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resistance to infection and that, in other cases, such a reaction 

 may be demonstrable but may be insufficient to account for 

 immunity. At this point it is natural to raise questions as to the 

 importance of complement and phagocytosis, either as acting 

 independently of an antigen-antibody reaction or as the agents 

 for carrying on the work initiated by such a reaction. 



Questions about complement and phagocytosis form part 

 of a larger problem, the mechanism of vital resistance, which, 

 though it must be viewed from many different aspects, cannot 

 safely be split up into smaller, water-tight compartments. There 

 is no question of choosing between a " cellular " and a " humoral " 

 theory, because the cells and the plasma are equally essential 

 parts of the mechanism, and the condition and contents of cells 

 and plasma are interdependent. Nor is the importance of the 

 cells confined to the phagocytes. Practically all the tissue cells 

 which take any part in the metabolism of the body must be 

 regarded as concerned with immunity in greater or less degree. 

 Particular mention may be made of the endothelial cells, which 

 are present in every part of the body and, apart from their work 

 as phagocytes, act as selective filters between the blood and 

 lymph streams and the tissues. It is therefore dangerous to 

 focus attention exclusively upon one type of cell, e.g., to conclude 

 from in vitro experiments upon leucocytes that the animal's 

 protective mechanism in a particular instance is, or is not, 

 phagocytosis of sensitised bacteria. 



It has long been known that circulating plasma and serous 

 exudates contain substances, other than antibodies, which play 

 a part in immunity, though the precise nature of these substances 

 has never been defined. In the older literature, substances of 

 this sort are often called complement or alexin, terms which, 

 when used with reference to the living body, cannot be accepted 

 as meaning more than postulated factors of unknown nature. 

 But to assume that these factors, or some of them, are identical 

 with the properties of fresh guinea-pig serum, which is the ordinary 

 laboratory sense in which the term complement is used, would 

 not be justifiable. For example, the absence of bacteriolysis 

 in a test-tube experiment, where fresh guinea-pig serum is one 

 of the reagents, would not justify any inference as to the 

 protective mechanism in the body of an animal infected with the 

 bacteria under investigation. It is not at all clear that this 

 laboratory reagent is, or contains, any special substance which 

 is a common constituent of all animal bodies. I have discussed 

 the question at length in relation to the Wassermann test for 

 syphilis.* 



It seems to me, therefore, that antigens and antibodies, even 

 when supplemented with postulated functions of complement 

 and phagocytosis, are not a sufficiently large stock-in-trade of 

 ideas to explain immunity. 



* Ministry of Health. Reports on Public Health and Medical Subjects. 

 No. 1. 1920. 



