Immunoglobulin class switching occurs quickly in the human body upon the activation of mature naive B cells, resulting in the expression of IgG, IgE, or IgA instead of IgM and IgD. IgM is the first class of immunoglobulin that is generated during an immune response. As the immune response advances, B cells create immunoglobulins that are primarily of the IgM class, but can also be of the IgG, IgE, or IgA classes, depending on the type of stimulation. Instead of changing the antigen specificity of the antibody, this mechanism merely modifies the constant portion of the heavy chain, leaving the variable portion unchanged. This leads to variations in the way the antibody binds to various effector molecules.