Structure of immunoglobulins Made Easy
An antibody also termed Immunoglobulin (Ig), was initially called a neutralizing substance. They were discovered in 1890 by two immunologists, Emil von Behring and Kitasato Shibasaburo. Antibodies are proteins that are produced in response to the exposure of the antigen. The antibodies belong to a group of serum proteins called Immunoglobulins (Igs).
So, Immunoglobulin is a glycoprotein that is made in response to an antigen and can recognize and bind to the antigen that caused its production. Let’s know about the characteristics of these specialized proteins,
- They are synthesized by plasma cells in response to antigens and function as antibodies.
- Immunoglobulin are gamma globulins.
- It constitutes about 25-30% of total serum proteins.
- They are a specialized group of proteins mostly associated with the Y globulin fraction.
- Immunoglobulins are a functional term while Y globulin is a physical term.
It is to be noted that, “All antibodies are Immunoglobulins, but all Immunoglobulins may not be antibodies”.
Structure of Immunoglobulins
- Basically, all immunoglobulin molecules are composed of 4 polypeptide chains:
• Two identical heavy chains.
•Two identical light chains (similar in all Immunoglobulins).
which we call as H2L2, a Y-shaped tetramer.
- Each heavy chain consists of 450 amino acids and the light chain consists of 212 amino acids.
- Both these chains are held together by disulfide linkages and non-covalent interactions.
- Light and heavy chains are subdivided into variable and constant regions.
– In light chain,
•Amino terminal half is variable constant (VL).
• Carboxy terminal half is a constant region (CL).
– In heavy chain,
• One-quarter of the amino-terminal region is a variable region (VH).
• Remaining three quarters is a constant region (CH1, CH2, CH3).
- The light chain has 3 hypervariable regions and the heavy chain has 4 hypervariable regions.
The amino acid sequence of variable regions of light and heavy chains is responsible for the specific binding of antibodies (immunoglobulin) with antigens. The hypervariable regions more specifically determine the antigen-antibody binding.
Types of Immunoglobulins
Mainly Immunoglobulins are classified into five classes:
•IgG – gamma heavy chains
•IgM – Mu heavy chains
•IgA – alpha heavy chains
•IgD – delta heavy chains
•IgE – epsilon heavy chains
Immunoglobulin G (IgG)
- This constitutes about 80% of total Immunoglobulin.
- These are present in the blood, plasma, and tissue fluids and contain less carbohydrates than other Immunoglobulins.
- IgG is the most abundant class in serum and has a half-life of 23 days(the longest of all of the other Immunoglobulins).
- It crosses the placenta and provides natural immunity to fetus and neonate at birth.
- IgG acts against bacteria, and viruses by opsonizing and also neutralizing toxins.
- Catabolism of IgG varies with its serum concentrations.
- The subclasses of IgG are IgG1, IgG2, IgG3, and IgG4.
•IgG1, IgG3, IgG4 – cross the placenta and protect the fetus.
•IgG3 – activates complement.
•IgG1, IgG3 – binds to Fc receptor on phagocytic cells, monocytes, and macrophages and mediate opsonization.
Immunoglobulin M (IgM)
- This is the first immunoglobulin to be produced in the primary response to an antigen.
- It accounts for 5-10% of total serum proteins.
- The molecular weight of IgM is 900,000-10,00,000(millionaire molecule) and the half-life for this is 5 days.
- About 80% of IgM present intravascularly and at low concentration in intercellular tissue fluids and cannot cross the placenta.
- The presence of IgM antibodies in the serum of newborns indicates congenital infection.
- It agglutinates bacteria and activates complement by the classical pathway.
- Also, it is responsible for protection against blood invasion by microorganisms.
- These are a predominant class of antibodies produced in the primary response to an antigen.
- A person having blood group A antigen will have anti-B antibodies in his circulation. These are produced without any known antigenic stimulation and are hence called natural antibodies.
Immunoglobulin A (IgA)
- IgA constitutes 10-15% of total Immunoglobulins.
- It is present in milk, saliva, tears, and mucous of the respiratory tract, digestive tract, and genitourinary tract.
- Half cycle for this ranges from 6-8 days.
- In serum, it exists as a monomer, and in external secretions, it exists as a dimer called a secretory piece.
- The monomer and the dimer are held together by a J chain.
- IgA is the most predominant antibody in colostrum.
- Secretory IgA present in breastmilk protects newborns during the first months and acts as important line of defense against various viruses and bacteria.
- It promotes phagocytosis and intracellular killing of microorganisms and provides local immunity.
Immunoglobulin D (IgD)
- IgD is a single Y-shaped monomer unit.
- It is present in low concentrations in circulation and also present on the surface of B cells where it acts as a receptor for antigen.
- The structure is similar to IgG.
- Its serum concentration is 30 micrograms per ml and constitutes about 0.2% of total immunoglobulins.
- The half-cycle is 3 days.
- IgD together with IgM is a major membrane-bound immunoglobulin on unstimulated B lymphocytes which acts as a recognition receptor for antigens.
Immunoglobulin E (IgE)
- Its structure is similar to IgG(a single Y-shaped monomer) and has 4 constant region domains.
- The molecular weight is 1,90,000 and its half cycle is 2 days.
- The normal serum concentration is 0.3g/ml, it is heat liable (inactivated at 56°C in 1 hour).
- This does not cross the placenta and is mostly present extracellularly.
- These are associated with the body’s allergic responses and are produced during hay fever, asthma, anaphylactic shock, etc.
- IgE tightly binds with Fc receptors on basophils and mast cells and which release histamine and cause allergy.
- It does not activate complement and agglutinate antigens.
- IgE plays a role in hypersensitivity reactions and in immunity against helminthic parasites.
- The physiological role of IgE is to protect against pathogens by mast cell degranulation and release inflammatory mediators.
So generally, Immunoglobulins are a group of proteins which produced by our body in response to the presence of foreign bodies. That is, if our body detects any foreign bodies or any antigen, our body will produce immunoglobulins. So, these are the ones that protect our bodies from any infection.
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