CD4-positive T-lymphocyte

In medicine, the CD4-positive T-lymphocyte is "a critical subpopulation of T-lymphocytes involved in the induction of  most immunological functions. The HIV virus has selective tropism for the T4 cell which expresses the CD4 phenotypic  marker, a receptor for HIV. In fact, the key element in the profound  immunosuppression seen in HIV infection is the depletion of this subset of T-lymphocytes."

The number of CD4-positive T-lymphocyte is called the CD4 lymphocyte count and it is reduced in Human Immunodeficiency Virus Type 1 and AIDS.