Antibodies are immunoglobulins, which react specifically with the antigen that stimulated their production. They make up about 20% of the plasma proteins. Antibodies generated in response to a single complex antigen are heterogeneous because they are formed by many different clones of cells. Each clone expresses an antibody capable of reacting with a different antigenic determinant on the complex antigen. These antibodies are called polyclonal. In contrast, immunoglobulins that arise from a single clone of cells, such as a plasma cell tumor (myeloma), are homogeneous and are called monoclonal antibodies. Monoclonal antibodies can be produced in vitro by fusing a myeloma cell with an antibody producing B lymphocyte.

T he immunoglobulin (Ig) molecules share common structural features; that is, all the Ig molecules are composed of light and heavy polypeptide chains. The terms light and heavy refer to their molecular weight. The light chains have a molecular weight of approximately 25,000, whereas the heavy chains have a molecular weight of approximately 50,000. Each Ig molecule consists of two identical light (L) chains and two identical heavy (H) chains linked by disulfide bridges. The L chains can be either κ (kappa) or λ (lambda) and their classification is made based on the amino acid differences in their constant regions (Figure 1). Both light chain types can occur in all classes of immunoglobulins (IgG, IgM, IgA, IgD, and IgE), but any one Ig molecule contains only one type of L chain. The amino terminal portion of each L chain contains part of the antigen-binding site. Similarly, the H chains are distinct for each of the five immunoglobulin classes and are designated γ (gamma), μ (mu), α (alpha), δ (delta), and ε (epsilon) (Table 1). The amino terminal portion of each H chain participates in the antigen-binding site; the other (carboxyl) terminal forms the Fc fragment (Figure 1). The Fc portion of the Ig molecule participates in various biologic activities such as complement activation.

Fig1. Schematic representation of an IgG molecule, indicating the location of the constant and the variable regions on the light and heavy chains. Fab fragment is fragment antigen binding, Fc fragment is fragment crystallizable.

Table1. Properties of Human Immunoglobulins

Therefore, an individual antibody molecule consists of identical H chains and identical L chains. The simplest anti body molecule has a Y shape (Figure 1) and consists of four polypeptide chains: two H chains and two L chains. The four chains are covalently linked by disulfide bonds.

When studying the Ig molecule structure, it was identified experimentally that an antibody molecule, such as IgG, can be split into two fragments by the proteolytic enzyme, papain. When this happens, the peptide bonds in the hinge region are broken. The antigen-binding activity is associated with one of these fragments, the Fab portion. The second fragment is the Fc portion that is involved in placental trans fer, complement fixation, attachment to various cells, and other biologic activities.

The L and H chains of an Ig molecule are subdivided into variable regions and constant regions. The regions are composed of three-dimensionally folded, repeating segments called domains. By using high-resolution x-ray crystallography, the structure of these domains has been determined. An L chain is composed of one variable domain (V_L ) and one constant domain (C_L ) whereas most H chains have one variable domain (V_H ) and three or more constant domains (C_H ). Each domain is approximately 110 amino acids in length. The variable regions of the Ig molecule are involved in antigen binding, whereas the constant regions are responsible for the biologic functions described later in this section.

Within the variable regions of both the L and H chains are subregions consisting of extremely variable amino acid sequences, called hypervariable, that cooperate in space to form the antigen-binding site. The hypervariable regions form the area of the Ig molecule complementary in structure to the antigenic determinant (epitope). This area is known as the complementarity-determining region (CDR). Only 5–10 amino acids in each hypervariable region constitute the anti gen-binding site. Antigen binding is noncovalent, involving van der Waals and electrostatic as well as other weak forces.

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مواضيع ذات صلة

B Cell Receptor for Antigen

Antigen Processing and Presentation

Methods of Enhancing Agglutination

Mechanisms of Agglutination

The Major Histocompatibility Complex

Cellular Basis of the Adaptive Immune Response

Mechanisms of Innate Immunity

Overview of Acute-Phase Proteins

C-Reactive Protein

Hematopoietic Stimulators

The Complement System: Diagnostic Evaluation

Biological Functions of Complement Proteins