This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General anatomy, Subject name: Anatomy
[{'content': 'Cell surface molecules involved in peripheral tolerance \nSelect and explain. \nA. B7 and CD28, B. CD40 and CD40L, C. CD 34 and CD51, D. B7 and CD3', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Ref Robbins 7/e p225 , Harrison 16/e p1907, Robbins 9/e p213 B7 is a type of peripheral membrane protein found on activated antigen presenting cells that, when paired with either a CD28 or CD152 surface protein on a T cell, can produce a costimulatory signal', 'role': 'assistant'}]