An agglutination test is an immunological assay in which antibodies bind to particulate antigens and cause them to clump together. The reaction is visible to the naked eye and can be used to identify either a specific antigen on cells or particles or antibodies samples.
Explanation
Agglutination occurs when antibodies with multiple binding sites cross‑link particles bearing the target antigen, forming lattices that precipitate out of suspension. Tests are performed on slides or in tubes and may be direct, in which an antigen is detected using known antibodies, or indirect, in which known particles coated with antigen detect patient antibodies. Variants include hemagglutination, which uses red blood cells; latex agglutination, which uses polystyrene beads; and co‑agglutination, which employs bacteria such as Staphylococcus aureus to display antibody Fc regions. Agglutination tests are widely used because they are simple, rapid and inexpensive. However, they require a sufficient concentration of antigen or antibody to form visible clumps, and nonspecific reactions can occur if samples are inadequately controlled.
Diagnostic applications and examples
Blood group determination relies on hemagglutination: adding anti‑A or anti‑B sera to a blood sample causes agglutination if the corresponding antigen is present. The Widal test measures antibodies to Salmonella typhi O and H antigens by mixing patient serum with antigen suspensions and observing agglutination, though specificity is limited. Latex agglutination kits detect microbial antigens such as capsular polysaccharide from Haemophilus influenzae type b or cryptococcal antigen in cerebrospinal fluid. Rapid staphylococcus or streptococcus tests use latex particles coated with antibodies to microbial antigens. Agglutination can also detect antibodies to rheumatoid factor or certain viruses by mixing patient serum with antigen‑coated particles.
Agglutination tests remain valuable tools for screening and point‑of‑care diagnostics. Their ease of use and immediate visual read‑out make them suitable for low‑resource settings, although confirmatory assays are often required for definitive diagnosis.
Related Terms: Serology, Hemagglutination, Latex agglutination, Antibody, Antigen