Haemagglutination

👉 Agglutination of red blood cells resulting in the formation of small clumps visible to the naked eye.

◈ There are 3 kinds of hemagglutination reactions:

1. In the first, used for the determination of blood groups, specific antibodies are made to act on the surface antigens of the red blood cells.
2. The second kind is represented by passive hemagglutination reactions. Here, the red blood cells play a role of inert support on which soluble antigens are artificially fixed; they will be agglutinated by specific antibodies, a reaction applied in the TPHA (Treponema Pallidum Hemagglutination Assay).
3. The third type used in virology. Some viral proteins, hemagglutinin, are able to bind to surface receptors in red blood cells. Thus, the attachment of a virion to two neighboring red blood cells leads to their agglutination.

◈ A positive reaction is manifested by the formation of a haze on the surface while the appearance of a pimple at the bottom of the cup is an indicator of a negative reaction.