Red blood cells are not the only particulate antigens which can be used for agglutination assays. In this example, microscopic latex particles are coated with an antibody specific for a bacterial (streptococcal Group A) antigen. In the absence of antigen, the antibody-coated particles exhibit a smooth milky distribution (left). In the presence of the antigen (a small amount of cultured bacteria) the particles clump into the pattern seen at the right. This "reverse agglutination" (so named because it is the antibody, not the antigen, which is immobilized on the particles) is one of many kinds of tests used to identify different bacteria.