Differences Between Immunoglobulin Classes
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.
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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
1. What are immunoglobulins and what is their primary function in the body?
Immunoglobulins (Ig), commonly known as antibodies, are large, Y-shaped protein molecules produced by plasma cells (differentiated B-lymphocytes). Their primary function is to identify and neutralise foreign objects like bacteria and viruses. Each antibody has a unique target, known as an antigen, to which it binds, marking the invader for destruction by other components of the immune system. For more information, you can read about the structure and types of antibodies.
2. What is the basic structure of an immunoglobulin (antibody) molecule?
An immunoglobulin molecule is a 'Y' shaped structure composed of four polypeptide chains. It consists of two identical heavy (H) chains and two identical light (L) chains, linked together by disulfide bonds. This structure is often represented as H₂L₂. Each arm of the 'Y' contains a variable region that forms the antigen-binding site (paratope), which is specific to a particular antigen. The stem of the 'Y' is the constant region, which determines the antibody's class and its mechanism of action. You can explore the detailed immunoglobulin structure here.
3. What are the five major types of immunoglobulins and their importance?
There are five major classes of immunoglobulins found in the human body, each with a distinct role:
- IgG (Immunoglobulin G): The most abundant antibody in blood and tissue fluids, providing long-term protection. It is the only immunoglobulin that can cross the placenta from mother to fetus.
- IgA (Immunoglobulin A): Found in mucosal secretions like saliva, tears, and breast milk (colostrum), providing localised protection on mucous membrane surfaces.
- IgM (Immunoglobulin M): The first antibody to be produced in response to a new infection (primary immune response). It is a large molecule that excels at clumping pathogens together.
- IgE (Immunoglobulin E): Involved in allergic reactions by binding to mast cells and basophils, and also provides protection against parasitic worms.
- IgD (Immunoglobulin D): Found in small amounts, it acts as an antigen receptor on the surface of B-cells, helping to activate them.
4. How do immunoglobulins contribute to passive immunity?
Immunoglobulins are central to passive immunity, where a person receives pre-made antibodies rather than producing them on their own. This provides immediate but temporary protection. Key examples include:
- Natural Passive Immunity: IgG antibodies cross the placenta during pregnancy, protecting the newborn for the first few months of life. Additionally, IgA antibodies are transferred to the infant through the mother's first milk (colostrum).
- Artificial Passive Immunity: This involves the injection of antibodies (immunoglobulins) from an immune donor, such as in tetanus antitoxin, to provide immediate protection after exposure. Learn more about the difference between active and passive immunity.
5. How can you differentiate between Immunoglobulin G (IgG) and Immunoglobulin M (IgM)?
While both are crucial antibodies, IgG and IgM differ significantly in their structure, timing, and function. IgG is a monomer (a single Y-shaped unit), is the most abundant Ig in the blood, and dominates the secondary immune response (re-exposure to an antigen). In contrast, IgM is a pentamer (five Y-shaped units joined together), making it much larger. It is the first antibody produced during a primary immune response, acting as the initial line of defence. For a detailed comparison, see the Difference Between IgM and IgG.
6. Why is the Y-shape of an immunoglobulin molecule so crucial for its function?
The Y-shape is critical for an immunoglobulin's effectiveness for two main reasons. Firstly, it provides two identical antigen-binding sites at the tips of the arms. This bivalency allows it to bind to two separate antigens (or two epitopes on the same pathogen) simultaneously, leading to efficient agglutination (clumping) of pathogens, which makes them easier targets for phagocytes. Secondly, the stem of the 'Y' (the Fc region) acts as a handle that can interact with other parts of the immune system, such as activating complement proteins or binding to Fc receptors on immune cells to trigger destruction of the pathogen.
7. What is the clinical importance of measuring immunoglobulin levels in the blood?
Measuring the levels of different immunoglobulins in the blood, as done in an IgG test, is a vital diagnostic tool. Abnormally high levels (hypergammaglobulinemia) can indicate chronic infections, autoimmune diseases (like lupus or rheumatoid arthritis), or certain types of cancer (like multiple myeloma). Conversely, abnormally low levels (hypogammaglobulinemia) suggest an immunodeficiency disorder, which can be congenital or acquired, leaving the patient highly susceptible to infections. Therefore, these levels provide a snapshot of the immune system's health and activity.
8. How do immunoglobulins ensure the specificity of an immune response?
The incredible specificity of the immune response is due to the variable regions of the immunoglobulin molecule. Within these regions are hypervariable segments known as Complementarity Determining Regions (CDRs). The unique sequence of amino acids in the CDRs of each antibody creates a precise three-dimensional structure. This structure acts like a lock that only a specific part of an antigen, called an epitope, can fit into, much like a key. The body can produce millions of B-cells, each making an immunoglobulin with a unique CDR, ensuring there is a specific antibody for nearly every possible pathogen.
