This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Complement system, Subject name: Pathology
[{'content': 'Endogenous chemoattractant is: \nSelect and explain. \nA. C5a, B. Bacterial products, C. Lipopolysaccharide A, D. C8', 'role': 'user'}, {'content': 'Answer: A \nExplanation: After exiting the circulation, leukocytes move in the tissues toward the site of injury by a process called chemotaxis, which is defined as locomotion along a chemical gradient. Both exogenous and endogenous substances can act as chemoattractants. The most common exogenous agents are bacterial products, including peptides that possess an N-formyl methionine terminal amino acid and some lipids. Endogenous chemoattractants include several chemical mediators including cytokines (e.g., IL-8); Components of the complement system (paicularly C5a) and arachidonic acid metabolites, mainly leukotriene B4 (LTB4).', 'role': 'assistant'}]