Immune Response

Even though we have a single immune system, it is diversified into two subsystems so we can combat the multitude of infectious agents we encounter in our lifetimes. This diversification is a result of the differing approaches that B and T cells have to ridding the body of infectious agents once they are found. B cells provide a response called humoral immunity, while T cells provide a cell-mediated immunity.

Humoral Immunity Blood and lymphatic fluid were referred to as "body humours" in medieval times; B cells fostered immunity and were given the name humoral immunity. When a B cell encounters an antigen, it immediately makes

Figure 11.13 Clonal populations. When a lymphocyte binds an antigen, it proliferates to produce many copies of the lymphocyte and its antigen receptor. This strengthens the immune system's ability to rid the body of that infectious agent.

copies of itself, resulting in a population of identical cells able to help fight the infection. This population of cells is called a clonal population.

The entire clonal population has the same DNA arrangement, and all the cells in a clonal population carry the same antigen receptor on their membrane. The cells of the clonal population, called memory cells, will help the body respond more quickly if the infectious agent is encountered again. Should subsequent infection occur, the large number of memory cells facilitates a quicker immune response (Figure 11.13).

In addition to the memory cells produced when an antibody binds to an antigen, B cells also produce plasma cells. Plasma cells secrete antibodies specific to an antigen. The antibodies secreted by plasma cells circulate within body fluids, including tears and saliva; when they encounter an antigen, they bind to it. This antigen-antibody complex then combines with proteins in the blood called complement proteins. When complement proteins attach to antibody-antigen complexes on the pathogen's plasma membrane, they cause the cell to break open (Figure 11.14). Antigen-antibody complex binding also increases phagocytosis and the overall ability of the immune system to destroy invaders.

Cell-mediated Immunity T cells also respond to infection by undergoing rapid cell division to produce memory cells and by becoming specialized cells. However, unlike B cells, T cells do not secrete antibodies; instead, they directly attack other cells. Two of these attacking cell types are the cytotoxic T cells and helper T cells. Cytotoxic T cells attack and kill body cells that have become infected with a virus. When a virus infects a body cell, viral proteins are placed on the surface of the host cell. Cytotoxic T cells recognize these proteins as foreign, bind to them, and destroy the entire cell. They do this before the virus has had time to replicate by releasing a chemical that causes the plasma membranes of the target cell to leak.

Helper T cells, also called T4 cells, can be thought of as boosters of the immune response. These cells detect invaders and alert both the B and T cells that infection is occurring. Without helper T cells, there can be almost no immune response. Helper T cells also secrete a substance that greatly increases the level of cytotoxic T cell response. The AIDS virus, HIV, infects helper T cells, thus crippling the body's ability to respond to any infection (see Chapter 9).

Humoral Immunity

B-cell receptors (antibodies)

Humoral Immunity

B-cell receptors (antibodies)

Antigens

Plasma cells secrete antibodies specific to the antigen.

Memory cells help the body respond more quickly if the antigen is encountered again.

Antigens

Plasma cells secrete antibodies specific to the antigen.

Memory cells help the body respond more quickly if the antigen is encountered again.

Clonal population

/ Clonal population

/ Clonal population

Clonal population

Complement proteins

Complement proteins

Antibodies bind to antigens, then bind to complement proteins and cause pathogen cells to burst.

Figure 11.14 Humoral immunity. B lymphocytes do not kill cells bearing antigens directly. Instead they make and secrete antibodies specific to antigens. The antibodies circulate in the blood and lymph. The antibody-antigen complex binds with complement proteins in the blood, and together the antibody and complement can burst bacterial plasma membranes.

Macrophages also help fight infection by transferring an antigen to their own plasma membranes, thus alerting the immune system, via the T cell, to a foreign antigen present in the body. When the macrophage presents an antigen to a T cell that has the correct receptor, the T cell replicates itself to produce more memory cells, more cytotoxic T cells, and more helper T cells. Figure 11.15 summarizes the cell-mediated immune response.

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