What is the Structure and Function of an Antibody?

Antibodies are Y-shaped glycoproteins, whose basic functional unit is an immunoglobulin (Ig) monomer. The Ig monomer consists of four polypeptide chains, including two equivalent heavy chains and two equivalent light chains. These chains comprise structural domains which vary in the number of amino acids that are featured with different sizes and functions.

Heavy and light chains

The sizes and compositions of heavy chains decide the types of antibodies. A single heavy chain can be divided into a constant region and a variable region, while a light chain only has one constant domain and one variable domain.

Such a Y-shaped structure with both constant and variable structural domains allows a dual antibody function of antigen binding and biological activity mediation. The stem part of the Y-shaped molecule is called the Fc segment (the fragment crystallizable region), and the two arms of the molecule are called antibody Fab fragments (the part for binding to other fragments and antigens), in which two whole light chains and twp half of the heavy chains are contained.

The Antigen Binding Function

The Fab regions are two points where an antibody binds to two antigens, and a Fab region includes a constant domain and a variable domain from each of the heavy and the light chain. The well-known concept of recognizing specific foreign objects and binding to antigens is based on the variable domain, also known as the Fv region.

The Immune Cell Activity Mediation Function

The Fc region, containing two half heavy chains, is usually responsible for binding to specific proteins to generate immune responses for given antigens. It also interacts with various cell surface receptors and some immune molecules to mediate physiological effects, such as opsonization, cell lysis, basophils, and eosinophils.

Five Different Isotypes of Antibodies

The differences between heavy chains are presented by the classification of antibodies. There are five types of antibody molecules found in serum, including IgG, IgM, IgA, IgE and IgD.

IgG has a typical immunoglobulin monomer structure with the smallest molecular mass and the highest content in human serum and extracellular fluid. It could provide protection for months and years after the presence of the antigen that has triggered their production. IgG also protects against bacteria, viruses, neutralizes bacterial toxins, triggers complement protein systems and binds antigens to enhance the effectiveness of phagocytosis, making it one of the most widely used research products in the laboratory. For instance, its simple structure facilitates the process of antibody labeling in experiments.

IgM is involved in the ABO blood group antigens on the surface of RBCs. Moreover, scientists also found the predominant circulating class of antibodies in the shark similar to the earliest produced IgM macroglobulins of fetal humans. As a result, current studies often use Chondrichthyes and Teleostei, for example, zebrafish antibodies, as model organisms to carry out research projects.

IgA is the first defense of mucosal surfaces of the intestines, nose, and lungs, which can bind to and aggregate antigens on microbes before they invade tissues and then expel with secretions.

IgE has the least content in normal human serum, and it usually binds to mast cells and basophils which participate in the immune response. Besides, it may be related to the body's anti-parasitic immunity. When parasites and allergens invade, the content of immunoglobulin E in the serum is significantly increased.

Immunoglobulin D (IgD) antibodies, similar to IgM, are expressed in the plasma membranes of immature B-lymphocytes, playing a role in inducing antibody production. A secreted form of IgD can also be found in blood serum with a small amount.

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