Agglutination is an antigen–antibody reaction in which a particulate antigen combines with its antibody in the presence of electrolytes at a specified temperature and pH resulting in for-mation of visible clumping of particles. Agglutination occurs optimally when antigens and antibodies react in equivalent proportions.
· Agglutination reactions are mostly similar to precipitation reactions in their fundamentals and share similar features. This reaction is analogous to the precipitation reaction in that antibodies act as a bridge to form a lattice network of antibodies and the cells that carry the antigen on their sur-face. Because cells are so much larger than a soluble anti-gen, the result is more visible when the cells aggregate into clumps.
· Agglutination differs from precipitation reaction in that since the former reaction takes place at the surface of the particle involved, the antigen must be exposed and be able to bind with the antibody to produce visible clumps.
In agglutination reactions, serial dilutions of the antibody solution are made and a constant amount of particulate antigen is added to serially diluted antibody solutions. After several hours of incubation at 37°C, clumping is recorded by visual inspection. The titer of the antiserum is recorded as the reciprocal of the highest dilution that causes clump-ing. Since the cells have many antigenic determinants on their surface, the phenomenon of antibody excess is rarely encountered.
Occasionally, antibodies are formed that react with the antigenic determinants of a cell but does not cause any agglu-tination. They inhibit the agglutination by the complete anti-bodies added subsequently. Such antibodies are called blockingantibodies. Anti-Rh antibodies and anti-brucella antibodies arefew examples of such blocking antibodies.
Agglutination reactions have a wide variety of applications in the detection of both antigens and antibodies in serum and other body fluids. They are very sensitive and the result of the test can be read visually with ease.