Functional separation of the two signals involved in stimulating immunological responses was achieved through the judicious use of two natural bacterial antigens. Native protoplasmic polysaccharide (NPP) extracted from Escherichia coli was immunochemically identical to the lipopolysaccharide (LPS) extracted from the same organism. However, NPP was not endotoxic, not mitogenic, did not fix complement, and was immunologically independent of T cells. The NPP, which appeared to contain only the antigenic signal, could induce a primary antibody response in mice and could sensitize mice for a secondary response. However, the antigenic signal contained in NPP was insufficient to trigger a secondary response in mice primed with either NPP or LPS. LPS, containing both the antigenic and second signals, was required to trigger a secondary response in primed mice.

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