When foreign bodies or antigens enters the body and blood stream,the immune system activates or produces antibodies in serum and body fluids and these antibodies react with antigens. The sera with high antibody levels after infection or immunization is called the immune sera.The term immunoglobulin means proteins of animal origin with known antibody activity and other chemically related proteins.
Structure of Immunoglobulin(antibody structure)
human Immunoglobulins are glycoproteins each made up of two pairs of different sizes of polypeptide chains.The smaller chains are called light( L) chains and the larger( H) chains.The chain is attracted to the H chain through the disulphide bond.The molecular weight of the L chain is about 25,000 and the H chain is about 50,000.H- chains are structurally and antigenically different for each class and are indicated by a Greek letter corresponding to immunoglobulin as described below
|Immunoglobulin class (classes of antibodies)||H chain|
|immunoglobulin m (Ig M )||μ (mu)|
The H- chains are similar in all immunoglobulin classes.They are of two kinds: lambda (λ) and kappa (κ ) .The antigen combination site of the molecule is located at its amino terminals which both contain L and H chains.The amino acid sequence of the carboxy terminal half occurs only in a constant sequence and is therefore referred to as the constant region, whereas the amino acid sequence in the amino terminal half of the chain is highly variable and is therefore referred to as the variable region.The antibody specificity range of immunoglobulins depends on the variability of amino acid sequences in the variable range of H
Classes of Immunoglobulin and their properties
Human sera contain the various immunoglobulin types IgG, IgA, IgM, IgD and IgE in decreasing order of concentration.
|Immunoglobulin classes||Properties and Functions|
|IgG||This is the most important composition of immunoglobulin in serum of about 80 percent of the total amount and is evenly distributed between intracellular and extracellular compartments. |
It contains less carbohydrates than other immunoglobulins. It has a half- life of approximately 23 days.
IgG is the only immunoglobulin that is transported via placenta and offers passive immunity in new bonds.IgG binds to microorganisms and intensifies their phagocytosis.
IgG protects against infectious agents that are active in the blood and tissue.
Passively administered IgG suppresses homologous antibody synthesis by an icedback process.This property is used for isoimmunization of women by administration of anti- Rh( D)-IgG during delivery. Four subclasses were identified: IgG, IgG2, IgG3, IgG4,
|IgA||The second most abundant class, constituting of about 10– 13 percent of serum Immunoglobulins and its half life is 6 – 8 days. It is the main immunoglobulin in colostrum, saliva and tears.|
The IgG variety plays an important role in Lovac immunity to and against intestinal pathogens and is resistant to digestive enzymes and reducing agents.
IgA antibodies inhibit the adhesion of microorganisms to the surface of mucosal cells by covering the organisms.It promotes phagocytosis and the intracellular killing of microorganisms.
|IgM||IgM constitutes of 5 – 8 percent of serum immunoglobulins with a half life of about five days|
It is synthesized by the foetus from 20 weeks of age.
It is not transported via the placenta, the presence of IgM in the fetus or newborn indicates intrauterine infection and its detection is useful in the diagnosis of congenital infections such as syphilis, rubella, HIV infection, and toxoplasmosis.
It is found in the intravascular space and is responsible for protection against blood invasion by microorganisms.
IgM deficiency is frequently associated with septicaemia.
|IgD||This is structurally similar to IgG. It’s intravascular and has a half- life of about three days. |
IgD and IgM occur on the surface of non- stimulated B lymphocytes and serve as recognition receptors for antigens
|IgE||This is structurally similar to IgG with a half- life of about two days. |
It is mostly extravascular and does not pass through placental barriers. Serum levels rise during asthma and eczema.
IgE is produced in the linings of the airways and intestinal tract.
Deficiency is associated with IgA deficiency in individuals with limited immunity.
IgE is responsible for the anaphylactic nature of the hypersensitivity.