Figure 26-22B cells and T cells.

Lymphocytes are the white blood cells responsible for the specific immune response. These cells can be found circulating in the blood and lymphatic systems, and they reside in lymphatic organs such as the lymph nodes and spleen. (See Section 21-11 to review the lymphatic system.) In contrast to blood, which contains a variety of white blood cells, 99% of the lymph is made up of just one type: the lymphocytes. Lymphocytes have antigen receptors, proteins on their plasma membranes that stick out from the cell surface and can bind to specific antigens—each lymphocyte can bind to just one type of antigen. The presence of antigen receptors allows lymphocytes, collectively, to recognize and react to a wide array of antigens.

There are two major types of lymphocytes. Both develop from stem cells in the bone marrow, but one type leaves the bone marrow and continues to mature in the thymus, a lymphatic organ located in the upper chest. Lymphocytes are named for where they mature, so those that mature in the bone marrow are named B lymphocytes, or B cells, and those that mature in the thymus are called T lymphocytes, or T cells (FIGURE 26-22). The other cells that normally circulate in the lymphatic system are dendritic cells, which use the lymphatic system for the sole purpose of meeting up with lymphocytes.

Both B and T cells have antigen receptors on their surfaces. Lymphocyte antigen receptors are structurally diverse. In fact, there are endless variations of these receptors. It is estimated that our lymphocytes can recognize billions of different pathogens. But each individual lymphocyte doesn’t have billions of different antigen receptors. Instead, each lymphocyte has just one type of receptor that recognizes just one antigen, and it has many copies of this receptor on its surface.

Like an intense military operation on land and sea, the specific immune system operates on two fronts in order to win the war. Lymphocytes fight pathogens in both body fluids (humoral immunity) and within cells (cell-mediated immunity) (FIGURE 26-23).

Figure 26-23Fighting invaders in body fluids and within cells. B cells are responsible for humoral immunity (antibody-mediated immunity), and T cells are responsible for cell-mediated immunity.

1. In humoral immunity, also called antibody-mediated immunity, the individual is protected against pathogens and toxins found in body fluids, such as blood and lymph. The word humoral comes from “humors”—the term that Greeks and Romans used for body fluids. Blood was one of the four humors (the other three were phlegm, yellow bile, and black bile). When an antigen is detected by a B cell with the matching antigen receptors on its surface, the B cell secretes antibodies into the blood, lymph, and other body fluids. The circulating antibodies defend against specific pathogens and toxins and make it easier for phagocytes to engulf them.

2. In cell-mediated immunity, the individual is protected against pathogens that have invaded and are located inside body cells. All your body cells have molecules on their cell membranes that identify them as “self” to the cells of your immune system—as cells that belong. Body cells that have been infected by a pathogen (and many cancer cells) present some different proteins (antigens) on their membranes. When a T cell recognizes one of these antigens (because the T cell has the matching antigen receptors on its surface), it binds to the antigen. This binding initiates an immune response that kills the infected cell and, usually, the pathogen within.

To defeat a specific pathogen, numerous identical lymphocytes (bearing the same antigen receptor) are needed. You may be starting to ask yourself, do we have trillions of immune cells? How does the body have room for all of these individual armies of lymphocytes ready to face any enemy it might—or might not ever—encounter? The answer is that the body doesn’t store vast quantities of identical lymphocytes. Rather, for each of the billion or more different antigen-recognizing receptors, it has just a small number of lymphocytes with those receptors. It makes copies of these lymphocytes only if the antigen is encountered. In the next section, we see how the body does this.