Agglutination is the clumping of particles. The word agglutination comes from the Latin agglutinare (glueing to).

Agglutination is a reaction in which particles (as red blood cells or bacteria) suspended in a liquid collect into clumps usually as a response to a specific antibody.

This occurs in biology in two main examples:

Hemagglutination is the process by which red blood cells agglutinate, meaning clump or clog. The agglutin involved in hemagglutination is called hemagglutinin. In cross-matching, donor red blood cells and the recipient's serum or plasma are incubated together. If agglutination occurs, this indicates that the donor and recipient blood types are incompatible.

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.

An example is the PH-L form of phytohaemagglutinin.

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.