Each heavy chain has two regions, the constant region and the variable region. The constant region is identical in all antibodies of the same isotype, but differs in antibodies of different isotypes. Heavy chains γ, α and δ have a constant region composed of three tandem Ig domains and a hinge region for added flexibility; heavy chains µ and ε have a constant region composed of four immunoglobulin domains.
The variable region of the heavy chain differs depending on the B cell that produced it, but is the same for all antibodies produced by a single B cell or B cell clone. The variable region of each heavy chain is approximately 110 amino acids long and is composed of a single Ig domain.