Key Concepts of Section 23.2

Key Concepts of Section 23.2

Immunoglobulins: Structure and Function

  • Most immunoglobulins (antibodies) are composed of two identical heavy (H) chains and two identical light (L) chains (H2L2). Each chain contains a variable (V) region and a constant (C) region. Proteolytic fragmentation yields monovalent F(ab) and bivalent F(ab′)2 fragments, which contain variable-region domains that retain antigen-binding capability (see Figure 23-9). The Fc fragment contains constant-region domains and determines their ability to activate complement components or bind to receptors specific for Fc regions expressed on leukocytes.

  • Immunoglobulins are divided into classes based on the constant regions of their heavy chains (see Figure 23-10). In mammals, there are five major classes: IgM, IgD, IgG, IgA, and IgE; the corresponding heavy chains are referred to as µ, δ, γ, α, and ε. There are two major classes of light chains, κ and λ, again characterized by the attributes of their constant regions.

  • IgM and IgA can form higher-order structures: IgM can form pentamers (five identical H2L2 copies), and IgA can form dimers (two identical H2L2 copies)

  • Each individual B lymphocyte expresses an immunoglobulin of unique sequence and is therefore uniquely specific for a particular antigen. Upon recognition of antigen, only a B lymphocyte that bears a receptor specific for it will be activated and expand clonally (clonal selection) (see Figure 23-12).

  • The antigen specificity of antibodies is conferred by their variable regions, which contain regions of high sequence variability, called hypervariable or complementarity-determining regions (see Figure 23-13a). These hypervariable regions are positioned at the tip of the variable region, where they can make specific contacts with the antigen for which a particular antibody is specific.

  • The repeating immunoglobulin domains that make up immunoglobulin molecules have a characteristic three-dimensional structure, the immunoglobulin fold, which consists of two β sheets held together in a sandwich by a disulfide bond (see Figure 23-13b).

  • The constant regions of the heavy chains endow antibodies with unique effector functions, such as the capacity to bind complement, the ability to be transported across epithelia, or the ability to interact with receptors specific for the Fc portion of immunoglobulins.