The present studies were undertaken to analyze the nature of the effect of bacterial lipopolysaccharide (LPS) on antibody production in vitro. We have done this by making comparative studies of the effects of LPS on in vitro primary and secondary antibody responses to soluble hapten-protein conjugates and to particulate and soluble sheep erythrocyte antigens. The results obtained demonstrate that the biological action of LPS in vitro may be predominantly manifested on the function of B lymphocytes or T lymphocytes depending on the conditions employed. In the absence of antigen, LPS appears to act primarily on B lymphocytes. In the presence of antigen, however, the data presented here show that LPS significantly influences specific helper T-cell function and it is this latter influence that is predominantly responsible for the adjuvant effects of LPS on antigen-specific antibody responses.

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