CD4 and graft versus host disease: The mechanisms of donor T cell (CD4+ T cell and CD8+ T cell)-mediated GVHD are multifactorial and include activation of macrophages and antigen-presenting cells (APC) by transplantation conditioning regimens to damage host tissue, releasing soluble cytokines such as TNF-α and IL-1; alloreactive T cell activation, proliferation and differentiation in response to host or donor APC; and alloreactive T cell infiltration and release of pro-inflammatory cytokines which leads to damage of the target tissue [2].