Crohn's disease is an inflammatory (swelling and soreness) disorder of the digestive tract. Affected patients suffer from abdominal pains, diarrhea (sometimes bloody), weight loss. It is a lifelong disease with frequent flares during the course of the disease. Crohn's disease is mostly treated with medications, sometimes surgery is needed. Infliximab is a medication for treating severe Crohn's disease. This medicine is effective by blocking special substance (tumor necrosis factor) released from certain white blood cells in the body. Infliximab is given via a vessel at week 0, 2, 6 initially, then every 2 monthly for maintenance. However, some of patients with Crohn's disease do not respond infliximab. Currently there is no test to reveal which patients will respond to treatment. This study aims to analyze and compare particular subgroup of white cells and its products during and after infliximab treatment which may determine the responsiveness to infliximab treatment.