What Are the Five Immunoglobulin Classes and Their Functions
Antibodies are produced in the bloodstream due to the response of an infection. When a human body detects the presence of any pathogen or a foreign particle, our immune system prepares to neutralize it by producing antibodies. These proteins are also called immunoglobulin (Ig). In this article, we will discuss two types of antibodies IgM and IgG, their differences, and the blood tests performed to detect them.
What are Antibodies?
When a human body detects the presence of antigen or a pathogen, the B lymphocytes start producing specific proteins called immunoglobulin (Ig). These Y-shaped proteins present in blood start a mitotic division to increase in number and attack the foreign bodies. They are of different kinds according to their structural features. The most prevalent one among these types is IgM antibodies.
The prime function of the antibodies (Ig) is to attack the antigens present in the blood. We can thus conclude that the production of the antibodies takes place due to the provocation of the antigens present in our circulatory system. A specific IgM test is performed to detect the presence of a particular type of pathogens in order to support the immune system with medications and to regain normal health conditions.
What is Immunoglobulin?
It is a Y-shaped protein made of 4 polypeptide subunits. One subunit has two identical heavy and light chains in its molecular structure. The ending of these antibody proteins forms a Y shape due to the antigen-bonding domains. This significant shape forms due to the antigen-binding domains at the N-terminus.
The C-terminus of these heavy chains of these antibodies aids in the interaction of these immunoglobulin proteins with the effector cells. The polypeptide subunits are thus in number and are bound together by non-covalent bonds and disulfide bonds.
These heavy and light chains form the H-structural chain. The differences in these chains create different kinds of antibodies.
(Image will be Uploaded soon)
Types of Antibodies
1. Immunoglobulin M or IgM
IgM antibodies are produced first when there is a microbial attack. The presence of foreign pathogens inside the circulatory system triggers the B lymphocytes to produce such proteins in the blood. It is also called the largest antibody found in terms of its pentameric molecular structure. The prime IgM antibody function is to provide the first defensive line against any pathogenic infection.
After the production of IgM, it constitutes nearly 6% of the antibodies in blood serum and circulates throughout the body. This antibody protein is involved in opsonization and agglutination. Due to its largest size, it has many antigenic sites to interact with the antigens and facilitates the activation of the immune response of an individual. An IgM test conducted can identify its type for identifying the pathogens present in the system.
2. Immunoglobulin G or IgG
This is the most abundantly found immunoglobulin protein present in the blood plasma by constituting at least 80% of the total content of antibodies. The prime function of IgG is to detoxify harmful substances present in our blood and help in the recognition of the antibody-antigen complex. Its heavy chain has only two antigen-binding sites.
It is always present in the blood circulatory system and is also transferred to the baby through the placental connection in the mother’s womb. IgG also protects the fetus from unwanted pathogenic infection inside the mother’s womb. It has four subclasses based on the structural features. They are IgG 1, 2, 3, and 4. Only IgG 3 and 4 can cross the placental barrier to protect the fetus.
3. Immunoglobulin A or IgA
This type of antibody protein is found in the secretion or liquids in our body such as milk, saliva, serum, and the fluids of the intestines. The prime function of this antibody depends on the source. For instance, breast milk IgA protects the gastrointestinal tract of an infant.
It constitutes 13% of the total antibody present in the human body. It has two subclasses IgA 1 and 2. IgA 1 is prevalent in the secretions of the human body and is also called secretory Ig. They exist in both monomeric and dimeric forms and provide the first defensive line to stop pathogens along with participating in the immune response.
4. Immunoglobulin D or IgD
This immunoglobulin constitutes only 1% of the entire content. It takes parts in the immune response by acting as a receptor on the B lymphocytes. It also acts as an important factor for the activation and differentiation of B lymphocyte cells.
5. Immunoglobulin E or IgE
This immunoglobulin is present the least in the system (0.02%). This type of Ig can be found in the inner lining of the intestinal tracts and respiratory tracts. They respond well to allergic reactions and help the immune system recognize and tackle foreign particles or pathogens.
