Murine. B-cell-stimulating factor type 1 (BSF-1) acts on resting B cells. and drives them to a transitional activated state characterized. by the expression of high levels ofIa antigens (4, 5).In. combination with antibodies to surface Ig receptors (anti-Ig), it induces B cells to synthesize DNA (3, 6-8). T cells are involved in cell-mediated immunity, whereas B cells are primarily responsible for humoral immunity (relating to antibodies). The function of T cells and B cells is to recognize specific "non-self" antigens, during a process known as antigen presentation.