CD4 and aplastic anemia: Alloantigen-specific CD4+ T cells can contribute to GvH-driven aplastic anemia either directly, by recognition of major histocompatibility alloantigens on bone marrow precursors (Sprent et al., 1994), or indirectly, by provision of help to minor histocompatibility alloantigen-specific CD8+ T cells (Chen et al., 2004).