and more than 80% of helper T cells (CD4+) in humans. However, some T cells lose CD28 expression during activation, particularly antigen-experienced T cells, which can be re-activated independently of CD28. These CD28^- T cells are often antigen-specific, terminally differentiated, and categorized as memory T cells (TMs). The proportion of CD28^- T cells increases with age.

As a homodimer