Concanavalin-A-activated lymphocytes suppress immune responses in vitro but are helper cells in vivo.

In vitro concanavalin A (Con A)-activated spleen cells suppressed the primary in vitro immune response of syngeneic spleen cells to sheep erythrocytes (SRBC) but enhanced the in vivo anti-SRBC responses when transferred to syngeneic untreated mice. Con-A-activated spleen cells injected into nude mice enabled these mice to respond to SRBC.