Agglutination type of reaction in antigen-antibody combination.
Q35 marksShort Essays
Answer
Agglutination is the visible clumping that occurs when a particulate antigen (whole cells, or a soluble antigen coated onto an inert carrier particle) reacts with its specific antibody, cross-linking the particles into aggregates. It is more sensitive than precipitation for detecting antibody because particulate antigens present more surface area.
Types of agglutination reactions
Direct (slide/tube) agglutination — antibody reacts directly with a particulate antigen already present on a cell surface. Examples: blood grouping (ABO, Rh typing using known antisera), Widal test (agglutination of Salmonella O and H antigens by patient serum in enteric fever), bacterial identification by slide agglutination with specific antisera.
Passive (indirect) agglutination — a soluble antigen is first artificially adsorbed onto the surface of an inert carrier particle (latex, RBC, bentonite), which then agglutinates in the presence of specific antibody. Examples: latex agglutination tests (RA factor, CRP, ASO titre, pregnancy test using anti-hCG-coated latex), passive haemagglutination (TPHA for syphilis).
Reverse passive agglutination — antibody (instead of antigen) is coated onto the carrier particle, used to detect antigen in the test sample. Example: detection of bacterial toxins/antigens in stool or CSF.
Coombs’ test (antiglobulin test) — detects incomplete (non-agglutinating) antibodies using anti-human globulin as a bridging reagent; direct Coombs’ test detects antibody already bound to RBCs in vivo (e.g., haemolytic disease of newborn, autoimmune haemolytic anaemia), indirect Coombs’ test detects free antibody in serum (e.g., antenatal Rh antibody screening, cross-matching).
Coagglutination — antibody is bound via its Fc portion to Protein A of Staphylococcus aureus (Cowan strain), used for rapid antigen detection (e.g., streptococcal grouping).
Haemagglutination inhibition — used for viral diagnosis (e.g., influenza, rubella) and titrating antiviral antibody by inhibition of virus-induced RBC agglutination.
