A model for CD4⁺ T-cell–dependent normal hematopoiesis. (A) In the presence of antigen-primed CD4⁺ T cell, cytokines are secreted and terminal differentiation of hematopoietic committed progenitors and precursors is efficient. (B) When CD4⁺ T cells are absent or their cognate antigens are not available, hematopoiesis is defective, resulting in (left) immature cell accumulation in the bone marrow or (right) leukopenia in the peripheral blood.