Immunoglobulin G (IgG) and IgA, but also nonantibody factors, account for in vitro neutralization of human immunodeficiency virus (HIV) type 1 primary isolates by serum and plasma of HIV-infected patients.

The factors present in serum and plasma samples of human immunodeficiency virus (HIV)-infected patients that are responsible for the neutralization of four HIV type 1 (HIV-1) primary isolates in vitro have been analyzed. Purification of immunoglobulins (Ig) by affinity chromatography showed that the activities were mostly attributable to IgG and less frequently to IgA. For two samples, we have shown that the high-level and broad-spectrum inhibitory activity was essentially caused by non-Ig factors interfering with the measurement of antibody-specific neutralizing activity.