigen to produce an immune complex.

Each of the five classes may be produced with specificity for a single antigen. Their structure varies according to function and they are present in different amounts in the blood and other body fluids (Box 6.3).

Box 6.3 Information

Classes of antibodies (immunoglobulins)

• IgM – only found in blood. The first immunoglobulin to appear in the bloodstream in the primary immune response to infection. It functions in complement activation and agglutination of antigens. Since it disappears quickly after the antigen disappears, it is an indicator of current or very recent infection.

• IgG – the most plentiful immunoglobulin. Produced in large quantities in both the primary and secondary immune responses, particularly important in the secondary response. Levels in the blood give an indication of recent infection. Functions to neutralise toxins. It is also important as a defence against infection in the first few weeks of life, being the only immunoglobulin that crosses the placenta to the fetus.

• IgA – secreted onto the mucosal surfaces of the respiratory, gastrointestinal and genitourinary tracts and present in body secretions.

• IgE – normally found on the surface membrane of basophils (in the skin, mucosae and the lungs) and mast cells. It is associated with hypersensitivity (allergic) reactions and is active in parasite infestations.

• IgD – present in small quantities bound to B lymphocytes where it aids in the immunological ‘memory’ function.

Primary and secondary antibody-mediated immune response

When a specific antigen is encountered for the first time it takes approximately 2 weeks for a corresponding antibody to be detected in the blood. The production of this antibody is called the primary response. Although the immune system reacts immediately to antigens, the synthesis of antibody takes some time. However, on subsequent exposures to the specific antigen the matching specific antibody is produced much faster.

Primary response

During this process the antigen binds to its specific receptor on the surface of the B lymphocyte, triggering the following sequence of events:

• The B lymphocyte is stimulated to differentiate into a plasma cell and to undergo multiple divisions so that identical plasma cells are formed.

• Plasma cells synthesise the specific antibody.

• Meanwhile, some B lymphocytes differentiate to become memory cells, which persist and replicate in the body long after the invading antigen has been destroyed (Figure 6.3).

• Once sufficient quantities of specific antibody have been produced to destroy the antigen, the plasma cells die, leaving memory cells ready to respond to any future encounter with that antigen.

• Antibodies bind to the antigen activating the events outlined above to produce an immune complex.

Figure 6.3 The primary immune response.

The time interval occurring between contact with the antigen and the production of IgM may allow disease to develop in the individual due to the effects of the antigen, e.g. microbial infection.

Secondary response

occurs when a person encounters an antigen for the second time. The memory cells respond rapidly by producing plasma cells, which then secrete the specific antibody. This response occurs within a few days and, together with any residual antibody from the primary response, usually prevents disease from developing. In other words, the person has developed immunity.

Immunity against infectious disease

Immunity to a specific infectious disease may be due to inherited qualities. Otherwise immunity develops by natural processes or is achieved by artificial means, i.e. vaccines. Both natural and artificial immunity may be passive or active (Figure 6.4).

Figure 6.4 Natural and artificial immunity.

Natural immunity

may be passive where maternal IgG crosses the placenta to the fetus and following birth is transferred in colostrum and breast milk. This type of passive immunity is short-lived but helps to protect the infant until their own immune responses are sufficiently developed.

Active natural immunity is acquired if the person has the specific infection. Because this involves the production of a specific antibody or T lymphocyte, active immunity tends to be of longer duration (see primary and secondary response, p. 181).

Artificial immunity

involves processes whereby immunity is initiated or augmented. Passive artificial immunity is achieved by the administration of immunoglobulins (usually obtained from the plasma of immune individuals). There are normal immunoglobulins produced from pooled plasma and disease-specific immunoglobulins including those for rabies, tetanus, hepatitis B and varicella-zoster. Immunoglobulins are used in disease outbreaks, e.g. measles, to protect non-immune individuals, or in the prophylaxis and/or management of specific diseases such as for people with tetanus-prone wounds. Passive immunity is short-lived because the person’s immune system is not stimulated to produce specific antibodies (see Further reading, e.g. Health Protection Agency 2008, Provan et al 2008). Note. Immunoglobulins obtained from animals (antisera) can result in serious allergic reactions, including serum sickness, and have largely been replaced by human immunoglobulins.

Active artificial immunity is achieved by immunisation and, because this involves the production of a specific antibody or T lymphocyte, tends to be of longer duration (see primary and secondary response, p. 181).

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