Strategy of the Adaptive Immune Response

On first exposure to a given microbe or any other antigen, systemic evidence of the adaptive immune response takes a week or more to develop; during this delay the host depends on the protection provided by innate immunity, which may not be sufficient to prevent disease. This first response to a particular antigen is called the primary response. As a result of that initial encounter, the adaptive immune system is able to "remember" the mechanism that proved effective against that specific antigen. As a result, when the same antigen is encountered later in life, there is an enhanced antigen-specific immune response called the secondary or anamnestic response. The efficiency of the secondary response reflects the memory of the immune system. ■ antigen, p. 372 The adaptive immune response uses two basic strategies for countering foreign material. One response, humoral immunity, works to eliminate antigens that are extracellular, for example, bacteria, toxins, or viruses in the bloodstream or in the fluid that surrounds tissues (figure 16.1). The other, called cellular immunity or cell-mediated immunity, deals with antigens residing within a host cell, such as a virus that has infected a cell. Humoral and cellular immunity are both powerful and, if misdirected, can cause a great deal of damage to the body's own tissues. Because of this, the adaptive immune response is tightly regulated; each lymphocyte, the primary participants in the adaptive response, requires a "second opinion" from a different type of cell before it can unleash its power. ■ lymphocytes, p. 378

Overview of Humoral Immunity

Humoral immunity is mediated by B lymphocytes, or B cells. Their name reflects the fact that they develop in an organ called the bursa in birds. In humans, however, B cells develop in the bone marrow. In response to extracellular antigens, B cells may be triggered to proliferate and then differentiate into plasma cells, which function as factories that produce Y-shaped molecules called antibodies. These molecules bind to antigens, providing protection to the host by mechanisms that will be described shortly. A high degree of specificity is involved in the binding, so a multitude of different antibody molecules are needed to bind to the wide array of antigens that are encountered throughout life. Some of the B cells form memory cells, long-lived cells that respond more quickly if the antigen is encountered again.

Antibody molecules have two functional regions—the two identical arms and the stem of the molecule. It is the arms of the Y that bind to a specific antigen; the amino acid sequence of the end of the arms varies from antibody to antibody, providing the basis for their specificity. The stem of the Y functions as a "red flag," tagging antigen bound by antibody and enlisting other components of the immune system to eliminate the bound molecule.

Antibodies that bind to an antigen protect the host by both direct and indirect mechanisms. Simply by coating an antigen, the antibodies prevent that molecule from binding to critical sites on host cells. For example, a viral particle that has been coated with antibody cannot bind to its intended receptor on a host cell and, therefore, because it cannot attach, it is unable to enter the cell. The indirect protective effect is due to the "red flag" region that facilitates elimination of the antigen by the innate defenses. Phagocytes, for example, have receptors for that region of the antibody molecule, enabling them to more easily engulf an antigen coated with bound antibodies; this is the process of opsonization that was described in chapter 15. ■ attachment of viruses, p. 349 ■ opsonization, p. 383

How does a B cell know when to replicate in order to eventually produce antibodies? Each B cell carries on its surface multiple copies of a membrane-bound derivative of the antibody it is programmed to make; each of these molecules is called a B-cell receptor. If the B cell encounters an antigen that its B-cell receptors bind, then the cell may gain the capacity to multiply. Clones, or copies, of the cell are produced that can eventually differentiate to become plasma cells that make and secrete copious amounts of antibody. Generally, however, before the B cell can multiply, it needs confirmation by another lymphocyte, an effector T-helper cell, that the antigen is indeed dangerous.

Overview of Cellular Immunity

Cellular immunity is mediated by T lymphocytes, or T cells; their name reflects the fact that they mature in the thymus. T cells include two subsets, T-cytotoxic cells and T-helper cells. Both of these cell types have multiple copies of a molecule called a T-cell receptor on their surface, which is functionally analogous to the B-cell receptor, enabling the cell to recognize a specific antigen. Unlike the B-cell receptor, however, the T-cell receptor does not recognize free antigen. Instead, the antigen must be presented by one of the body's own cells. Like B cells, T cells must receive confirmation by another cell that the antigen they recognize signifies danger before they can be triggered to multiply. The resulting T-cell clones differentiate to form effector T cells, which are armed to perform a distinct protective role. The cell type generally responsible for providing the "second opinion" to T cells is the

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Effector T-helper cell

Activates macrophages that have engulfed antigen

Effector T-helper cell

Stimulates effector T-cytotoxic cells that bind antigen

Effector T-cytotoxic cell

Destroys infected host cells

Virus

Virus

Host cells routinely present samples of cytoplasmic proteins.

Those cells presenting viral proteins or other abnormal proteins that signify danger are destroyed.

Host cells routinely present samples of cytoplasmic proteins.

Figure 16.1 Overview of Humoral and Cellular Immunity Cellular immunity is also called cell-mediated immunity (CMI).

dendritic cell, a component of innate immunity. Like B cells, both subsets of T cells are able to form memory cells, which respond more quickly if the same antigen is encountered later in life. However, T cells never produce antibodies. ■ dendritic cells, p. 378

In response to intracellular agents such as viruses, effector T-cytotoxic cells destroy the cells that are harboring the intruder. While this response obviously harms one's own cells, the sacrifice of infected "self" cells ultimately protects the body. For example, destroying virally infected cells prevents those cells from being used by the virus to produce and release more viral particles. Sacrifice of the cells also releases unassembled viral components. This can strengthen the overall immune response by stimulating production of more antibodies that can then block further cellular infection.

The difficulty for the immune system is to distinguish and destroy only those "self" cells that are infected or otherwise tainted; failure to do this can result in an autoimmune disease.

As a mechanism for internal quality control, host cells regularly display short fragments of proteins within their cytoplasm in specialized molecules on the cell surface; this effectively "presents" the proteins for inspection by effector T-cytotoxic cells. If a "self" cell presents an abnormal protein that signifies danger, such as a viral protein, an effector T-cytotoxic cell will induce that cell to undergo apoptosis. ■ apoptosis, p. 387

Rather than killing tainted "self" cells, effector T-helper cells help orchestrate the various responses of humoral and cellular immunity. They provide direction and support to either B cells or T cells; they also provide direction for activation of macrophages.

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