Antigen–Antibody Reactions Made Easy | ELISA, Agglutination, Prozone & Hook Effect (MBBS)

Still mixing up affinity vs avidity or getting trapped by prozone/postzone MCQs? In this crisp, high-yield session, we’ll decode Ag–Ab concepts with visuals and bedside examples you can recall in seconds.

What you’ll learn ✅

Antigen basics: epitope/paratope, immunogenicity (size, foreignness, complexity), hapten–carrier concept

Antibody structure & classes: Fab/Fc, IgG/IgM/IgA/IgE/IgD—location, half-life, placenta, complement activation, opsonization & neutralization

Affinity vs Avidity: what examiners test + memory hooks

Primary vs Secondary response: IgM → IgG switch, lag period, anamnestic response

Ag–Ab reactions & tests:

Precipitation (in gels: Ouchterlony, immunodiffusion), Immunoelectrophoresis

Agglutination: slide/tube, latex, Coombs (DAT/ICT), hemagglutination inhibition

Complement fixation, Neutralization (toxins/viruses)

Immunofluorescence (DFA/IFA), Western blot (confirmatory logic)

ELISA (direct/indirect/sandwich/competitive), CLIA, Lateral-flow (POC: HCG, Dengue NS1, COVID Ag)

Titration concepts: antibody titre, seroconversion, paired sera (≥4-fold rise)

Common traps: Prozone (excess Ab), Postzone (excess Ag), High-dose hook effect in sandwich assays

Clinical pearls: when to pick IgM vs IgG, choosing the right assay for the clinical question

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