This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pathology
[{'content': 'Antigen involved in post transplant rejection - \nSelect and explain. \nA. HLA - Antigen, B. Nuclear antigen, C. Polysaccharide, D. DHA', 'role': 'user'}, {'content': "Answer: A \nExplanation: Ans. is 'a' i.e., HLA - antigen For rejection of graft, there must be some antigen that is recognized as foreign and the most impoant antigen is HLA antigen of grafted tissue.", 'role': 'assistant'}]