Which immunoglobulin domains are altered by isotype switching?

• A. 1
• B. 1 and 2
• C. 2, 3, and 4
• D. 5 and 6

Answer: (C)

Isotype switching, also known as class switching, is a biological process that enables a B cell to produce antibodies of different classes (isotypes), such as IgM, IgG, IgA, or IgE, while maintaining the same antigen specificity. This process involves alterations in the constant region of the heavy chain of the immunoglobulin.

In immunoglobulin molecules, the heavy chain is composed of multiple regions or domains: the variable region (which includes domain 1) and the constant regions (which include domains 2, 3, and 4). During isotype switching, the DNA encoding the constant region is rearranged, allowing for the expression of a different heavy chain isotype. This alteration typically involves the constant regions, specifically domains 2, 3, and 4, which are involved in determining the particular isotype of the antibody.

Domains 5 and 6 refer to additional constant region domains in certain isotypes and are not commonly discussed in the context of the basic principles of isotype switching. Ultimately, since isotype switching modifies the constant region of the heavy chain (domains 2, 3, and 4), these are the domains that are specifically changed during this process.