This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Immunomodulators, Subject name: Pharmacology
[{'content': 'The following monoclonal antibody is directed against alpha 4 integrin? \nSelect and explain. \nA. Efalizumab, B. Natalizumab, C. Ibalizumab, D. Toclizumab', 'role': 'user'}, {'content': "Answer: B \nExplanation: NatalizumabNatalizumab is a humanized IgG4 monoclonal antibody that binds to the a4-subunit of a4b1 and a4b7 integrins expressed on the surfaces of all leukocytes except neutrophils and inhibits the a4-mediated adhesion of leukocytes to their cognate receptor. It is indicated for patients with multiple sclerosis and Crohn's disease who have not tolerated or had inadequate responses to conventional treatments.Ref: Katzung 13th ed.pg 993.", 'role': 'assistant'}]