Humoral Immunity and Antibody Structure at a Glance
- Humoral immunity, mediated by antibodies produced by B lymphocytes, is a form of specific immunity directed primarily toward extracellular antigens.
- Antibody molecules consist of two identical light chains covalently linked to two identical heavy chains. The variable region of the antibody molecule is responsible for antibody binding, and the constant region mediates most effector functions.
- The five antibody classes serve distinct functions. Immunoglobulin (Ig) M is involved in primary antibody responses, IgD is an antigen receptor on naive B cells, IgA is critical for mucosal immunity, IgG is the major Ig in the circulation and is important in secondary antibody responses, and IgE mediates immunity to parasites.
- An individual is capable of generating millions of distinct antibodies in millions of distinct B-cell clones through the processes of gene rearrangement and junctional diversity.
During evolution, jawed vertebrates developed the capacity to respond with exquisite specificity to foreign organisms.1 Specific immunity is characterized by an enormous diversity of possible responses and by refinement in the immune response with successive exposures to the organism.2 The cells that can discriminate with fine specificity through their vast repertoire of receptors are lymphocytes. Specific immunity, also called adaptive immunity because it develops as an adaptation to infection, can be segregated into humoral immunity, mediated by antibodies produced by B lymphocytes, and cellular immunity, mediated by T lymphocytes. These two forms of specific immunity developed to serve different functions. Humoral immunity is directed primarily toward extracellular antigens such as circulating bacteria and toxins. Cellular immunity is directed primarily toward antigens that infect or inhabit cells (see Chapter 10). To combat extracellular pathogens, the defending agent needs to be abundant and widely distributed in the body, particularly at its interfaces with the environment. Antibodies fulfill these characteristics by being capable of being secreted in great quantity from the cells that produce them and by being distributed in blood, mucosa, and interstitial fluid. In addition, antibodies can attach through Fc receptors (FcRs) to the surface of certain other cells of the immune system, such as mast cells, conferring antigen specificity to cells that do not have their own endogenously produced antigen-specific receptors. In addition to their major function in humoral immunity as antibody producers, B lymphocytes have a role in antigen presentation, regulation of T-cell subsets and dendritic cells, organization of lymphoid tissues, and cytokine and chemokine production.3,4
Antibodies, or immunoglobulins (Ig), are a family of glycoproteins that share a common structure.2,5,6 The antibody molecule has a symmetric Y-shape consisting of two identical light chains, each about 24 kDa, that are covalently linked to two identical heavy chains, each about 55 or 70 kDa, that are covalently linked to one another (Fig. 37-1). Within the light and heavy chains are variable and constant regions. The major function ...