Immunoglobulins (abbreviated Ig) are an essential part of the body's immune system. They attach to foreign substances, such as bacteria and destroy them. There are 5 classes of immunoglobulins:

• IgA or immunoglobulin A: A major class of immunoglobulins found in serum and external body secretions such as saliva, tears, and sweat as well as in the gastrointestinal, respiratory, and genitourinary tracts.
• IgD or immunoglobulin D: A class of immunoglobulins found as antibodies on the surface of B cells (B-lymphocytes). Almost nothing is known about the normal function of IgD.
• IgE or immunoglobulin E: A class that includes the antibodies elicited by an allergic substance (allergen). A person who has an allergy usually has elevated blood levels of IgE. IgE antibodies attack the invading army of allergens.
• IgG or immunoglobulin G: A major class including many of the most common antibodies circulating in the blood. Also known as gamma globulin. Intravenous IgG is sometimes given to aid in fighting infections or immune diseases.
• IgM or immunoglobulin M: A major class of immunoglobulins. IgM includes the antibodies that are produced first in an immune response and are later replaced by other types of antibodies. Their presence indicates acute or recent infection.