An antibody is formed of four polypeptide chains: two heavy and two light chains bound in a Y shape.

An antibody is a molecule that recognizes a specific antigen; this recognition is a vital component of the adaptive immune response. Antibodies are composed of four polypeptides: two identical heavy chains (large peptide units) that are partially bound to each other in a “Y” formation, which are flanked by two identical light chains (small peptide units). The area where the antigen is recognized on the antibody is known as the variable domain or variable region. This is why there are numerous antibodies that can each recognize a different antigen. The antibody base is known as the constant domain or constant region. The portion of an antigen that is recognized by the antibody is known as the epitope.

Practice Exam

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MCAT Official Prep (AAMC)

Practice Exam 2 B/B Section Passage 10 Question 56

Key Points

• Antibodies contain four polypeptides: two identical (to each other) heavy chains in a “Y” formation and two identical (to each other) light chains on the outside of the top of the “Y” portion

Key Terms

antigen: a substance that binds to a specific antibody; may cause an immune response

epitope: that part of a biomolecule (such as a protein) that is the target of immune response; the part of the antigen recognized by the immune system

antibody: protein produced in the blood to fight against an antigen

adaptive immune response: a specific type of immunity developed over time

the variable domain: provides antibodies with their specificity for binding antigen, includes the ends of the light and heavy chains