Possible source of "second signal" to a B-cell bound by specific antigen include -

Ans. is 'c' i.e., Antigen-specific T cells Role of T cell in antibody formation: . Antigen with class II M fiC on antigen presenting cells activates the CD4 helper T cells. CD4 helper T cells produce cytokines that stimulate B cells which have already exposed to similar antigen. . Though antibody is produced by B cells ( after differentiating into plasma cells), coordination of helper ( CD 4) T cells is also necessary. In antibody formation T cells are involved earlier than B cells. . After antigen exposure, antigen presenting cells present antigen to CD 4 helper T cells, which then secrete cytokines that induce differentiation of B cells ( which have already been exposed to same antigen) to plasma cells. . Plasma cells form antibody against that specific antigen.