T Lymphocytes Antigen Recognition and Response

T lymphocytes, or T cells, share several important characteristics with B cells and, in fact, are identical in appearance microscopically. Like the B cells, T cells have multiple copies of a receptor on their surface that recognizes a specific antigen. The T-cell receptor (TCR) consists of two polypeptide chains, designated alpha (a) and beta (b), each with a variable and constant region (figure 16.15). As in the B-cell receptor, the variable regions supply the antigen-binding sites. The specificity of the T-cell receptor is like that of the B-cell receptor; of the approximately 1010 T cells in the body, only one or a few will recognize a given epitope.

Alpha chain

Beta chain

Alpha chain

Beta chain

16.7 T Lymphocytes: Antigen Recognition and Response 407

Antigen: MHC-binding site

Variable

Antigen: MHC-binding site

Variable region

- Constant region

Figure 16.15 Two T-Cell Receptors Composed of Alpha and Beta Polypeptide Chains Each chain has one variable (V) and one constant (C) region. The two chains are connected by a disulfide bond. Unlike antibodies, which have two binding sites for antigen,T-cell receptors have only one.

region

- Constant region

Figure 16.15 Two T-Cell Receptors Composed of Alpha and Beta Polypeptide Chains Each chain has one variable (V) and one constant (C) region. The two chains are connected by a disulfide bond. Unlike antibodies, which have two binding sites for antigen,T-cell receptors have only one.

Despite the similarities in certain characteristics, the role of T cells is very different from that of B cells. For one thing, T cells never produce antibody. Instead, the effector form directly interacts with another cell, the target cell, to cause distinct changes in that cell. Another important difference is that the T-cell receptor never interacts with free antigen. Instead, the antigen must be "presented" by another host cell. Antigen presentation can be viewed as a host cell seeking a T cell's opinion about the significance of a certain protein. The host cell does this by degrading, or processing, the antigen and then displaying individual peptides from the proteins; each fragment is cradled in the groove of a surface glycoprotein called the major histocompatibility complex molecule, or MHC molecule. The MHC molecule is shaped somewhat like an elongated bun and holds the peptide lengthwise, like a bun holds a hot dog. T cells will recognize an antigen only if it has been processed and presented by an MHC molecule; the T cell is actually recognizing both the peptide and MHC molecule simultaneously. In other words, the T-cell receptor recognizes the "whole sandwich," the peptide: MHC complex.

Although all T cells are identical in appearance micro -scopically, two distinct functional populations have been identified: T-cytotoxic cells and T-helper cells. The most practical way for scientists to distinguish the cells is to take advantage of various surface proteins called cluster of differentiation (CD) markers (or molecules). Most T-cytotoxic cells have the CD8 marker (CD8+) and are frequently referred to as CD8 T cells; most T-helper cells carry the CD4 marker (CD4+) and are often called CD4 T cells. Note that CD4 is the receptor for HIV, which explains why the virus infects T-helper cells.

We will first discuss the types of T cells and their respective effector functions. Then, we will examine the events that lead to the activation of naive T-cells, enabling them to proliferate and develop the functions we will have already described.

408 Chapter 16 The Adaptive Immune Response

Functions of Effector T-Cytotoxic (CD8) Cells

Effector T-cytotoxic cells induce apoptosis, or programmed cell death, in "self" cells infected with a virus or other intracellular microbe; they also destroy cancerous host cells. In addition, effector T-cytotoxic cells produce various cytokines that allow neighboring cells to become more vigilant against intracellular invaders. One of the cytokines, for example, stimulates antigen processing and presentation in nearby cells, facilitating the detection of infected cells by effector T-cytotoxic cells. Another cytokine selectively activates local macrophages whose toll-like receptors have been triggered. Note that a more efficient mechanism of macrophage activation involves effector T-helper cells and will be discussed in more detail shortly.

How do effector T-cytotoxic cells distinguish dysfunctional cells from their normal counterparts? The answer lies in the significance of antigen presentation by a type of MHC molecule called MHC class I (figure 16.16a).

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