Prolonged C3 depletion by cobra venom factor in thymus-deprived mice and its implication for the role of C3 as an essential second signal for B-cell triggering.

Cobra venom factor (CoF) is a potent immunogen in intact mice, and its capacity to deplete C3 is neutralized by antibody. In thymus-deprived (T times B) mice antibody was not elicited; C3 depletion by CoF was prolonged, and subsequent injections were also effective. The effect of prolonged C3 depletion was examined on the antibody response to two T cell-independent immunogens, levan and SIII. The spleen PFC responses to levan were unaffected by C3 depletion or thymus deprivation; those to SIII were unaffected by thymus deprivation but were diminished (not abolished) by C3 depletion. It is argued that the capacity to activate C3 is neither sufficient nor necessary to cause an immunogen to be T cell-independent.