This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Immunology, Subject name: Microbiology
[{'content': 'Which of the following is very difficult to induce antibody - \nSelect and explain. \nA. Polysaccharide, B. Protein, C. Antigen, D. Effector', 'role': 'user'}, {'content': 'Answer: A \nExplanation: An antigen is any substance that causes an immune system to produce antibodies against it. Antigens are usually peptides, polysaccharides or lipids. In general, molecules other than peptides (saccharides and lipids) qualify as antigens but not as immunogens since they cannot elicit an immune response on their own. Ref: Ananthanarayan & Panikers textbook of microbiology 9th edition pg:88', 'role': 'assistant'}]