Antibodies are proteins found in the blood that help protect us against infection and are part of our immune system. Merck Manuals describe the action of antibodies as helping cells envelop antigens (foreign substances), attacking toxic substances that are caused by bacteria, attacking bacteria and viruses, and stimulating other parts of the immune system. There are five classes of antibodies.
IgM (Immunoglobulin M)
According to Merck Manuals, IgM antibodies are made when you come into contact with a foreign substance for the first time. Your body's response to this antigen is the primary immune response. IgM is found in the blood, but not in the tissues of your body.
IgG (Immunoglobulin G)
This is the most common antibody and is made in large amounts when your body comes into contact with an antigen for the second time. This is called the secondary immune response, according to Merck Manuals, and the antibodies produced, which are mostly IgG, are highly effective. This is the only antibody that can be passed through the placenta from mother to fetus, according to Biology Reference. IgG fights off fungi, bacteria and viruses.
IgA (Immunoglobulin A)
IgA can be found in tears, breast milk, sweat, saliva and mucus. It helps defend the body from any bacteria that may be trying to enter the body through surfaces lined with a mucous membrane, such as your nose or lungs. This antibody can also be found in the blood.
IgE (Immunoglobulin E)
This antibody helps protect the body from parasites and the infections they cause. IgE attaches to white blood cells and, when antigens are encountered, stimulates the white blood cells to cause inflammation. This often causes allergy symptoms.
IgD (Immunoglobulin D)
IgD binds to immature B cells (which help fight off infection) and helps them mature. According to Merck Manuals, it is not known how these antibodies work in fighting infection otherwise.
