Some of the most severe are:

serious infections
reactivation of hepatitis B
reactivation of tuberculosis
lethal hepatosplenic T-cell lymphoma (generally only when combined with 6-mercaptopurine)
drug-induced lupus
demyelinating central nervous system disorders
psoriasis and psoriasiform skin lesions
new-onset vitiligoCases of leukopenia, neutropenia, thrombocytopenia, and pancytopenia (some fatal) have been reported with infliximab. The FDA issued a warning to doctors appearing in the respective product labeling of infliximab instructing them to screen and monitor potential patients more carefully. The FDA issued a warning to doctors that there is an increased risk of lymphoma and other cancers associated with the use of infliximab and other tumor necrosis factor blockers in children and adolescents.Maintenance therapy with the drug (versus intermittent or sporadic therapy) lessens the likelihood of developing antibodies to infliximab which are known to reduce the efficacy of the drug. Combination treatment with methotrexate (an antifolate drug which suppresses the immune system) has been shown to reduce the formation of these antibodies in patients with rheumatoid arthritis and combination therapy with other immunosuppressants has been shown to reduce the likelihood of these antibodies being formed in Crohns disease. The use of immunosuppressants may not be necessary in all diseases for which infliximab is indicated, and indiscriminant uses of these other immunosuppressants carry their own risks. Infliximab was studied in monotherapy (without concomitant immunosuppressants such as methotrexate or azathioprine) in psoriasis, psoriatic arthritis, and ankylosing spondylitis.
These differences may account for the differential actions of these drugs in both efficacy and side effects.Infliximab has high specificity for TNF-α, and does not neutralize TNF beta (TNFβ, also called lymphotoxin α), an unrelated cytokine that uses different receptors from TNF-α. Biological activities attributed to TNF-α include induction of proinflammatory cytokines (such as interleukins IL-1 and IL-6), enhancement of leukocyte movement or migration from the blood vessels into the tissues (by increasing the permeability of endothelial layer of blood vessels), and increasing the release of adhesion molecules. Infliximab prevents disease in transgenic mice (a special type of mice biologically engineered to produce a human form of TNF-α and which are used to test the results of these drugs that might be expected in humans). These experimental mice develop arthritis as a result of their production of human TNF-α, and when administered after disease onset, infliximab allows eroded joints to heal.Other monoclonal antibodies targeting TNF-α are golimumab, adalimumab, and certolizumab pegol. Etanercept also binds and inhibits the action of TNF-α, but is not a monoclonal antibody (it is instead a fusion of TNF-receptor and an antibody constant region).