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