In the presence of infection certain reaction products are developed in the infected individual. Their development is excited by the infective bacteria. This immunizing process may be due to the action of the cells or of the fluids or of both. If the process more particularly affects the cells the result is termed cellular immunity; if it affects the fluids, humoral immunity. Some of the substances formed are protective to the individual; others apparently have little demonstrable protective influence, and still others sensitize the subject. The substances formed in humoral immunity may be divided into: (1) agglutinins, (2) precipitins, (3) complement-binding substances and (4) opsonins.
The demonstration of these substances forms the basis of the serum reactions which have attained so much importance in recent years. The detection of agglutinins attained its greatest importance in the diagnosis of infections caused by typhoid, paratyphoid, and colon bacilli, in which they are