FIGURE 23-10 Immunoglobulin isotypes. The different classes of immunoglobulins, called isotypes, may be distinguished biochemically and by immunological techniques. In mice and humans, there are two light-chain isotypes (κ and λ) and five heavy-chain isotypes (µ, δ, γ, ε, α). Each isotype defines a class of immunoglobulin based on the identity of the heavy chain. IgG, IgE, and IgD (not shown) are monomers with generally similar overall structures. IgM and IgA can occur in serum as pentamers and dimers, respectively, accompanied by an accessory subunit, the J chain, in covalent disulfide linkage. This volume-rendered depiction of the immunoglobulins highlights their modular design, with each barrel representing an individual Ig domain. Different isotypes have different functions. See Figure 23-13 for definitions of abbreviations.