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: Microbiology
[{'content': 'The chemotactic factor is: \nSelect and explain. \nA. C2a, B. C3b, C. C4a, D. C5a', 'role': 'user'}, {'content': 'Answer: D \nExplanation: C5a is a chemotactic molecule specifically recognized by polymorphonuclear leukocytes or phagocytic cells.\nThis substance causes leukocytes to migrate to a tissue in which an antigen–antibody reaction is taking place.\xa0\nAt that site, a phagocytic cell recognizes opsonized particles and ingests them.\nC5a not only has a chemotactic effect on neutrophils, but also activates these cells causing their reversible aggregation and release of stored enzymes, including proteases.\nC5a also enhances the adhesiveness of neutrophils to the endothelium.\n\nParija SC. Textbook of Microbiology & Immunology. Elsevier Health Sciences; 2014. Page:120', 'role': 'assistant'}]