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-lymphocytes is called the CD4 lymphocyte count and it is reduced in AIDS.