Depending on the type of heavy chain, antibodies are categorized into different classes, and assume different roles in the immune system. The most abundant and typical antibody has a gamma heavy chain and is called immunoglobulin G (IgG) . IgG has four different subclasses, but as a whole, IgG is found mainly in blood serum. About 75% of the serum antibodies are IgG, and these are critical to stimulate immune cells to engulf invading pathogens. IgG is the only antibody able to transfer across the placenta during pregnancy.
The second most common antibody in serum is secretory IgA. This antibody is also found in mucosal secretions as well as colostrum and breast milk. It is extremely important in fighting respiratory and gastrointestinal infections, especially in infants, where gastrointestinal illnesses are particularly deadly.
The third most common is IgM, which is usually found as a pentamer. The unusual structure of IgM provides multiple binding sites for antigens (10 in IgM versus two in IgG). This structure makes IgM good for clumping microorganisms, and then stimulating immune cells to digest the entire complex.
IgD is found at low levels, and its role is still uncertain.
IgE is the least common antibody in serum and is primarily found attached to mast cells. IgE is the antibody that stimulates allergic responses by releasing the histamines that cause all the common symptoms of allergies, including runny noses, sneezing, and coughing.