Which statement best describes an epitope?

• A. A peptide that must be at least 10,000 Da
• B. An area of an immunogen recognized only by T cells
• C. A segment of sequential amino acids only
• D. A key portion of the immunogen

Answer: (D)

An epitope is the exact part of an antigen that binds to a receptor on immune cells, such as an antibody or a T cell receptor. This region can be a linear sequence of amino acids or a conformational surface feature formed by the protein’s 3D shape, but what matters is that this specific portion is what the immune system recognizes and interacts with. The statement that best fits this idea is that an epitope is a key portion of the immunogen—the part that the immune receptor targets. The other options misstate the concept: there is no required minimum size for an epitope, epitopes are not recognized only by T cells (antibodies can recognize epitopes directly), and epitopes are not limited to sequential amino acids since conformational epitopes exist as well.