Chapter 12: Disorders of the Immune Response – HIV/AIDS

The twelfth chapter explores the complexities of the human immune system when it malfunctions, causing injury or increased susceptibility to disease through disorders of the immune response. These are broadly categorized into immunodeficiency disorders (abnormal loss of function), hypersensitivity reactions (excessive responses), transplant rejection, and autoimmune disorders. Immunodeficiency disorders can be primary (inherited, such as Severe Combined Immunodeficiency Disorders or SCID) or secondary (acquired, like HIV/AIDS or nutrient loss), affecting humoral immunity (B-cells/antibodies) against bacteria or cell-mediated immunity (T-cells) against viruses, fungi, and intracellular bacteria. Specific defects detailed include X-linked agammaglobulinemia (XLA), DiGeorge syndrome, Wiskott–Aldrich syndrome (WAS), and deficiencies within the complement system (leading to enhanced infection risk and autoimmunity like SLE) or phagocytic function (e.g., Chronic Granulomatous Disease). The chapter meticulously classifies hypersensitivity reactions into four types: Type I (immediate, IgE-mediated, seen in anaphylaxis and allergic rhinitis), Type II (IgG or IgM against specific host cells, causing cellular destruction or dysfunction, as in Graves disease or Rh incompatibility), Type III (immune complex deposition leading to inflammation and vasculitis, like serum sickness), and Type IV (delayed, cell-mediated responses involving T lymphocytes, illustrated by allergic contact dermatitis). Mechanisms underlying autoimmune disease are discussed as a failure of immunologic self-tolerance, maintained through central and peripheral processes, with breakdown potentially triggered by factors like genetics (HLA/MHC molecules), release of sequestered antigens, or molecular mimicry. Finally, the text provides an extensive review of Acquired Immunodeficiency Syndrome (AIDS), caused by the Human Immunodeficiency Virus (HIV), a retrovirus that selectively destroys CD4+ T lymphocytes. The progression from primary infection through a long latency period to overt AIDS is characterized by high viral replication, falling CD4+ counts, and the onset of opportunistic infections. Diagnosis relies on antibody and antigen testing (EIA/ELISA, Western blot, PCR), while treatment focuses on Highly Active Antiretroviral Therapy (HAART) to interrupt the viral replication cycle.