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Practice MCAT Question - Q126 - Answer!

Antibodies are “Y-shaped” molecules consisting of four polypeptide chains: two larger (heavy) chains and two smaller (light) chains. Each chain has a constant region and a variable region that contains a unique amino acid sequence. Which region of the antibody will most likely form the binding site with the foreign antigen?

A.  Constant regions
B.  Variable regions
C.  Both A and B
D.  Neither A nor B

The correct answer is (B). Since the variable regions have their own unique amino acid sequences, these are the parts of antibodies that can be designed to “fit” the unique antigen structures of each invading organism. This helps account for the “specificity” of antibody-mediated immunity. The constant region is usually one of five types that help us classify antibodies with respect to where they are made and where they are found.

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