This subject has been sufficiently illustrated at pp. 17 and 18, where it is pointed out that, as a part of the generic, specific, racial, and individual characters, there are great differences in the reception which the pathogenic agents receive from different animals. Indeed, the designation pathogenic is to be used relatively to particular animals. A microbe is pathogenic in some animals and non-pathogenic in others, or, in other words, some animals are susceptible to a particular infection and others are immune.

Coming closer to the matter we have to inquire into the meaning of these varying degrees of natural immunity. As all the activities of the body are directly related to the cells, so immunity must be ultimately resolvable into Cellular action.

Phagocytosis is a term devised by Metschnikoff to designate the mode in which, according to his view, immunity comes about. Phagocytosis is a direct cellular action, and it is at least one of the modes in which the cells deal with the infective agents. Metsch-nikoffs first observations were made on a disease of the water-flea or daphne, in which fungus spores are visible in the transparent body of the animal. He found that when the spores were present in small numbers the amoeboid leucocytes, collecting around them, took them into their substance, destroyed their power of germination, and finally disintegrated them, while if many spores were present, some of them, which were not taken up by the leucocytes, germinated and grew through the body. Thus it appeared that the active cells of the animal have the power of attacking, and in some cases englobing, the intruding organisms, which are afterwards disposed of by a process of digestion.

In the further development of this doctrine MetschnikofF recognizes a difference between the leucocytes and the fixed cells of the tissues. Both may become amoeboid and both have the power of taking up solid particles and digesting them. In view of this power he names them Phagocytes ( =to devour). These two forms of cells present different reactions to the various infective agents, and for the sake of distinction the leucocytes which have divided nuclei are called Microphags, and the cells derived from the fixed cells of the tissues, which are larger and have large oval nuclei, are Macrophags. In all diseases of this class there is liable to be a struggle between the infective agents and the phagocytes, whether the microphags or the macrophags.

This process of phagocytosis undoubtedly proceeds with more or less success in many infective diseases. In erysipelas the microbe produces a severe inflammation, and the leucocytes englobe the bacteria and dispose of them. The anthrax bacillus, introduced into the frog, which is immune to this form, is taken up by the leucocytes, so that in a few hours none are free. The leucocytes convey the microbes to the liver and spleen, where they are disposed of within the cells, presenting various stages of degeneration. In other animals the number of bacilli undergoing disintegration in the spleen is in inverse proportion to the susceptibility of the animal to anthrax. Observations on relapsing fever and tuberculosis seem to show that immunity largely depends on the power of the living cells to englobe and dispose of the pathogenic microbes.

But this is not necessarily the whole explanation of natural immunity. Cells may act in other ways than by englobing, they may prevent the entrance of microbes by exercising a repellent influence on them. The layers of cells at the various surfaces of the body, cutaneous and mucous, have an important office in preventing the intrusion of microbes, and immunity to various forms of disease, such as some manifestations of tuberculosis, has probably this explanation. Cellular activity is not limited to the one mode of action.