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B cells are initially activated to secrete antibodies after the binding of antigens to antigen‐specific membrane immunoglobulin (Ig) molecules (BCRs), which are expressed by these cells. It has been estimated that each B cell expresses approximately 100,000 BCRs of exactly the same specificity. Once ligated, the B cell receives signals to begin making the secreted form of this immunoglobulin, a process that initiates the full‐blown antibody response whose purpose is to eliminate the antigen from the host. Antibodies are a heterogeneous mixture of serum globulins, all of which share the ability to bind individually to specific antigens. All serum globulins with antibody activity are referred to as immunoglobulins (see Chapter 6). These molecules have common structural features, which enable them to do two things: (1) recognize and bind specifically to a unique structural entity on an antigen (namely, the epitope), and (2) perform a common biological function after combining with the antigen. Immunoglobulin molecules consist of two identical light (L) chains and two identical heavy (H) chains, linked by disulfide bridges. The resultant structure is shown in Figure 1.3. The portion of the molecule that binds antigen consists of an area composed of the amino‐terminal regions of both H and L chains. Thus each immunoglobulin molecule is symmetrical and is capable of binding two identical epitopes present on the same antigen molecule or on different molecules.

Figure 1.3. Typical antibody molecule composed of two heavy (H) and two light (L) chains. Antigen‐binding sites are noted.

In addition to differences in the antigen‐binding portion of different immunoglobulin molecules, there are other differences, the most important of which are those in the H chains. There are five major classes of H chains (termed γ, μ, α, ε, and δ). On the basis of differences in their H chains, immunoglobulin molecules are divided into five major classes—IgG, IgM, IgA, IgE, and IgD—each of which has several unique biological properties. For example, IgG is the only class of immunoglobulin that crosses the placenta, conferring the mother’s immunity on the fetus, and IgA is the major antibody found in secretions such as tears and saliva. It is important to remember that antibodies in all five classes may possess precisely the same specificity against an antigen (antigen‐combining regions), while at the same time having different functional (biological effector) properties. The binding between antigen and antibody is not covalent but depends on many relatively weak forces, such as hydrogen bonds, van der Waals forces, and hydrophobic interactions. Since these forces are weak, successful binding between antigen and antibody depends on a very close fit over a sizeable area, much like the contacts between a lock and a key.

Besides the help provided by T cells in the generation of antibody responses, noncellular components of the innate immune system, collectively termed the complement system, play a key role in the functional activity of antibodies when they interact with antigen (see Chapter 4). The reaction between antigen and antibody serves to activate this system, which consists of a series of serum enzymes, the end‐result of which is lysis of the target in the case of microbes such as bacteria or enhanced phagocytosis (ingestion of the antigen) by phagocytic cells. The activation of complement also results in the recruitment of highly phagocytic polymorphonuclear (PMN) cells or neutrophils, which are active in innate immunity.