Immunoglobulins (Ig) or antibodies serve as key detection molecules in the immune system. Antibodies refers to proteins that are produced in B-lymphocytes by the body’s immune system in response to foreign substances or infections, including bacteria or viruses.
There are five major classes (isotypes) of immunoglobulins:
- IgA (immunoglobin A)
- IgD (immunoglobin D)
- IgE (immunoglobin E)
- IgG (immunoglobin G)
- IgM (immunoglobin M)
IgM is the largest immunoglobulin among all with a size of 970 kDa and the first immunoglobulin developed during human fetal development at around 20 weeks. In addition, IgM are the first antibodies to be produced in respond to any infection and account only 10% of the total volume of the serum.
IgG is the most abundant type of antibody (75% of the total volume of the serum) and is found in all body fluids and protects against bacterial and viral infections.
IgG antibodies are smaller (150 kDa) and the only type of antibody that can cross the placenta, providing mother’s immunity to the infant while his/her humoral response is inefficient.
Why is my Doctor checking for antibodies?
An important difference between the two antibodies is related to exposure. While IgM antibodies are the first antibodies to fight against, and attack of viruses or bacteria, they are usually found in a human body right after it has been exposed to a disease. Afterward, IgM antibodies disappear within 2 or 3 weeks and are replaced by IgG antibodies which lasts for life providing lasting immunity.
Therefore, Doctors check for antibodies because IgM is an indicator of a current infection while presence of IgG antibodies against a certain antigen indicates a recent or past exposure to the illness.
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