In antibody responses, B cells switch from the expression of immunoglobulin (Ig) μ and δ heavy (H) chains to that of other Ig classes (α, γ, or ɛ), each with a distinct effector function. Membrane-bound forms of α, γ, and ɛ, but not μ and δ, have highly conserved cytoplasmic tails. Mutant mice unable to express membrane γ1 H chains or producing tailless γ1 H chains failed to generate efficient IgG1 responses and IgG1 memory. H chain membrane expression after class switching is thus required for these functions, and class switching equips the B cell antigen receptor with a regulatory cytoplasmic tail that naı̈ve B cells lack.