Immunity and Resistance 635 



chronic P. inui or P. knowlesi infections also are beneficial to monkeys 

 with the homologous infection (46, 47). Likewise, serum therapy has pro- 

 tected canaries against P. cathemerhim (86) and P. circumflexum (110) 

 and chickens against P. lophurae when adequate dosage was continued 

 over a long enough period (180). 



In certain parts of Africa, the comparative incidence of malaria in 

 infants and in older children has suggested to some workers that passive 

 immunization may be important in man. Cases of malaria are relatively 

 rare in young infants but become more and more common toward the end 

 of the first year. On this basis, it has seemed possible that resistant mothers 

 transmit to their infants an immunity which is rather effective during the 

 first few months after birth and then gradually disappears. 



FACTORS INVOLVED IN ACQUIRED 

 RESISTANCE 



Antibodies 



The development of an acquired immunity may involve both a 

 specific intensification of the host's normal defensive reactions and the 

 appearance of defensive factors not present in the normal animal. The 

 mechanism of resistance may include an increased phagocytic activity, 

 specific for the homologous parasite, as well as the production of specific 

 antibodies affecting the parasite directly. Substances which induce such 

 reactions upon parenteral introduction into an animal are known as 

 antigens. In general, an antigen may be considered a protein which, if it 

 is to show antigenic properties in a particular animal, must be chemically 

 foreign to that animal. Since Protozoa, like other microorganisms, con- 

 tain more than a single type of antigen, any strain should be considered 

 an antigenic complex rather than a pure antigen. There is probably a 

 certain amount of overlapping among related Protozoa. One or more 

 similar, or possibly identical, antigens (group antigens) may occur in 

 several strains or in several species. Other antigens (species-specific or 

 strain-specific) are limited to a single species or a single strain. Among 

 bacteria, strain-specificity may depend upon certain non-protein sub- 

 stances (Jmpteyies) which modify the antigenicity of proteins. Such sub- 

 stances have not yet been investigated extensively in Protozoa. However, 

 possibly specific polysaccharides have been reported from leptomonad 

 stages of Leishmania tropica (153) and also from Trypanosoma cruzi, 

 Leishmania brasiliensis, L. donovani, Endotrypanum schaudinni, Lepto- 

 monas culicidarurn, and L. oncopelti (119a). It is interesting that the 

 polysaccharide fractions from Leptomonas gave negative precipitin tests 

 with antisera for the other flagellates, while T. cruzi showed fairly strong 

 cross-reactions with anti-Leishmania sera. It has been suggested that the 

 lipoid fraction, rather than the carbohydrate fraction, is related to the 

 antigenic peculiarities of trypanosomes (97a). 



