many possible serotypes, first obtaining a polyvalent match can reduce the number of tests required.

The binding between a surface antigen and the antiserum can be experimentally observed in many forms. A number of bacteria species, including Streptococcus pneumoniae, display the Quellung reaction visible under a microscope. Others such as Shigella (and E. coli) and Salmonella are traditionally detected using a slide agglutination test. HLA types