Which of the following refers to the force of attraction between an antibody and a single antigenic determinant?

• A. Affinity
• B. Avidity
• C. Van der Waals attraction
• D. Covalence

Answer: (A)

The concept being tested is how strongly a single antibody binding site interacts with one antigenic determinant. This strength is called affinity. It describes the binding between one paratope and one epitope, and it is often quantified by a dissociation constant: lower Kd means higher affinity.

Why that term fits best: the question focuses on a single antigenic determinant, so the relevant measure is the binding strength of that one paratope–epitope pair, i.e., affinity. Avidity, by contrast, refers to the overall strength when multiple binding sites engage several epitopes, which isn’t what’s described here. Van der Waals attraction is a general type of weak intermolecular force that can contribute to binding but isn’t the immunology-specific descriptor for a single interaction. Covalence would imply covalent bonds, which are not the usual mode of antigen–antibody binding in physiological conditions.