Table of contents
| 1. | Introduction |
| 2. | Structure |
| 3. | Types |
| 4. | Secretion |
| 5. | Regulation of secretion |
| 6. | Functions |
Introduction
Immunoglobulin (Ig) is also known as an antibody. But there are some differences. Antibodies are glycoproteins of the immunoglobulin family, and in a strict sense, antibodies are soluble secretions without B-cell receptors.
An antibody is a large Y-shaped protein produced by the immune cells responding to an antigen. When an antigen attaches to B lymphocyte receptors (BCR), the B lymphocyte totalphysiology.com /white blood
cells differentiate into a plasma cell that secretes soluble antibodies.
The soluble antibodies are released into the blood and tissue fluids and almost all body secretions. Tissue fluids and body secretions are known as humor. As soluble antibodies are present in humor, this is known as humoral immunity.
Antibody formation is a primary physiological response to an antigen. Antigen stimulates the formation of a particular antibody and combines the antibody.
Antibodies are Immunoglobulins that circulate in the blood.
Structure
Antibodies migrate to the last portion -gamma protein-in electrophoresis serum protein test. Almost all gamma globulins are antibodies. Therefore gamma globulins are used as synonyms.
Antibodies are heavy proteins arranged in three globular regions and roughly look like Y shape molecules.
An antibody molecule comprises four polypeptide chains, distinct subunits held by disulfide bonds ‘ -s-s-. ‘When an antibody molecule reacts with a reducing agent, polypeptide chains are separated.
Two identical heavy and two similar light chains are present in four polypeptide chains.
Structure of Immunoglobulin
There are two types of light chains -Kappa (k) and lambda (λ), of which an individual antibody molecule has only one variety. For example, if an antibody molecule supposes ‘Z’ has kappa light chains, there are no lamda chains in that antibody molecule.
Properties of light chains are common to all immunoglobulins.
Five types of heavy chains provide subclass-related physicochemical properties to the antibody molecules.
Types of heavy chains are ¥ gamma, µ mu, α alpha, ɛ epsilon, and ɗ delta. The heavy chains are subclass specific.
The main types and subtypes of immunoglobulins (Ig) or antibodies are:
IgG–¥ gamma, subtypes – IgG 1,2,3,4:
IgM– µ mu
IgE– ɛ epsilon
IgD– ɗ delta subtypes – IgD and IgD 2.
IgA– α alpha subtypes – IgA 1 and 2.
They have a similar structure with minor differences within the main classes and subclasses.
An immunoglobulin molecule has two parts, Fab and Fc.
1. Two antigen-binding sites, Fab. and
2. One Fc component (complement binding site) is responsible for antigenic differences between the Ig classes and class-related functions.
Detail of immunoglobulins
IgG constitutes about 70-80 % of immunoglobulins in normal serum and is equally distributed in blood and extracellular fluid (ECF). Only IgG passes through the placenta to the fetus by active transportation and provides passive immunity against common diseases in infants for about the first six months.
About one-fourth, i.e.,25% of Ig G, passes from the bloodstream, and the same amount returns via the thoracic duct.
Main functions
- Antibacterial and
- Neutralizes soluble toxins produced by microorganisms, e.g., toxins of tetanus.
IgM is the second most abundant immunoglobulins present in humans. It is only intravascular. This has five Ig units linked with a J (joining ) chain. Therefore, it has ten antigen-binding sites.
Main functions
It is very efficient in linking particulate antigens for agglutination and phagocytosis.
IgA forms about 15-20% of total immunoglobulins and has three molecular forms-
1. 7s monomer form is present in the blood, and its size is similar to IgG.
2.10s Dimeric form of IgA is secreted by the plasma cells in the mucosa and has J -chains. This is transported across the epithelium into saliva, tears, gastrointestinal secretions, and respiratory secretions.3. 11s -During transepithelial transport of 10s, another polypeptide secretory component is incorporated to form 11s IgA.
Main functions
Lamina propriae underlying the mucosa secrete IgA.
This is a secretory antibody and protects the mucosa from infections.
Its function in blood and tissues are not very important.
IgD contraction is less than 1% of total immunoglobulins. It is intravascular and present only on the surface of immature B lymphocytes. IgD is involved in B lymphocyte’s maturation and regulation of their proliferation. The average concentration is 3-5 mg/dl of blood.
IgE is less than 0.002%, a negligible amount in the serum. It is mainly extravascular and has a considerable affinity for cell surfaces, especially Mast cells and basophils. Therefore, IgE binds firmly with mast cells and basophils, causing the release of their contents.
Its physiological functions are not clearly defined, but it plays an essential role in defense against worm infestation and is vital for immediate hypersensitivity reactions, e.g., Hey fever.
Site of antibody synthesis
Except for the thymus, all lymphoid tissues of the body produce antibodies. B lymphocytes are activated by different mechanisms to form plasma cells. In normal conditions, the plasma cells are not present in the circulation. Instead, the plasma cells synthesize immunoglobulins.
B-lymphocytes → Plasma cells →Immunoglobulins
Theory of antibody production :
- Template theory: According to this theory, antigen molecule acts as a template for Ig immunoglobulin formation. This theory explains why immunoglobulin formed explicitly combined with the antigen.
- Clonal selection theory of Burnet: According to this theory, there are many immunologically competent cells -probably lymphocytes and mesenchymal cells. These cells respond to one or very few molecules of specific antigens patterns by making antibodies against this pattern.
- This antigenic stimulation promotes the proliferation of cells of the appropriate reactivity and forms a clone of cells, all with similar specific reactivity. Antigen acts as a trigger for the proliferation of cells.
- But in fetal life, antigen inhibits cell activation instead of stimulating it. Therefore antigens present during the development of fetal life do not produce antibody formation.
There are some ‘Forbidden clones’ that promote Auto-antibody formation in some cases of autoimmune diseases.
What is a clone?
Clone is the population of cells descended by asexual reproduction from a single cell.
Activation of B lymphocytes occurs by two methods :
1. Direct activation of some unusual physiochemically antigens (for example, pneumococcal polysaccharides ) stimulates B lymphocytes to form plasma cells directly.
2. T cell-dependent activation needs two signals-
a) B-lymphocytes are activated when an antigenic determinant binds with B cell receptors.
b) T cells produce some activating signals, probably IL-4, which bind with surface receptors of B lymphocytes and stimulate. Once stimulated, B cells acquire several new surface receptors for growth factors and increase in number.
Functions
Protection against infections and neutralizes the toxins produced by the microorganism.
Sometimes it is harmful and gives rise to autoimmune diseases.
FAQ
Q. Describe different types of immunoglobulins?
A. There are five types of immunoglobulins, IgG, IgM, IgA, IgD, and IgE.
Q. Most abundant type of immunoglobulin?
A. Immunoglobulin G is the most abundant immunoglobulin. It is about 70-80 % of immunoglobulins in normal serum.
Q. Which immunoglobulin is exclusively intravascular?
A. Immunoglobulin M is intravascular.
Please comment whether this article is helpful or not.
learn-and-fly.co.in 