HOST RESPONSE TO ANTIGENIC ACTION 477 



engendered by some strains of pneumococci, varying with the spe- 

 cies and age of the animal host. There are also normal opsonins, 

 which prepare the invading bacteria for ingestion by the normal 

 phagocytic cells. In addition to the opsonins protective substances 

 are present in the serum of normal animals to ward off the attack, 

 but these natural protective antibodies may be effective for one or 

 more but not for all serological types of Pneumococcus. The pos- 

 session or lack of these secondary, humoral defensive antibodies 

 appears to parallel the natural susceptibility or resistance of dif- 

 ferent species of animals to pneumococcal infection. Within the 

 species, new-born members are devoid of these circulating anti- 

 bodies, but with increase in age there is a corresponding augmenta- 

 tion of these factors. 



During pneumococcal infections in man, when native resistance 

 is overcome, the body cells, aroused to activity by the stimulating 

 components of the bacteria, elaborate motile immune substances, 

 which comprise agglutinins, precipitins, opsonins, and protective 

 antibodies. A substance, as yet of unknown nature, appears early 

 in the serum of the pneumonia patient, which is capable of precipi- 

 tating the C carbohydrate of Pneumococcus, but the true specific 

 immune substances develop as the disease progresses, usually ap- 

 pearing in greatest quantity at the time of crisis, persisting in the 

 blood stream for a while, and then disappearing during or after 

 convalescence. The humoral antibodies thus evoked are specific for 

 the serological type of the infecting strain, but it is conceivable 

 that the recovered patient may possess for a time some residual 

 immunity to organisms of a type other than that of the infecting 

 pneumococcus. The presence of specific humoral antibodies does 

 not always spell recovery for the patient, but their absence usu- 

 ally presages death. From the pneumococcal cell, as a result of its 

 disintegration in the body, polysaccharides are introduced into the 

 blood and, when in sufficient concentration, inhibit or block the ac- 

 tion of the antibodies. 



The humoral effects and cellular reactions taking place during 



