The isotype of an immunoglobulin antibody is defined by which component?

• A. Is defined by the heavy chain.
• B. Is defined as different alleles of the same antibody type (e.g., IgG).
• C. Is constant for all immunoglobulins of an individual.
• D. Is the variation within the variable region.

Answer: (A)

The isotype of an immunoglobulin is defined by the constant region of the heavy chain. This constant region determines the antibody class (IgM, IgG, IgA, IgD, or IgE) and its effector functions, such as how it activates complement or interacts with Fc receptors. The variable region, located at the tips of the antibody, provides antigen-binding specificity and does not define the isotype. Light-chain type (kappa or lambda) is a separate attribute and does not set the immunoglobulin class. During class switching, the heavy-chain constant region can change to another isotype while preserving antigen specificity, illustrating that isotype is about the constant heavy-chain part rather than the variable region.