This is a monocentric, non-profit prospective cohort study with longitudinal biological sampling. The study will include patients with IBD starting biological therapy with infliximab. During four routine clinical visits in the induction phase (standard or accelerated) and the first maintenance visit, two saliva samples (pre- and post-infusion) and one plasma sample (pre-infusion) will be collected. Plasma samples will be obtained from leftover blood collected during routine clinical practice, with no additional blood draws required. The induction regimen (standard or accelerated) will be determined by the treating physicians based on the patient's clinical and laboratory characteristics and will not be influenced by study participation. The study focuses on the first four infliximab infusions (three induction and one maintenance). Standard induction lasts 14 weeks (infusions at weeks 0, 2, 6, and 14), while accelerated induction lasts 8 weeks (infusions at weeks 0, 1, 4, and 8).

The primary objective of the study is to compare infliximab levels measured in plasma with infliximab levels in saliva in a sample of 15 patients receiving the drug during the first four infusions after a diagnosis of IBD. The study is purely exploratory, and the collected data will not be used for diagnostic or therapeutic purposes.

Inclusion Criteria:

• Age between 3 and 18 years
• diagnosis of Crohn's disease or ulcerative colitis
• starting biological therapy with infliximab;
• Signed informed consent.

Exclusion Criteria:

• Patients in whom the diagnosis of IBD has not been confirmed according to standardized endoscopic and histological criteria;
• Patients who refuse to participate in the study.