Immunoglobulin class switching is the process which determines whether a B-cell secretes antibodies of the IgM, IgG, IgA or IgE class (or isotype). IgE is the antibody that mediates the allergic response by sensitising mast cells to allergens at the mucosal barrier. Class switching proceeds by three successive steps, culminating in the synthesis and secretion of antibody: these are germline gene transcription, DNA recombination and B-cell differentiation. We review here the present state of knowledge concerning the mechanisms involved in each of these steps, with particular reference to IgE. Intervention in the mechanisms that specify the selection of IgE may offer a means to combat allergy.