Immunodiagnostic or serodiagnostic testing studies antigen–antibody reactions for diagnosis of infectious disease, autoimmune disorders, immune allergies, and neoplastic disease. These modalities also test for blood groups and types, tissue and graft transplant matching, and cellular immunology. Blood serum is tested for antibodies against particular antigens—hence, the term blood serology testing.
Antigens are substances that stimulate and subsequently react with the products of an immune response. They may be enzymes, toxins, microorganisms (e.g., bacterial, viral, parasitic, fungal), tumors, or autoimmune factors. Antibodies are proteins produced by the body’s immune system in response to an antigen or antigens. The antigen–antibody response is the body’s natural defense against invading organisms. Red blood cell groups contain almost 400 antigens. Immune reactions to these antigens result in a wide variety of clinical disorders, which can be tested (e.g., Coombs test).
Pathologically, autoimmune disorders are produced by autoantibodies—that is, antibodies against self. Examples include systemic rheumatic diseases (SRDs), such as rheumatoid arthritis (RA) and lupus erythematosus.
Immunodeficiency diseases exhibit a lack of one or more basic components of the immune system, which includes B lymphocytes, T lymphocytes, phagocytic cells, and the complement system. These diseases are classified as primary (e.g., congenital, DiGeorge syndrome) and secondary (e.g., human immunodeficiency virus/acquired immunodeficiency syndrome [HIV/AIDS]).
Hypersensitivity reactions are documented using immediate hypersensitivity tests and are defined as abnormally increased immune responses to some allergens (e.g., allergic reaction to bee venom or pollens). Delayed hypersensitivity skin tests are commonly used to evaluate cell-mediated immunity. Histocompatibility antigens (transplantation antigens) and tests for human leukocyte antigen (HLA) are important diagnostic tools to detect and prevent immune rejection with transplantation.
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