To abolish ALL chance of a response to protein, what size do they need to be?

• A. Greater than 5 kD
• B. 2-3 kD
• C. 10-20 kD
• D. < 1 kD

Answer: (D)

Recognizing an antigen requires peptide fragments of enough length to bind MHC molecules and be seen by T cells (and, in turn, by B cells). Very small fragments don’t form stable MHC-peptide complexes and are unlikely to be recognized as specific antigens, so they fail to elicit a response. Peptides smaller than about 1 kDa (roughly 9 amino acids) are generally too small to serve as effective T cell epitopes, effectively abolishing an adaptive immune response to the protein. Therefore, degrading a protein to fragments under 1 kD would prevent a response. Larger fragments, like those in the other options, can still be processed into epitopes and evoke some immune recognition.