What do IgG and IgM indicate?

Q: What do IgG and IgM indicate? Why are these tests conducted?

A:IgG and IgM are short for immunoglobulin G and immunoglobulin M. Immunoglobulins are also known as antibodies and are substances produced by the body's immune system in response to foreign substances such as bacteria, viruses, fungi or other substances like animal dander or cancer cells. Antibodies combine or attach to the foreign substances, causing them to be destroyed or neutralised by the cells of the immune system. Antibodies are usually specific to each type of foreign substance e.g., antibodies produced in response to a tuberculosis infection attach only to tuberculosis bacteria. Antibodies also play a role in allergic reactions and occasionally may be produced against a persons own tissues in what is called an autoimmune disease. There are five major types of antibodies - IgA, IgG, IgM, IgD and IgE. IgG antibodies are the smallest antibody and are found in all body fluids. They are composed of two heavy chains and two light chains, and each molecule has two antigen binding sites. They are the most abundant immunoglobulin, comprising about 75-80% of all the antibodies in the body. IgG antibodies are important for fighting bacterial and viral infections. IgG antibodies are the only type of antibody that can cross the placenta, therefore the IgG antibodies of a pregnant woman can also help protect her baby in its initial weeks of life before its own immune system has developed. IgM antibodies are the largest type of antibody and are found in blood and lymph fluid and are the first type of antibody produced in response to an infection. They also cause other immune system cells to produce compounds that can destroy invading cells. IgM antibodies normally comprise about 5% to 10% of all the antibodies in the body. IgM antibodies form when an infection occurs for the first time, and high levels of IgM can indicate a new infection is present. High levels of IgM in a newborn usually indicate that the baby has an infection that started in the uterus before delivery. Immunoglobulins are unique for different illnesses e.g., the IgM antibody for rubella is different from the IgM antibody for herpes. Therefore doctors can look for an immunoglobulin unique to a specific illness as surrogate markers to help diagnose that illness. Different antibodies can be used to distinguish between a new and past infection. For example, the presence of IgM antibodies for rubella with or without IgG antibodies indirectly indicates a new infection, whereas IgG antibodies without IgM indicate a past infection.