The complement-fixation test might be applied to the determination of unknown bacteria, using the unknown culture as antigen and trying it with the sera of different animals immunized against a variety of organisms, some one of which might prove to furnish _specific amboceptor_ for the unknown organism and hence indicate what it is. The test used in this way has not been shown to be a practical necessity and hence is rarely employed. It has been used, however, to detect traces of unknown proteins, particularly blood-serum proteins, in medico-legal cases in exactly the above outlined manner and is very delicate and accurate. CHAPTER XXX. PHAGOCYTOSIS--OPSONINS. It has been mentioned that Metchnikoff, in a publication in 1883, attempted to explain immunity on a purely cellular basis. It has been known since Haeckel’s first observation in 1858 that certain of the white corpuscles do engulf solid particles that may get into the body, and among them bacteria. Metchnikoff at first thought that this engulfing and subsequent intracellular digestion of the microörganisms were sufficient to protect the body from infection. The later discoveries (discussed in considering Ehrlich’s theory of immunity) of substances present in the blood serum and even in the blood plasma which either destroy the bacteria or neutralize their action have caused Metchnikoff to modify his theory to a great extent. He admitted the presence of these substances, though giving them other names, but ascribed their formation to the phagocytes or to the same organs which form the leukocytes--lymphoid tissue generally, bone marrow. It is not within the province of this work to attempt to reconcile these theories, but it may be well to point out that Ehrlich’s theory is one of _chemical substances_ and that the _origin_ of these substances is not an _essential_ part of the theory, so that the two theories, except in some minor details, are not necessarily mutually exclusive. [Illustration: PLATE V ELIE METCHNIKOFF] Sir A. E. Wright and Douglas, in 1903, showed that even in those instances where immunity depends on phagocytosis, as it certainly does in many cases, the phagocytes are more or less inactive unless they are aided by chemical substances present in the blood. These substances _act on the bacteria, not on the leukocytes_, and change them in such a way that they are more readily taken up by the phagocytes. Wright proposed for these bodies the name _opsonin_, derived from a Greek word signifying “to prepare a meal for.” Neufeld and Rimpau at about the same time (1904), in studying immune sera, observed substances of similar action in these sera and proposed the name _bacteriotropins_, or bacteriotropic substances. There is scarcely a doubt that the two names are applied to identical substances and that Wright’s name _opsonin_ should have preference.