Various streptococcal species produce an haemosensitizing factor during the logarithmic phase of growth. A variety of mammalian cells sensitized with this factor become agglutinated following the addition of antistreptoccocal serum and also undergo cytopathic changes in the presence of complement. The haemosensitizing factor is thermostable and is unaltered by trypsin, papain, chymotrypsin, lipases or ribonucleases. Attempts to destroy the binding sites on the cell membrane by treatment with phospholipase C from Clostridium welchii or by neuraminidase failed. Treatment with trypsin or papain on the other hand markedly increased the binding capacity of red blood cell for the haemosensitizing factor.
Studies on the nature of the binding sites on the erythrocyte membrane of the haemosensitizing factor suggest that cholesterol and phospholipids constitute some of the binding sites for this factor.