Cellular Immunity

Objectives

When you have completed this section, you should be able to

• list the types of lymphocytes involved in cellular immunity and describe the roles they play;

• explain how antigen-presenting cells activate T lymphocytes;

• explain how interleukins coordinate the actions of immune system cells; and

• explain the role of memory cells in cellular immunity.

Cellular immunity involves four classes of T cells:

1. Cytotoxic T (TC) cells are the "effectors" of cellular immunity which carry out the attack on foreign cells. They are also called killer T cells, but are not the same as natural killer cells.

2. Helper T (TH) cells promote the action of TC cells as well as playing key roles in humoral immunity and nonspecific defense. All other T cells are involved in cellular immunity only.

3. Suppressor T (TS) cells limit the cell-mediated attack and keep the immune system from running out of

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control. Th and TS cells are regulatory lymphocytes that act somewhat like an accelerator and brake on TC action.

4. Memory T cells are descended from the cytotoxic T cells and are responsible for memory in cellular immunity.

Th cells are also known as T4, CD4, or CD4 + cells, because they have a surface glycoprotein called CD4. Cytotoxic (TC) and suppressor (TS) cells are collectively known as T8, CD8, or CD8+ cells after their glycoprotein, CD8. (CD stands for cluster of differentiation, a classification system for many cell surface molecules.) These glycoproteins enable T cells to bind to other cells in the events to be described shortly.

With the foregoing introduction to the "actors" of the immune system, we can now discuss the "plot"—the mechanisms of immunity. Both cellular and humoral immunity occur in three stages that we can think of as recognition, attack, and memory (or "the three Rs of immunity"—recognize, react, and remember). In cellular immunity, the events of each stage are as follows.

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