Serum bacterial reaction by the alternative pathway of complement: potentiation by target multiplicity and modulation by serum factors.

The bactericidal reactions of normal human serum and of ethylene glycol-bis(beta-aminoethyl ether)-N,N-tetraacetic acid (trisodium salt)-Mg2+-treated human serum differ in a number of aspects. The latter reaction was shown (i) to require the integrity of the alternative pathway of complement, (ii) to display two levels of efficiency depending on the concentration of target bacteria, and (iii) to be modulated by the addition of thermostable serum fractions with inhibitory and stimulatory activities. The inhibitory activities were characterized to a certain extent; they comprised several glycoprotein species of molecular weights between 60,000 and 20,000. The stimulatory activities could be measured only after depletion of the inhibitors by concanavalin A-Sepharose or heat-killed bacteria and could be fractionated into several peaks by gel filtration.