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
Which of the following statements concerning alveolar macrophages is true? 
Select and explain. 
A. 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
B: They secrete elastase.
Answer: B: They secrete elastase. 
Explanation: Alveolar macrophages secrete elastase. Normally, a1-antitrypsin, a serum protein, interacts with elastase, thereby protecting the lung against damage that may lead to emphysema.