This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Systemic histology, Subject name: Anatomy
[{'content': 'Which of the following statements concerning alveolar macrophages is true? \nSelect and explain. \nA. They secrete a1-antitrypsin., B. They secrete elastase., C. They originate from blood neutrophils, D. They may play a role in causing hyaline membrane disease', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Alveolar macrophages secrete elastase. Normally, a1-antitrypsin, a serum protein, interacts with elastase, thereby protecting the lung against damage that may lead to emphysema.', 'role': 'assistant'}]