See image on pg 16: T-and B-cell activation:
Two signals are required. First, alloantigen binds to antigen-specific receptors- the TCR (T cells) or surface IgM (B cells). The second, or costimulatory, signal is provided by IL-1 released by the antigen-presenting cell. CD4 helper T cells (Th) release IL-2, IL-4, and IL-5, which provide help for CD81 T cells (Tc) and for B-cell activation.