The synthesis of IgE by B cells occurs at a low rate compared with that of other antibodies, even in allergic subjects. One rate-limiting step is the class switch, by which B lymphocytes switch to produce immunoglobulin ε heavy chains rather than μ or γ heavy chains. We propose an additional rate-limiting step: survival of the B lymphocyte after the switch to IgE. The hypothesis we present here is that the survival of the IgE-switched B cell is compromised, particularly in an active germinal center. Antigenic stimulation in the absence of a danger signal fails to induce a mature germinal center, which allows IgE-switched B cells to escape and mature into plasma cells. Antigenic stimulation in the presence of a danger signal (or under nonhygienic conditions) induces germinal centers, which eliminate IgE-switched B cells. Thus the essence of an allergen is antigenic stimulation in the absence of conditions that generate mature germinal centers. Because germinal centers are important for the generation of B-cell memory, the IgE immune response is characteristically poor in memory (but might be long lasting because of the generation of long-lived plasma cells). In addition to this direct route to IgE, typical for atopic sensitization, another type of T_H2 response exists. On chronic allergen exposure with the concomitant induction of germinal centers, IgG4-switched B memory cells are induced that are slow to differentiate into plasma cells. These IgG4-switched B memory cells might occasionally undergo a secondary switch to IgE.