The proteins encoded in the HLA (Human Leukocyte Antigen) system are largely responsible for the compatibility in organ transplants. To date, the molecular determinants involved in recognizing HLA antigens by recipient antibodies are unknown. Here we explore flexibility as a potential determinant. For this purpose, we compare in terms of N-RMSF (Normalized Root Mean Square Fluctuation) amino acids labeled as confirmed eplets (regions defined around polymorphic amino acids) against amino acids that have not been reported as eplets. We found that eplet amino acids tend to be less flexible than non-eplet amino acids, which would indicate that the antibodies would have a preference for binding with less mobile regions.