Practice question

Immunodiffusion relies on diffusion of soluble antigen and antibody through agar gel matrix. When both reagents meet at optimal proportion, they form large cross-linked complexes that exceed solubility limit and precipitate as visible opaque line within gel. This line represents insoluble lattice trapped in agar pores, observable without staining. Excess of either reactant produces soluble small complexes that remain invisible. Unlike color emission or direct cellular agglutination, precipitin formation is static band reflecting equivalence zone migration. Its position, thickness and intensity encode relative concentration and specificity, useful for analyzing antigen-antibody interactions.