Microbiology · MU Sofia

38. Antigen-antibody reaction

Explains how an antibody's variable region binds its antigen to form a complex, which can then recruit complement or phagocytic cells via the constant region. Covers specificity, epitopes and paratopes, affinity, bivalency and IgM pentamers, and avidity. Also describes immune lattice theory, the zone of equivalence, and links to neutralisation, diagnostic tests and Type III hypersensitivity.

Inside: 1 section · 3 labelled pictures · 14 flashcards · 2 min

Topic 38 of 113 in the MU Sofia syllabus for Microbiology (Medicine, Year 3). The free AceMed note covers it in 1 section, about 304 words (2 min of reading), with 14 flashcards.

In this note

  1. Antigen-antibody reaction - Mechanism and general characteristic. Immune lattice theory.

Antigen-antibody reaction - Mechanism and general characteristic. Immune lattice theory.

The antigen-antibody reaction is a fundamental process in adaptive immunity. It involves the specific binding of an antibody to its corresponding antigen.

Mechanism:

  • The antigen binds to the variable region (Fab) of the antibody.
  • This binding forms an antigen-antibody complex.
  • The constant region of the antibody can then activate other immune components, such as the complement system or phagocytic cells.

General Characteristics:

  • Specificity: The antibody binds only to the specific antigen it was generated against. This is determined by the epitope on the antigen (the part recognised by the antibody) and the corresponding binding site (paratope) on the antibody.
  • Affinity: This refers to the binding strength between the antibody and the epitope.
  • Multivalency: Most antibodies are bivalent (can bind two antigen molecules or epitopes), which enhances the interaction and stability of the complex. IgM antibodies can form pentamers, providing 10 antigen-binding sites.
  • The functional binding strength, considering affinity and the number of binding sites, is called avidity.

Immune Lattice Theory:

  • This theory explains the formation of a lattice structure when multiple antigen-antibody complexes interact.
  • The lattice forms when antibodies, having multiple binding sites, cross-link multiple antigen molecules.
  • Lattice formation is the basis for observable reactions like precipitation (with soluble antigens) and agglutination (with particulate antigens).
  • For effective lattice formation, there must be an optimal proportion of antigen and antibody, known as the zone of equivalence.
  • An excess of either antigen or antibody can lead to weak or no lattice formation.

Related Reactions:

  • The antigen-antibody reaction is crucial for neutralization, such as when antitoxins (antibodies) bind specifically to toxins to prevent their harmful effects.
  • It is also the basis for many diagnostic tests, including precipitation reactions, agglutination reactions (like blood typing), and labeled antibody tests like ELISA.
  • The deposition of small, soluble antigen-antibody complexes in tissues can cause inflammatory reactions, contributing to Type III hypersensitivity.

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