Which cells secrete B cell growth factor?

• A. Cytotoxic T cells
• B. B cells
• C. T helper cells
• D. Macrophages

Answer: (C)

B cell growth depends on signals from other immune cells, with helper T cells providing the main stimulus. They secrete a cytokine known as B cell growth factor, most notably interleukin-4, which acts on B cells to promote their proliferation, differentiation into antibody-secreting cells, and class switching. This helper signal is characteristic of TH2-type responses that support B cell–mediated immunity. Cytotoxic T cells focus on destroying infected targets and release cytotoxic molecules rather than driving B cell growth. B cells can produce antibodies and some cytokines, but they are not the primary source of the growth factors that stimulate their own expansion. Macrophages produce inflammatory cytokines that modulate the response but do not serve as the main B cell growth factor source.