CD4 and neoplasm: Two other main cell types contributing to the immunosuppression found in tumors are: myeloid-derived suppressor cells (MDSCs), which deplete essential amino acids (L-arginine and cysteine) necessary for the function of T cells, and regulatory T (Treg) cells which play a vital role in self-tolerance maintenance and can also act as an immunosuppressive barrier (a subset of CD4+ T cells only), inhibiting anticancer immunity in tumor-bearing hosts and impeding protective immunosurveillance against neoplasia.58