Rheumatoid arthritis is an autoimmune disease in which the immune system attacks the tissues in and around joints. This leads to inflammation and destruction of cartilage, bone, and ligaments, causing stiffness, pain, and disability. Although the cause of rheumatoid arthritis is not known, it affects approximately 1.3 million patients in the United States and it is more common in women and patients who are 40 to 60 years of age. Since there is no cure for rheumatoid arthritis, treatment is focused on relieving inflammation and pain and preventing joint destruction. In patients who do not respond adequately to anti-inflammatory, immunosuppressive, and antirheumatic drugs, genetically engineered biological agents such as infliximab have been developed that block the action of specific proteins that promote inflammation. Infliximab (Remicade, Centocor Ortho Biotech Inc.) is a genetically engineered antibody that binds to the protein tumor necrosis factor alpha (TNF-α) to inhibit inflammation. Another disorder that does not have a cure but that often responds to biological agents such as infliximab is Crohn disease, a progressive inflammatory disorder that affects the bowel. Like rheumatoid arthritis, the cause of Crohn disease is unknown; however, it is generally believed to be an autoimmune disorder. In adult Americans, the prevalence of Crohn disease is 201 per 100,000 versus 43 per 100,000 in Americans 20 years of age or younger.