Immunoglobulins also are known as antibodies are proteins that are produced by specific immune cells called plasma cells in response to bacteria, viruses, and other microorganisms as well as from exposure to other substances that are recognized from the body as “non-self” harmful antigens.
Our body makes few different types of immunoglobulin antibodies including these:
- Immunoglobulin A (IgA)
- Immunoglobulin G (IgG)
- Immunoglobulin M (IgM)
- Immunoglobulin E (IgE)
- Immunoglobulin D (IgD)
Among all of these, IgG and IgM are often measured together to give doctors an insight about the immune system functioning and to detect and monitor an excess or deficiency of these immunoglobulins especially relating to infection or autoimmune disease.
IgG and IgM are two antibodies that are found in the blood and at a basic level, the main difference between them is the onset of action. IgG antibody is involved in the early onset of action, which means it is the first antibody to be developed immediately when the body is exposed to any foreign particle during acute infection while IgM antibodies are produced after a long term response to the chronic infection caused by new bacteria and germs.
|Name||IgG refers to the Immunoglobulin G||IgM refers to the Immunoglobulin M|
|Definition||IgG is one of the classes of immunoglobulins containing the most abundant type of antibodies that circulate in the blood that gives a long term response from any disease and thus protect our body from bacterial and viral attacks||IgM is a class of immunoglobulins composed of a pentamer structure that includes the primary antibodies released immediately when any foreign particle is introduced and serves as a first line of defense|
|Molecular weight||Smaller than IgM (150,000MW or 150kDa)||Larger in size (900,000MW or 900kDa)|
|Size||It is a monomer consisting of a single Y-shaped unit with two antigen-binding sites||It is a pentamer consisting of five Y-shaped units joined together by a polypeptide chain with ten antigen-binding sites|
|Presence in serum||75% of the total serum||10% of the total serum|
|Subclasses||Four subclasses are found (IgG1, IgG2, IgG3, and IgG4)||No more subclasses|
|Role||It is produced as a long term response for any disease and protects our body from bacterial and viral attacks||It is the first antibody produced in the early stages of immune response when any foreign particle is exposed|
|Abundance||Most abundant type of immunoglobulin found in all body fluids such as, blood, lymph, and intestine||Less abundant than IgG found in lymph fluid and blood|
What is IgG?
It is the most abundant type of antibody that constitutes about 70 – 80% of total blood serum. It is a monomer (smallest antibody) consisting of a single Y-shaped unit that has 2 antigen-binding site and heavy chain of gamma (γ) as the well light chain of kappa (κ) and lambda (λ).
Properties of IgG
- Molecular weight: 150,000
- Serum concentration: 10 – 16 mg/ml
- Per cent of total immunoglobulin: 75%
- Glycosylation (by weight): 3%
- Distribution: Intravascular and extravascular
- Response: secondary
Subclasses of IgG
There are four subclasses of IgG differing on the basis of a number of disulfide bonds and the length and flexibility of the hinge region. Determination of IgG subclasses can be a valuable tool in indicating a potential antibody deficiency. For instance, severe and prolonged infections can provide an insight about the manifestation of disease.
- IgG1 (60 – 65% of the total main subclass)
- IgG2 (20 – 25% of the main subclass)
- IgG3 (5 – 10% with the highest affinity to antigens)
- IgG4 (less than 4%)
Role of IgG
In adults, IgG is the major immunoglobulin found in blood, lymph fluid, peritoneal fluid, and cerebrospinal fluid that plays a key role in humoral immune response against an antigen and promoting opsonization and phagocytosis.
In pregnancy, due to its small size IgG is the only antibody that can cross the placenta and transfers the mother’s immunity to the developing embryo.
It is an important immunoglobulin in the passive immunization due to its longevity in the serum.
What is IgM?
IgM, previously known as Y-macroglobulin is the default antibody produced by B cells. It is considered as default because it is the first antibody to be produced in response to an antigen.
Unlike IgG, IgM is the largest pentameric immunoglobulin that has five or six units each composed of two heavy chains (mu-chains) and two light chains that are joined together by disulfide bonds. IgM constitutes about 5 – 10% of serum immunoglobulins in adults.
Forms of IgM
There are two forms of IgM
- Monomeric IgM
It is expressed as a membrane-bound antibody on B cells.
- Pentameric IgM
It is secreted by plasma cells.
Properties and Function of IgM
- IgM is also called as macroglobulin or millionaire molecule because of its high molecular weight.
- IgM is the first immunoglobulin that is produced in a primary response to an antigen. The presence of IgM antibodies in the serum of patient indicates recent/acute infection and helps in the diagnosis of disease.
- IgM is the first immunoglobulin synthesized by the fetus at about 20 weeks of age and is useful in the diagnosis of congenital infections such as syphilis, rubella, toxoplasmosis, cytomegalovirus, and HIV infection.
- Because of its large size, IgM is mainly (about 80%) confined to the bloodstream.
- IgM is more efficient than IgG in activating classic pathway of complement.
- IgM is highly effective against viruses.
- IgM is a better agglutinin than IgG.
- The effect of IgM is temporary (2 – 3 weeks after their production).
- High level of IgM may give cause kidney damage, rheumatoid arthritis, hepatitis
Key Differences between IgM and IgG
- IgG is the smallest antibody that responds later causing the permanent eradication of antigen with the lasting effect while IgM provides immediate response when any antigen enters the body.
- IgG is a monomer with 2 antigen-binding sites whereas IgM is a pentamer with 10 antigen-binding sites.
- The molecular weight of IgG is lighter about 150,000MW while the molecular weight of IgM is 900,000MW.
- IgM accounts for only 10% of the total volume of the serum while IgG occupies 75% of the total volume of serum.
- IgG is of four types such as, IgG1, IgG2, IgG3, and IgG4 while IgM has no such types.
- IgG is abundantly found in the intestine, lymph, and blood whereas IgM is produced by plasma cells and is found mostly in blood and lymph fluid.
- IgG has the heavy chain of gamma while IgM has the heavy chain as mu but both have the light chain as kappa and lambda.
Antibodies are the proteins produced by B-lymphocytes to fight against any infections. IgM is the first antibody to fight against the attack of viruses of bacteria whereas IgG reacts later but helps in the permanent eradication of antigens by enhancing phagocytosis or opsonization.