There are two subclasses of T-cells: CD4 cells and CD8 cells. CD8 cells are known as cytotoxic (cell toxic) T-cells because they kill infected cells directly. In contrast, CD4 cells activate other cells to fight an infection. CD4 T-cells are responsible for initiating the antibody response, and they can also tell cells known as macrophages to destroy any bacteria they are carrying.
CD4 is often referred to as a co-receptor because it works together with other receptors during the course of the immune response. It is probably one of the most well-known receptors in the body because of the CD4-HIV connection.
Over time, HIV infection leads to a decline in the number of CD4 cells. Because of the importance of these cells to the immune response, a loss of CD4 cells causes people to become more susceptible to infection. That is why CD4 count is one of the criteria used to determine if a person has progressed to AIDS.