Cell-Mediated Immunity Another part of the immune response, which depends on the action of macrophages and several types of T cells, is called cell-mediated immunity. This part of the immune system defends the body against some viruses, fungi, and single-celled pathogens that do their dirty work inside body cells. T cells also protect the body from its own cells if they become cancerous.

When a cell is infected by a pathogen or when a macrophage consumes a pathogen, the cell displays a portion of the antigen on the outer surface of its membrane. This membrane attachment is a signal to circulating T cells called helper T cells. Activated helper T cells divide into more helper T cells, which go on to activate B cells, activate cytotoxic T cells, and produce memory T cells.

FIGURE 35–12 Cytotoxic T cell

FIGURE 35–13 Memory T cell

Cytotoxic T cells hunt down body cells infected with a particular antigen and kill the cells. They kill infected cells by puncturing their membranes or initiating apoptosis (programmed cell death). Memory helper T cells enable the immune system to respond quickly if the same pathogen enters the body again.

Another type of T cell, called suppressor T cells, helps to keep the immune system in check. They inhibit the immune response once an infection is under control. They may also be involved in preventing autoimmune diseases.

Although cytotoxic T cells are helpful in the immune system, they make the acceptance of organ transplants difficult. When an organ is transplanted from one person to another, the normal response of the recipient's immune system would be to recognize it as nonself. T cells and proteins would damage and destroy the transplanted organ. This process is known as rejection. To prevent organ rejection, doctors search for a donor whose cell markers are nearly identical to the cell markers of the recipient. Still, organ recipients must take drugs—usually for the rest of their lives—to suppress the cell-mediated immune response.

35.2 Assessment

Review Key Concepts

1. 1. Review List the body's nonspecific defenses against pathogens.

   2. Sequence Describe the steps of the inflammatory response.

2. 1. Review How does the immune system identify a pathogen?

   2. Compare and Contrast How are the roles of B and T cells different? How are their roles similar?

3. 1. Review What are the two main styles of action of the specific immune response?

   2. Apply Concepts Why would a disease that destroys helper T cells also compromise the humoral response?

VISUAL THINKING

4. These two T cells are attached to a cancer cell. What type of immune response are these cells a part of?

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