Difference Between IgG and IgM
The prime difference between IgG and IgM is that the former one is present in the highest amount in the serum whereas the second one is produced as an action against the detection of a pathogen or any foreign participle. IgM is heavier in size and has more antigenic sites than IgG.
Tests Related to the Immunoglobulin
Tests related to IgA, IgG and IgM are done comprehensively to get a clear idea of what the immune response of an individual is. For instance, the type of IgM blood test done for dengue and typhoid is different from each other.
The level of these Ig proteins tells us the stage of the diseases and helps us plan proper treatment. Dengue IgM positive in the test will signify the presence of dengue pathogens in the system.
FAQs on Immunoglobulin Types and Their Functions in Human Immunity
1. What are the different types of immunoglobulins?
The five main types of immunoglobulins are IgG, IgA, IgM, IgE, and IgD.
- IgG – Most abundant antibody in blood; provides long-term protection.
- IgA – Found in mucosal secretions like saliva, tears, and breast milk.
- IgM – First antibody produced during an initial immune response.
- IgE – Involved in allergic reactions and defense against parasites.
- IgD – Functions mainly as a receptor on immature B cells.
2. What is the function of IgG?
The main function of IgG is to provide long-term immunity by neutralizing pathogens and marking them for destruction.
- Most abundant antibody in blood plasma.
- Crosses the placenta to provide passive immunity to the fetus.
- Enhances phagocytosis through opsonization.
- Activates the complement system.
3. What is the role of IgA in the immune system?
The primary role of IgA is to protect mucosal surfaces by preventing pathogens from attaching to and entering epithelial cells.
- Found in saliva, tears, mucus, and breast milk.
- Exists mainly as secretory IgA in body fluids.
- Provides first-line defense in the respiratory, digestive, and urogenital tracts.
4. Why is IgM the first antibody produced in an infection?
IgM is the first antibody produced because it is expressed on naïve B cells and rapidly secreted during the primary immune response.
- Appears early after initial exposure to an antigen.
- Exists as a pentamer, making it highly effective at agglutination.
- Strong activator of the classical complement pathway.
5. What is the function of IgE in allergic reactions?
The function of IgE is to mediate allergic responses and defend against parasitic infections.
- Binds to mast cells and basophils.
- Triggers release of histamine during exposure to allergens.
- Important in immunity against helminth parasites.
6. What is the role of IgD in the body?
The main role of IgD is to function as a receptor on the surface of immature B lymphocytes.
- Present in small amounts in blood plasma.
- Acts as a B cell receptor (BCR).
- Involved in B cell activation and regulation.
7. What is the difference between IgG and IgM?
The main difference between IgG and IgM is that IgM is produced first in an initial infection, while IgG provides long-term immunity.
- Structure: IgM is a pentamer; IgG is a monomer.
- Timing: IgM appears early; IgG appears later.
- Placental transfer: IgG crosses the placenta; IgM does not.
- Duration: IgG persists longer in circulation.
8. How are immunoglobulins classified?
Immunoglobulins are classified based on differences in their heavy chain structure.
- Gamma (γ) chain – IgG
- Alpha (α) chain – IgA
- Mu (μ) chain – IgM
- Epsilon (ε) chain – IgE
- Delta (δ) chain – IgD
9. What is the basic structure of an immunoglobulin?
The basic structure of an immunoglobulin consists of two identical heavy chains and two identical light chains arranged in a Y-shape.
- Contains variable regions that bind specific antigens.
- Contains constant regions that determine antibody class.
- Forms antigen-binding sites at the tips of the Y arms.
10. How do immunoglobulins help in the immune response?
Immunoglobulins help in the immune response by specifically binding to antigens and facilitating their neutralization or destruction.
- Neutralization – Block toxins and viruses from entering cells.
- Opsonization – Mark pathogens for phagocytosis.
- Complement activation – Trigger lysis of pathogens.
- Agglutination – Clump pathogens for easier clearance.
