What term describes an antibody that reacts with more than one antigen?

• A. Cross-reactivity
• B. Specificity
• C. Affinity
• D. Avidity

Answer: (A)

Cross-reactivity describes an antibody's ability to bind more than one antigen because the antigens share similar epitopes. When the binding site recognizes a common structural feature, it can pair with related antigens, leading to reactivity beyond a single target. This fits the scenario of an antibody reacting with multiple antigens. Specificity refers to binding to a single antigen with minimal cross-binding. Affinity is the strength of a single antibody–antigen interaction, not the range across different antigens. Avidity describes the overall strength when multiple binding sites engage, combining several interactions. Therefore, cross-reactivity best describes an antibody that reacts with more than one antigen.