Immunoglobulin structure is a glycoprotein that is made in response to an antigen and can recognise and bind to the antigen that caused its production. The basic structure of immunoglobulin is composed of 4 polypeptide chains. These are composed of two identical heavy (H) and two identical light (L) chains. They can be separated into domains that bind antigens. Immunoglobulins are synthesised by plasma cells. 

Basic protein structure of immunoglobulin 

• These immunoglobulins are composed of 4 polypeptide chains.

• 2 heavy (H) and 2 light (L) chains.

• These chains are linked by disulphide bonds. 

• Light and heavy chains are subdivided into variable and constant regions.

• Each heavy and light chain contains amino terminal in region and carboxy terminal in constant regions. 

• Each immunoglobulin peptide chain has an intra chain disulphide bone that forms a loop.

• Each loop is compactly folded to fork a globular structure domain.

• Light chain contains a single variable domain and a single constant domain. 

• Heavy chain contains one variable domain and 3 constant domains (CH1, CH2, CH3).

• Hinge region is the segment in the heavy chain between CH1 and CH2.

Classification 

Based on structure and the nature of antigen of the Heavy chain the immunoglobulin is classified into 5 classes. 

• Ig G (gamma)
• Ig A (alpha)
• Ig M (mu)
• Ig D (delta)
• Ig E (epsilon)

Immunoglobulin G (Ig G)

• Ig G is the most abundant class in the serum.
• This constitutes 80% of total immunoglobulin.
• Ig G presents in the blood, plasma and tissue fluids.
• Ig G contains less carbohydrate than other immunoglobulins.
• It has a half-life of 23 days, which is the longest of all the immunoglobulin protein isotypes.
• Ig G can cross the placenta membrane and this provides immunity to the foetus during pregnancy.
• Ig G acts against the bacteria and viruses.
• Catabolism in ig G is different in that it varies with its serum concentration.

Subclasses of Ig G

Subclasses of Ig G are ig G1, ig G2, ig G3, ig G4.

Biological function of subclasses 

• Ig G1, ig G3, ig G4 can cross the placental barrier hence helps in foetus nutrition and development.

• Ig G3 activates complement.

• Ig G1, ig G3 binds to the fc receptor on phagocytic cells, monocytes and mediated opsonisation.

Immunoglobulin A (Ig A)

• Constitutes of 10-15 % of total immunoglobulin.
• Ig A presents in milk, saliva, tears, mucous of respiratory tract, genital tract and digestive tract. 
• Ig A exists in a serum called monomer.
• Ig A exists in an external secretion called the secretory immunoglobulin.
• Half-life of Ig A Is 6-8 days. 

Formation of secretory Ig A

• Dimeric ig A binds to the receptor on the surface of the epithelial cells and is transported across the luminal surface. 

• Secretory piece protects ig A from digestive enzymes.

Functions

• Ig A provides local immunity 

• Secretory ig A binds to the surface antigens of microorganisms and prevents attachment and invasion on the mucosal surface of the respiratory and digestive tract.

• It helps to defend against salmonella, Vibrio cholerae, N. gonorrhoeae and influenza virus.

Immunoglobulin M (IG M)

• Ig M is the polymer of pentamer.
• Ig M is held together by disulphide bonds and ‘J’ chain.
• Half-life of M is 5 days.

Functions

• It helps in agglutination of bacteria 

• Activates the complement system by classical pathway.

• IgM causes opsonisation and immune haemolysis.

Immunoglobulin E (ig E)

• Ig E has a structure similar to ig G. 
• It has 4 constant region domains
• Molecular weight 1,90,000
• Half-life: 2 days.
• Ig E is heat labile (inactivated at 56 degree C)
• Normal serum concentration is 9.3ug/ml
• Mostly present in extracellular surface 
• Ig E cannot cross the placenta membrane 
• IgE binds to the fc receptor of blood and mast cells.
• When two ig E molecules on the surface of these cells are cross linked by binding on the same antigen are called cell degradation.

Functions 

• Ig E is known as the reagin antibody.
• Ig E is responsible for symptoms of anaphylactic shock, hay fever or asthma.
• If E plays an important role in immunity against helminth parasites.

Immunoglobulin D (ig D)

• Structure is similar to ig G

• Serum concentration: 30mg/ml

• Half-life: 3 days

Conclusion 

Antibody molecules are glycoproteins of one or two or more units, each containing four polypeptide chains. Typically, the immunological response to an antigen is heterogeneous, resulting in many different cells. Which produces antibodies to the same antigen. As a consequence of this heterogeneous response serum forms an immunised animal will contain numerous antigen specific antibody clones.