In Passive Hemagglutination, sheep red blood cells (SRBC) are coated with a purified antigen (in this case from T. pallidum, the bacteria which causes syphilis). Allowed to settle in a well of a 96-well plate, such cells well settle into a small button, as seen in the last six wells of the top row (vairous negative controls). In the presence of antibody, such red cells will agglutinate, yielding the pattern seen at the top left (a 1:160 dilution of a control positive serum). Each of the sixteen patients' sera (numbered 5-19) is added to such "sensitized" SRBC (top of each pair of wells) as well as normal, unsensitized cells (bottom).