<prompt> Possible source of second signal to a B-cell bound by specific antigen include -EB virusEndotoxinAntigen-specific T-cellsPlasma cells  <response>: 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. 