Discussion
Immunoglobulins are classified into the following isotypes: IgG, IgM, IgA, IgD, IgE. What region of the immunoglobulin is used to determine this classification?
Answer: A
The isotype is decided by the constant region of the heavy chain only.
There are five kinds of heavy-chain constant region, gamma, mu, alpha, delta and epsilon, and they define IgG, IgM, IgA, IgD and IgE. The heavy-chain constant region also sets each class's effector functions, such as complement fixation by IgM or mast-cell binding by IgE. Class switching changes only this region while the antigen-binding site stays the same.
The light-chain constant region comes in two types, kappa and lambda, and either can pair with any isotype, so it plays no part. The variable regions of both chains form the antigen-binding site and decide specificity, not class.
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