Agglutination is a reaction in which particles suspended in a liquid collect into clumps usually as a response to a specific antibody). It is an antigen-antibody reaction that occurs when an antigen (i.e., a molecule capable of triggering the adaptive immune response) is mixed with its corresponding antibody at a suitable pH and temperature. These antibodies are called agglutinins because they bind multiple antigens together and form a lattice-like structure seen as clumping by the naked eye. Agglutination is commonly used as a method of identifying specific bacterial antigens and the identity of such bacteria, and therefore is an important technique in diagnosis).

The purpose of agglutination is to detect the presence of specific antigens or antibodies in samples of bodily fluids, such as blood, saliva, urine, or cerebrospinal fluid (CSF); as well as to identify specific strains of bacteria in microbiological cultures. Excessive amounts of antigens or antibodies in the solution may prevent agglutination; a phenomenon called prozone and postzone effect, respectively.

Agglutination can occur in different contexts, such as hemagglutination, which is the process by which red blood cells agglutinate, meaning clump or clog). When a person produces antibodies against their own red blood cells, as in cold agglutinin disease and other autoimmune conditions, the cells may agglutinate spontaneously. This is called autoagglutination and it can interfere with laboratory tests such as blood typing and the complete blood count). Leukoagglutination occurs when the particles involved are white blood cells).

In summary, agglutination is the clumping of particles together, usually as a response to a specific antibody, and it is commonly used as a method of identifying specific bacterial antigens and the identity of such bacteria, and therefore is an important technique in diagnosis).