There are two major subpopulations of T cells with different names, functions, and cell-surface markers— helper T cells and cytotoxic T cells (see Fig. 43.3). Helper T cells do just that—they help other cells of the immune system by secreting cytokines. One of their key roles is to activate B cells to secrete antibodies. Although B cells can work on their own, particularly against encapsulated bacteria, in most cases they require the participation of helper T cells. Helper T cells also activate macrophages, cytotoxic T cells, and other cells of the immune system.

Cytotoxic T cells kill other cells—that’s what “cytotoxic” means. Like B cells, they are activated by cytokines released from helper T cells. They are particularly effective against altered host cells, such as those infected with a virus or that have become cancerous.

These two types of T cell are distinguished by the presence of different glycoproteins on their surface—CD4 on helper T cells and CD8 on cytotoxic T cells. The ratio of these two classes of T cell can be used to assess immune function. In healthy individuals, the CD4 : CD8 ratio is usually about 2:1. A decreased ratio, indicating the presence of fewer helper T cells relative to cytotoxic T cells, is typical of individuals infected with HIV. HIV infects and kills helper T cells, disarming a key player in the immune system. In addition to helping distinguish the two types of T cell, CD4 and CD8 assist receptors on T cells carry out their functions, a topic we discuss next.