Although the function of HLA-DR on the surface of T cells is poorly understood, studies on infectious pathogens including Mycobacterium tuberculosis (the causative agent of tuberculosis, or TB) and Human Immunodeficiency Virus (HIV), show that HLA-DR+CD4+ T cells have the ability to persist longer in the periphery and are more resistant to suppression mechanisms initiated by regulatory T cells (63–65). Here, CD4 is linked to tuberculosis.