Recognizing “Nonself” In addition to recognizing “self,” the immune system recognizes foreign organisms and molecules as “other,” or “nonself.” That's remarkable, because we're surrounded by an almost infinite variety of bacteria, viruses, and parasites. Once the immune system recognizes invaders as “others,” it uses cellular and chemical weapons to attack them. And there's more. After encountering a specific invader, the immune system “remembers” it. This immune “memory” enables a more rapid and effective response if that same pathogen, or a similar one, attacks again. This specific recognition, response, and memory are called the immune response.

Antigens How does the immune system recognize “others”? Specific immune defenses are triggered by molecules called antigens. An antigen is any foreign substance that can stimulate an immune response. Typically, antigens are located on the outer surfaces of bacteria, viruses, or parasites. The immune system responds to antigens by increasing the number of cells that either attack the invaders directly or that produce proteins called antibodies.

The main role of antibodies is to tag antigens for destruction by immune cells. Antibodies may be attached to particular immune cells or may be free-floating in plasma. The body makes up to 10 billion different antibodies. The shape of each type of antibody allows it to bind to one specific antigen.

Lymphocytes The immune system guards the entire body, which means its cells must travel throughout the body. The main working cells of the immune response are B lymphocytes (B cells) and T lymphocytes (T cells). B cells are produced in, and mature in, red bone marrow. T cells are produced in the bone marrow but mature in the thymus—an endocrine gland. Each B cell and T cell is capable of recognizing one specific antigen. A person's genes determine the particular B and T cells that are produced. When mature, both types of cells travel to lymph nodes and the spleen, where they will encounter antigens.

FIGURE 35–6 B Lymphocyte

FIGURE 35–7 T Lymphocyte

Although both types of cells recognize antigens, they go about it differently. B cells, with their embedded antibodies, discover antigens in body fluids. T cells must be presented with an antigen by infected body cells or immune cells that have encountered antigens.

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