A 29-year-old man with Crohn's disease, maintained on azathioprine, presents with a 3-day history of fever, sore throat, and malaise. His blood tests show: Hb 125 g/L, WBC 1.8 x 10^9/L (Neutrophils 0.6 x 10^9/L), Platelets 150 x 10^9/L. What is the most important immediate action?