Complement components required for virus neutralization by early immunoglobulin antibody

Virus neutralization by early antisera usually requires or is enhanced by the presence of complement. The particular components of the complement system which are needed for neutralization of Newcastle disease virus by early IgM antibodies were studied. It was found in this system that participation of only the first four of the nine complement components (C1, C2, C3, and C4) was necessary and sufficient for neutralization to occur. It was concluded that the role of complement in neutralization was most likely that of contributing bulk in the form of large protein molecules to the virus-antibody complex.