The previous article dealt in detail with immunoglobulin and how they help in phagocytosis. This article is about the types of immunoglobulins, their functions.
The types of immunoglobulins are based on the types of light and heavy chains. There are two types of light chains namely the kappa and the lambda. An immunoglobulin contains either kappa (K-K) or lambda (L-L) but does not have a mixture of both (K-L not possible). About 60% of the immunoglobulins in humans have kappa chains.
So, the classes of immunoglobulins are based on the heavy chain. So based on this condition, there are five classes of immunoglobulins namely:-
- Immunoglobulin G (IgG) – gamma
- Immunoglobulin M (IgM) – mu
- Immunoglobulin A (IgA) – alpha
- Immunoglobulin D (IgD) – delta
- Immunoglobulin E (IgE) – epsilon
These immunoglobulins have certain configurations and play different roles in the human body. The immunoglobulin G is present the most. It constitutes about 80% of the total immunoglobulin. These are mostly present in the blood, plasma, and other body fluids. This immunoglobulin has the lowest carbohydrate content when compared to the rest. This immunoglobulin has a half-life of 23 days which is the longest of all. Some of the unique features and functions of this immunoglobulin:-
- This is the only immunoglobulin which can cross the placenta (this is a unique feature because this immunoglobulin provides immunity to the foetus inside the womb and also after birth for some months. Presence of others may indicate infection)
- This helps in killing bacteria and viruses by opsonisation (the process of covering the pathogen with a protein coat such that the pathogens become more presentable to the immune cells)
- Neutralize toxins
- Activate complement by classical pathway (The complement system, also known as complement cascade, is a part of the immune system that enhances the ability of antibodies and phagocytic cells to clear microbes and damaged cells from an organism, promote inflammation, and attack the pathogen’s cell membrane)
- Unique catabolism (breaking down of molecules) based on concentration
- There are four sub classes (G1, G2, G3 and G4) out of which 1,3 and 4 cross the placenta and offer immunity
- Also involves in the Rh immunization (there are two types’ Rh+ve and Rh-ve based on the presence of Rh factor in blood). The mother being Rh+ve and child the opposite is not a problem in the first pregnancy but can be fatal in second, killing the foetus.
The immunoglobulin M constitutes about 5-10% of total proteins. This is a pentamer structure with a J chain. This weighs about 900000-1000000 and is the heaviest of all. They have 5 days of half-life. Some of its features-
- Presence in newborn indicate congenital infection as they don’t cross placenta
- Short lived, so their presence indicates recent infection
- First Ig to participate in primary response
- classical pathway
- bacteria agglutination
- Play an important role in ABO blood grouping (discovered by Landsteiner). There are 8 types of blood groups based on antigen, antibody and Rh factor
Immunoglobulin A is also known as the secretory immunoglobulin and is mostly present in body secretions (tear, saliva, sebum, mucous, and milk) in which they are dimer and are monomer in blood. They constitute 10-15% of the proteins. They also have a J chain and secretory piece. Their half-life is 6-8 days.
- The secretory piece protects the Ig from enzymes and juices
- Complement activation by alternate pathway
- Promote phagocytosis
- Intracellular microorganism killing
- First line of defense against some microbes
Immunoglobulin E is a dimer similar to IgG. This is present in low concentrations (about 0.3) and has the weight of about 1,90,000. These have a half-life of about 2 days and can become inactivated at 56 C.
- Present extra-cellularly
- Associated with allergic reactions like asthma, hay fever and anaphylactic shock
- Bind with the Fc of mast cells and basophils resulting in degranulation and release histamine which causes allergy
- Mediate the some immunity reactions
- No complement activation
- Provide immunity against helminthes
The last is immunoglobulin D. It is present in low concentrations and on the surface of B lymphocytes. They constitute 0.2% of proteins. They have a half-life of 3 days. The IgM and IgD bind on the B lymphocyte to help in antigen identification.
Hence these were the different types of immunoglobulins and the mechanisms by which they help with immunity. The next article is about the process of inflammation.