Currently one of the most widely publicized diseases, acquired immunodeficiency syndrome (AIDS) is marked by progressive failure of the immune system. Although it’s characterized by gradual destruction of cell-mediated (T-cell) immunity, it also affects humoral immunity and even autoimmunity because of the central role of the CD4⁺ T lymphocyte in immune reactions. The resultant immunodeficiency makes the patient susceptible to opportunistic infections, unusual cancers, and other abnormalities that define AIDS.

A retrovirus—the human immunodeficiency virus (HIV) type I—is the primary causative agent. Transmission of HIV occurs by contact with infected blood or body fluids and is associated with identifiable high-risk behaviors. It’s therefore disproportionately represented in homosexual and bisexual men, I.V. drug users, neonates of HIV-infected women, recipients of contaminated blood or blood products (dramatically decreased since mid-1985), and heterosexual partners of people in the former groups. Because of similar routes of transmission, AIDS shares epidemiologic patterns with hepatitis B and sexually transmitted diseases.

HIV is transmitted by direct inoculation during intimate sexual contact, especially associated with the mucosal trauma of receptive rectal intercourse;
transfusion of contaminated blood or blood products (a risk diminished by routine testing of all blood products); sharing of contaminated needles; and transplacental or postpartum transmission from infected mother to fetus (by cervical or blood contact at delivery and in breast milk).

HIV isn’t transmitted by casual household or social contact. The average time between exposure to the virus and diagnosis of AIDS is 8 to 10 years, but shorter and longer incubation times have been recorded. Most people develop antibodies within 6 to 8 weeks of contracting the virus.

Acute leukemia begins as a malignant proliferation of white blood cell (WBC) precursors, or blasts, in bone marrow or lymph tissue. It results in an accumulation of these cells in peripheral blood, bone marrow, and body tissues.

The most common forms of acute leukemia include acute lymphoblastic (lymphocytic) leukemia (ALL), characterized by abnormal growth of lymphocyte precursors (lymphoblasts); acute myeloblastic (myelogenous) leukemia (AML), which causes rapid accumulation of myeloid precursors (myeloblasts); and acute monoblastic (monocytic) leukemia, or Schilling’s type, which results in a marked increase in monocyte precursors (monoblasts). Other variants include acute myelomonocytic leukemia and acute erythroleukemia.

Untreated, acute leukemia is invariably fatal, usually because of complications resulting from leukemic cell infiltration of bone marrow or vital organs. With treatment, the prognosis varies.

With ALL, treatment induces remission in 95% of children (average survival time: 5 years) and in 65% of adults (average survival time: 1 to 2 years). Children between ages 2 and 8 have the best survival rate—about 50%—with intensive therapy.

With AML, the average survival time is only 1 year after diagnosis, even with aggressive treatment. Remission lasting 2 to 10 months occurs in 50% of children; adults survive only about 1 year after diagnosis, even with treatment. Duration of remission is linked to age; younger patients have a greater chance of obtaining remission.

The exact cause of acute leukemia is unknown; however, radiation (especially prolonged exposure), certain chemicals and drugs, viruses, genetic abnormalities, and chronic exposure to benzene are likely contributing factors.

Although the pathogenesis isn’t clearly understood, immature, nonfunctioning WBCs appear to accumulate first in the tissue where they originate (lymphocytes in lymph tissue, granulocytes in bone marrow). These immature WBCs then spill into the bloodstream. They then overwhelm the red blood cells (RBCs) and the platelets. From there, immature white blood cells infiltrate other tissues.

Aplastic and hypoplastic anemias are potentially fatal and result from injury to or destruction of stem cells in bone marrow or the bone marrow matrix, causing pancytopenia (anemia, leukopenia, thrombocytopenia) and bone marrow hypoplasia.

Aplastic anemias usually develop when damaged or destroyed stem cells inhibit red blood cell (RBC) production. Less commonly, they develop when damaged bone marrow microvasculature creates an unfavorable environment for cell growth and maturation. About one-half of such anemias result from drugs (such as chloramphenicol), toxic agents (such as benzene), or radiation. The rest may result from immunologic factors (suspected but unconfirmed), severe disease (especially hepatitis), viral infection (especially in children), and preleukemic and neoplastic infiltration of bone marrow.

Disseminated intravascular coagulation (DIC) is also known as consumption coagulopathy and defibrination syndrome. DIC occurs as a complication of diseases and conditions that accelerate clotting, causing thrombosis of small blood vessels, organ necrosis, depletion of circulating clotting factors and platelets, and activation of the fibrinolytic system. This, in turn, can provoke severe hemorrhage.

Clotting in the microcirculation usually affects the kidneys and extremities but can occur in the brain, lungs, pituitary and adrenal glands, and GI mucosa. Other conditions, such as vitamin K deficiency, hepatic disease, and anticoagulant therapy, can cause similar hemorrhage.

DIC is usually acute but can be chronic for patients with cancer. The prognosis depends on early detection and treatment, the severity of the hemorrhage, and treatment of the underlying disease or condition.

DIC may result from:

Why such conditions and disorders lead to DIC is unclear, as is whether they lead to DIC through a common mechanism. In many patients, the triggering mechanisms may be the entrance of foreign protein into the circulation and vascular endothelial injury.

Regardless of how DIC begins, the typical accelerated clotting results in generalized activation of prothrombin and a consequent excess of thrombin. Excess thrombin converts fibrinogen to fibrin, producing fibrin clots in the microcirculation. This process consumes exorbitant amounts of coagulation factors (especially platelets, factor V, prothrombin, fibrinogen, and factor VIII), causing thrombocytopenia, deficiencies in factors V and VIII, hypoprothrombinemia, and hypofibrinogenemia.

Circulating thrombin activates the fibrinolytic system, which lyses fibrin clots into fibrinogen degradation products (FDPs). The hemorrhage that occurs may result largely from the anticoagulant activity of FDPs as well as from depletion of plasma coagulation factors.

A hereditary bleeding disorder, hemophilia results from deficiency of specific clotting factors. Hemophilia A (classic hemophilia), which affects more than 80% of all hemophiliacs, results from deficiency of factor VIII; hemophilia B (Christmas disease), which affects 15% of hemophiliacs, results from deficiency of factor IX. Other evidence suggests that hemophilia may result from nonfunctioning factors VIII and IX, rather than from deficiency of these factors.

Hemophilia produces abnormal bleeding, which may be mild, moderate, or severe, depending on the degree of factor deficiency. After a hemophiliac forms a platelet plug at a bleeding site, the lack of clotting factors impairs formation of a stable fibrin clot. Immediate hemorrhage isn’t prevalent, but delayed bleeding is common.