Agglutination occurs when an antibody interacts with antigen, resulting in cross-linking of the antigen particles by the antibody. This eventually leads to clumping. "Agglutination" comes from the Latin root agglutinare. "Agglutinare" means, "to glue". Cross-linking, or cross-matching is done to determine matches. Agglutination may occur when an unideterminate, multivalent antigen interacts with a single antibody. It may also occur if a multi determinate, univalent antigen interacts with at least two distinct antibodies.
Cross Agglutination: when an antibody that is raised against a similar antigen agglutinates that antigen.
Group Agglutination: when a collection of similar organisms is agglutinated by an agglutinin that is specific for that collection.
Applications for the agglutination test
BLOOD TYPING There are different blood groups like A,B, AB and O. The different blood types are differentiated according to the types of proteins on the surfaces of the red blood cells. Human red blood cells may either posses both of epitopes A and B on their surfaces. Individuals possessing only epitope A have anti-B antibody in their serum, while individuals possessing only epitope B on their red blood cells have circulating anti-A antibody. Some individuals have neither epitope and both antibodies present, while others have both epitopes and neither antibody. Blood type determination can be performed by mixing a sample of the individual’s blood with w solutions: one containing anti-A antibody and the other containing anti-B antibody. Agglutination occurs if the antibodies match the epitopes of the blood cells. By observing the agglutination patterns, the blood type of the individual can be determined.
DETERMINING MATING TYPES IN ORGANISMS Agglutination can be used to determine mating types in organisms such as Chlamydomonas. Extracts from cloned Chlamydomonas cells of either of the minus or plus type are added to cells of the opposite mating type as well as of the same mating type. What is observed is that, in the cells of similar type, no agglutination occurs. However, in the type with opposite mating type, agglutination does occur. Thus, this proves that the cells of different mating types are chemically different.