Antibodies are proteins called immunolglobulins (Ig). Five different Ig classes (IgG, IgM, IgD, IgA, IgE) are based on differences in the constant region of the heavy chain. Each class of antibody has different abilities to find and help remove antigen.

An allergy is an immune response to a harmless antigen, such as pollen, animal dander or a specific food. Mast cells bind IgE, and when antigen (or allergen) binds with IgE, mast cells release histamines. This produces the familiar "hay fever" or allergic reactions, including dilation of blood vessels, inflammation and sometimes difficult breathing. The most serious allergic reactions trigger anaphylatic shock where a massive release of histamine causes shock and potential death.

Whether or not we suffer from allergies largely depends on whether we make an IgE response to antigen. Since IgE is present at very low levels in our blood (.05 µg/ml), hay fever sufferers can be desensitized by giving increasing doses of the antigen causing the allergy. Frequent doses of antigen usually lead to a strong IgG response. The IgE does not disappear, but since IgG levels are at around 10 mg/ml, the IgG can remove most of the offending antigen before IgE containing mast cells are activated.

Before binding antigen, B cells contain IgM molecules only. Following antigen binding, when plasma cells are produced, class switching occurs. Class switching refers to a DNA rearrangement changing the heavy chain constant gene in memory cells. Loss of coding regions for the constant part of the heavy chain causes IgG, IgA, and IgE to be produced. Below is an example of class switching to produce IgA.