Immunoglobulins – Structure, Classes, Functions of Immunoglobulins

Immunoglobulins (IG)

The immunoglobulins are γ-globulins, called antibodies. All antibodies are immunoglobulin but all immunoglobulins may not be antibodies. They constitute about 20 percent of all the plasma proteins. Immunoglobulins are produced by plasma cells and to some extent by lymphocytes.

Structure of Immunoglobulins

Immunoglobulins are glycoproteins made up of light (L) and heavy (H) polypeptide chains. The term “light” and “heavy” refer to molecular weight. Light chains have a molecular weight of 25,000 whereas heavy chains have a molecular weight of 50,000 to 70,000. All immunoglobulins have the very same standard structure.

The fundamental immunoglobulin is a ‘Y’ shaped particle and consists of 4 polypeptide chains, two H and two L chains. The four chains are connected by disulfide bonds. L chain may be either of 2 types, Kappa (κ) or Lambda (λ) however not both. The heavy chains may be of 5 types and are designated by Greek letter:

  1. Alpha (α).
  2. Gamma (γ).
  3. Delta (δ).
  4. Mu (μ).
  5. Epsilon (ε).

Immunoglobulins are named based on their heavy chain type as IgA, IgG, IgD, IgM, and IgE. The L and H chains are further divided into variable and constant regions. L chain consists of one variable (VL) and one constant (CL) domain or region. Most H-chains consist of one variable (VH) and 3 constant (CH-1, CH-2, and CH-3) domains.

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Each immunoglobulin molecule has a hinge region between CH-I and CH-2, which allows a better fit with the antigen surface. The variable areas of both the light and heavy chains form the antigen-binding sites. The enzyme (papain) food digestion splits the immunoglobulin particle into two pieces called Fab (Fragment for antigen binding) and Fc (crystallizable fragment or fragment for complement binding).

Immunoglobulin Classes
IgG (Heavy chain γ)

IgG is a smaller sized monomeric molecule with two antigen-binding sites. IgG is the major class of immunoglobulin which represents 70 percent of the total serum immunoglobulins. IgG is the only antibody that crosses the placenta and is the maternal antibody that protects the fetus. It is produced mainly in the secondary response and provides defense against bacteria and viruses.

IgA (Heavy chain α)

IgA is the second most abundant class constituting about 20 percent of serum immunoglobulins. It is monomer or dimer. Dimeric IgA particles formed by two monomer units, joined together at their carboxy terminals by a protein described J-chains (J for joining). IgA is found in external secretions such as colostrum, saliva, tears, and respiratory, digestive, and genital system secretions.

It prevents the attachment of microorganisms like bacteria and viruses to mucous surface areas and assists to protect mucous surfaces from antigenic attack and prevent access of foreign substances to the circulation.

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IgM (Heavy chain μ)

It is a pentamer including 5 similar immunoglobulin molecules, collaborated by disulfide bridges. This pentamer is closed in a ring structure by J-chain.

IgM is the very first antibody to be produced in reaction to an antigen and is necessary for defense against bacteria and viruses. IgM present on the surface of B lymphocytes is a monomer, where it operates as an antigen-binding receptor.

IgD (Heavy chain δ)

It is a monomer and looks like IgG structurally. IgD has no known antibody function but might work as an antigen receptor. Like IgM, it is present on the surface of numerous B lymphocytes. The circulating concentration of IgD in the blood is extremely low.

IgE (Heavy Chain ε)

IgE is a monomeric particle comparable to IgG. It is sometimes called reagin. Although IgE exists in trace quantities, (roughly 0.004%) in normal persons with allergic activity have considerably increased amounts. IgE is accountable for the anaphylactic (instant) type of hypersensitivity and allergic reaction. Its activity is moderated by histamine. IgE also takes part in defense against particular parasites, e.g., helminths (worms).

Functions of Immunoglobulins (Antibody)

The main function of antibodies is to protect against contagious representatives.

  • In addition to these functions, antibodies can function as an enzyme to catalyze the synthesis of ozone (O3) that has microbicidal activity.
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Disorders of Immunoglobulins

Abnormally big quantities of specific immunoglobulins may be found in the plasma in several diseases of humans. As well as deficiency of γ-globulins is also found in rare hereditary diseases.

Multiple Myeloma

Multiple myeloma, a plasma cell cancer results due to an abnormally high concentration of serum immunoglobulins, normally IgG or IgA.

Amyloidosis

Amyloidosis is the accumulation of immunoglobulin light chain fragments in the tissues of numerous myeloma patients.

Bence Jones Proteins

In several myelomas, more light chains are produced than heavy chains and enter into the bloodstream, because they are of relatively low molecular weight, they pass through the glomerular membrane and appear in the urine. These protein chains of low molecular weight are referred to as Bence Jones Proteins.

Bence Jones proteins have the remarkable characteristic of precipitating on heating urine from 45 ° to 60 ° C and redissolve when the heating is continued above 80 ° C. Multiple myeloma with Bence Jones protein in the urine is called “light chain disease.” The chain may be of either of the κ-(kappa) or the λ-( lambda) type.