Normal Variations in the Susceptibility of Human Fibrin to Streptococcus Fibrinolysin. ∗

There are marked differences in the susceptibilities of different samples of presumably normal human plasma-clot to the thrombolysin formed or secreted by certain strains of S. hemolyticus. While one clot may be completely liquefied within 30 minutes after admixture with a given lytic culture, the clot from a second, presumably normal individual may show no sign of softening even after 24-hours incubation with this culture. 1

To test the possibility that this difference may be due to quantitative differences in fibrin-content, parallel fibrinolytic titrations were made with different concentrations of the same isolated fibrin. Typical data thus obtained are recorded in Table I.

From this table it is seen that the rate and completeness of fibrinolysis with a given lytic filtrate varies inversely with the fibrin-content. Complete insusceptibility to lysis, however, does not result from an increase in fibrin-content even to 4 times that of the average normal human plasma-clot. This result is in line with the statistical evidence reported by Hadfield, Magee and Perry. 3 These investigators found no correlation between fibrin-content and fibrinolytic susceptibility in different blood samples.

To test the possibility that the observed differences in susceptibility may be due to qualitative differences between “normal” and “immune” fibrins, parallel fibrinolytic titrations were made with fibrins isolated from susceptible and resistant plasma-clots. A typical comparison of this type is recorded in Table II.

From this table it is seen that serum-free fibrins isolated from resistant and susceptible plasmas are very nearly identical in their fibrinolytic susceptibilities. The slightly delayed softening of the “immune” fibrin is presumably due to adsorbed antibodies. This is confirmed by the fact that susceptible fibrinogen redissolved in and afterwards reisolated from resistant serum, has approximately this same slight degree of insusceptibility. This conclusion is in line with data reported by Fuchs 4 in his study of “normal” and “immune” fibrins by other technical methods.