The bloods of two apparently healthy human beings, of 25 studied, failed to produce a strong bactericidal test for type-specific antibody to the M protein of group A streptococci under in vitro conditions wherein most human blood leukocytes rapidly phagocytize and destroy virulent organisms in the presence of anti-M antibody and accessory plasma factors.
The defect in bactericidal activity of these two individuals is associated with the plasma rather than with the blood leukocytes. Leukocytes suspended in these atypical plasmas showed a characteristic delay in the rate of activation of phagocytosis.
Although previously the bloods of laboratory animals (except monkeys) had been reported to be much less active than human blood in this system, occasional exceptions were encountered in rabbits in this study. Two rabbits were found whose bloods were as strongly bactericidal against streptococci, in the presence of type-specific antibody, as the blood of the average "normal" human being.
The atypical behavior of some human and rabbit bloods in the bactericidal test may be explained by variations in accessory plasma factors that are as yet unidentified and that influence the rate of phagocytosis of virulent streptococci in vitro in the presence of type-specific antibody.