Immunoglobulin class switching occurs quickly in the human body following the activation of mature naive B cells, changing the production of IgM and IgD to IgG, IgE, or IgA. IgM is the first type of immunoglobulin to be created during an immune response. The immunoglobulins produced by B cells shift from being predominately IgM to IgG, IgE, or IgA classes as the immune response develops, depending on the type of stimulus. In actuality, throughout this process, only the constant-region of the heavy chain of the antibody is altered while the variable portion of the heavy chain remains unchanged, leading to variations in antibody binding to various effector molecules rather than variations in antigen specificity.