Rabbit and rat platelets contain two non-dialyzable components (analogous to Myrvik's components 1 and 2 of serum) which, together with bicarbonate ions, produce a potent bactericidin for B. subtilis. These platelet factors appear to be identical with the serum bactericidal substances, suggesting that disruption of platelets during blood coagulation releases the platelet factors into serum. Normal human platelets contain no measurable component 1 or 2.

Since bovine serum contains component 2 but little or no component 1, estimation of the concentration of component 1 in sera or platelet suspensions of other species can be made by diluting the materials to be tested in bovine serum. The level at which a bactericidal effect is lost represents the end-point of the titration of component 1.

Rabbit serum has a high concentration of component 1 and a lower concentration of component 2. It can be diluted to a point at which bactericidal activity is lost, owing to a depletion of component 2, but at which component 1 is still present in effective concentration. At this point the addition of another serum or a platelet homogenate causes a restoration of the bactericidal activity and permits an estimate of the concentration of component 2 in the added material.

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