What is an Antibody? Types of Immunoglobulin
Antibody: An immunoglobulin, a specialized immune protein,
An antibody, also known as an Antibody, is a large, Y-shaped protein produced mainly by plasma cells / B cells of the immune system to neutralize pathogens such as pathogenic bacteria and viruses.
The antibody recognizes a unique molecule of the pathogen, called an antigen. There are five kinds of antibodies, such as Immunoglobulins M, G, E, D and A. An antibody is a serum protein that is generally found in blood.
This is how Antibodies works : Animation Video.
The immunoglobulin can be divided into
five different classes, based on difference in the amino acid sequence in the constant region of the heavy chains.
1. IgG – Gamma heavy Chains.
2. IgM – Mu heavy chains.
3. IgA – Alpha heavy chains.
4. IgD – Delta heavy chains.
5. IgE – Epsilon heavy chains.
(Short tricks – GAMED )
Immunoglobulin types –
Immunoglobulin can also be classified by the type of light chain that they have .
Light chain types are based on difference in the amino acid sequence in the constant region of the light chain .
- 1. Kappa light Chains.
2. Lambda light Chains.
Antibodies also called immunoglobulin or Igs. ( with molecular weight of 150 -900Md) Constituents the gamma globulin part of the blood protein. They are soluble protein secreted by the plasma offspring( clones) of primed B.Cells.
IgE – Responsible for autoimmune response , such as allergies and diseases like arthritis , multiple sclerosis and systemic lupus erythematosus.
IgG – is the only antibodies that can cross the placental barrier to the fetus and it is responsible for 3 to 6 month immune protection of newborn that is Conferred by the mother .
IgM – is the dominant antibody produced in primary immune response & physically much larger than the other immunoglobulin.
IgG – dominant in secondary immune response.
Structure and some properties of Immunoglobulin classes :
A. ImmunoglobulinG (IgG) –
1. Structure – All IgG’s are monomers ( 7S immunoglobulin )
2. Half life – 23 days.
3. % total serum antibodies – 75-80 ( most abundant in serum)
4. Properties – IgG is the most versatile immunoglobulin because it is capable of carrying out all of the functions of immunoglobulin molecules.
- I. IgG is the Major Ig in serum – 75% of Serum Ig is IgG .
- II. IgG is the major ig in extra vascular space and in blood and lymph .
- III. Placental transfer – Transfer is mediated by a receptor on placental cell for the Fc region of IgG.
- IV. Fix Complement – Not all subclasses fix equally well; IgG4 does not fix complement.
B. Immunoglobulin M (IgM ) –
1. Structure – IgM normally exists as a Pentamer (19S immunoglobulin) but it can also exists as a monomer.
2. Half life – 05 days
3. % total serum Antibodies – 5 -10 %
4. Properties –
- I. IgM is the third most common serum Ig .
- II. IgM is the first Ig to be made by the fetus and the first Ig to be made by a virgin B cells when it is stimulated by antigens.
- III. As a consequences of its pentameric structure , IgM is a good complement fixing Ig . Thus IgM antibodies are very efficient in leading to the lysis of microorganisms .
- IV. As a consequences of its structure, IgM is also a good agglutinating Ig .thus , IgM antibodies are very good in clumping microorganisms for eventual elimination from the body.
C. Immunoglobulin A ( IgA) –
1. Structure – serum IgA is a monomer but IgA found in secretion is a diamer .
2. Half Life – 06 days
3. % total serum Antibodies – 10 -15%
4. Properties –
- I. IgA is the 2nd most common serum Ig.
- II. IgA is the major class of the Ig in Secretion – tears , saliva ,
colostrums , mucus , IgA is importent
in Local ( mucosal) immunity.
- III. Normally , IgA does not fix coplement , unless aggregated.
D) Immunoglobulin D (IgD) –
1. Structure – IgD exist only as a monomer.
2. Half life – 03 days
3.% total serum Antibody – 0.2%
4. Properties –
- I. IgD is found in low leval in serum, its role in serum is uncertain
- II. IgD is primarily found on B cell surface where it functions as a receptor for antigens.
- IV. IgD does not bind complement
E. Immunoglobulin E (IgE ) –
1. Structure – IgE Exists as a monomer and has an extra domain in the constant region.
2. Half life – 02 days
3. % total serum antibody – 0.002%
4. Properties –
- I. IgE is the least common serum Ig.
- II. involved in allergics reaction .
- III. IgE also plays a roles in parastic helminth diseases . Since serum IgE levals rise in parasitics diseases, Measuring IgE levals is helpful in diagnosing parasitic infections.
- IV ) IgE does not fix Complement.
TYPS OF ALLERGIC REACTION –
1. Type I Anaphylaxis –
*Characteristics – IgE bind to mast cell Or basophills causes degranulation of mast cell and release histamine.
*signs appear within.
– less than 30 min.
Example – Anaphylactic shock from drugs, venoms, common allergic conditions as hay fever, asthama.
2. Type II Cytotoxic –
*Characteristic – Antigen causes formation of IgM and IgG that bind to target cell, when combined with action of complement destroy target cell.
*Signs appear wihin – 5 – 12 hrs.
Example – Transfusion
3. Type III Immune Complex –
* Characteristics – Ag ( antigen) and Ab ( antibody ) form complex that causes inflammation .
* Signs appears within – 3 – 8 hrs
Example – Arthus reaction, serum Sickness .
4. Type IV Cell Mediated or delayed type
* Characteristics – Aantigen causes formation of Tc that kills target cell.
Examples – Rejection of transplanted tissue poison ivy , such chronic diseases like tuberculosis, leprosy.