Agglutination: Antibody-Mediated Red Cell Clumping in Blood Typing and Disease

Agglutination is the visible clumping of red blood cells (RBCs) when antibodies bind to surface antigens, forming antigen-antibody lattices.

Mechanism

IgM (pentamer) or IgG (with anti-IgG enhancement) bridges adjacent RBCs. Requires antigen density, antibody affinity, and optimal ionic strength/pH.

Clinical Applications

• ABO/Rh typing: Forward (known antisera + patient cells) and reverse (known cells + patient serum).
• Crossmatch: Donor cells + recipient serum (major) to prevent hemolytic transfusion reaction.
• Antibody screening/identification: Detect unexpected alloantibodies (e.g., anti-K, anti-Duffy).
• Direct antiglobulin test (DAT/Coombs): Detects in vivo IgG/C3 on patient RBCs (AIHA, HDFN, drug-induced).
• Infectious serology: Febrile agglutinins (Brucella, Salmonella), cold agglutinins (M. pneumoniae).

Grading

0 (negative) to 4+ (single large clump). Microscopic confirmation for weak reactions.

Interference

Rouleaux (stacking from high protein) mimics agglutination; disperses with saline dilution. Prozone effect (excess antibody) → false negative; resolved by dilution.