Studies on the nature of the immunity reaction.—I. An experimental study of pneumococcal immunity

Pneumococcal infection, both in man and animals, has been the subject of study by the writer during several years past. A careful histological investigation of the morbid histology of pneumonia, carried out in conjunction with Gaskell (1), in 1913-14, using his method of gelatin-embedded sections, led to the conclusion that infection of the lungs, in this disease, is by way of the air passages and not, as formerly believed, by the blood stream. Experimental proof of this view was afforded by the successful production of pneumonia in rabbits by intratracheal insufflation (2) (1914). The existence, in London, of serological types of pneumococcus, identical with those found in America and South Africa, was demonstrated (3) in 1919-20. This work was extended, the year afterwards, with the aid of the “ absorption of agglutinin ” test, to the serological study of the pneumococcus group ; various sub-types of the recognised disease-producing strains were then identified (4). A study of the means by which immunity from pneumococcal infection is acquired was a natural sequence to these researches. It is already well known that inoculation of laboratory animals, rabbits and mice, with killed cultures of disease-producing pneumococci, renders these animals proof against subsequent infection with living organisms of the same strain. Furthermore, it has been found that the blood serum of an artificially immunised rabbit and that of the human patient after crisis and recovery from lobar pneumonia equally possess immunising properties ; when inoculated into the naturally susceptible mouse, these sera confer protection against subsequent infection with living pneumococcus of the homologous strain.