Agglutination is the clumping together of antigens and antibodies. It occurs when the antibodies bind to particulate antigens. This causes the antigens to crosslink and form visible aggregates. Common applications of agglutination tests include blood typing (ABO and Rh), diagnosis of typhoid (Widal test), and identification of antibodies against Rh antigens (Coombs test). The titer or end point of an agglutination test refers to the highest dilution at which antigen-antibody clumping is still visible.