MBP (85-99) peptide antagonist, or J5, is a 15 amino acid peptide fragment designed on the binding motif of MBP 85-99 (myelin basic protein 85-99) to HLA-DR2 (human leukocyte antigen-DR2), which competes with and inhibits the binding of MBP (85-99) to HLA-DR2 and IL-2. MBP 85-99 is the the immunodominant epitope of MBP and a major candidate autoantigen in multiple sclerosis (MS), making this peptide a useful tool for MS research.

Myelin basic protein (MBP), the second most abundant protein in central nervous system myelin, is responsible for adhesion of the cytosolic surfaces of multilayered compact myelin.