Serological reactions like precipitation, agglutination, complement fixation and neutralization tests are used to detect antibodies and antigens. Precipitation reactions form visible precipitates when antigen and antibody interact and form lattices. Immunodiffusion techniques like Ouchterlony and immunoelectrophoresis allow antigen-antibody reactions to be visualized as precipitin lines or arcs in agar gels. Agglutination reactions cause visible clumping when particulate antigens react with specific antibodies.