Immunology

A child receives a vaccine and, years later, their immune system recognizes the same pathogen and destroys it before a single symptom appears. A patient receives a mismatched blood transfusion and goes into shock within minutes. Both events are driven by the immune system; one a triumph of immunological memory, the other a catastrophic hypersensitivity reaction.

Immunology is the study of how the body defends itself against infection, how that defense can go wrong, and how we harness immune mechanisms for diagnosis and treatment.

This section covers:

• Innate and adaptive immunity: physical barriers, phagocytosis, natural killer cells, T and B lymphocytes, and the logic of clonal selection
• Antibody structure and function: immunoglobulin classes, antigen-antibody interactions, and the significance of IgM versus IgG in acute versus past infection
• Complement system: pathways, effector functions, and clinical consequences of deficiency
• Hypersensitivity reactions: Type I through Type IV, with clinical examples including anaphylaxis, serum sickness, contact dermatitis, and transplant rejection
• Immunodiagnostic tests: ELISA, agglutination, precipitation, immunofluorescence, and the principles behind serological interpretation

Immunology confuses students because the same terms (antigen, antibody, complement) appear in multiple contexts with subtly different meanings. Every article in this section is written to make those connections explicit rather than leaving them as an exercise for the reader.