**Anti-Infliximab Antibodies (ATIs)** are immune proteins produced by the body that mistakenly target and neutralize infliximab, a biologic medication used to treat autoimmune conditions like Crohn's disease and rheumatoid arthritis.

When a patient develops ATIs, the antibodies bind to the drug, clearing it from the bloodstream much faster than normal. This primary response leads to a significant drop in drug efficacy, often causing a return of disease symptoms (loss of response). Additionally, the formation of these immune complexes increases the risk of acute infusion reactions, such as fever, chills, or rashes, during treatment.
An Anti-Infliximab Antibodies (ATI) diagnostic test measures the level of these neutralizing antibodies in a patient's blood, typically alongside infliximab drug levels. This dual assessment, known as therapeutic drug monitoring (TDM), is crucial when a patient is losing response to treatment.