Vedolizumab is a new medication being used for the treatment of Crohn's disease and Ulcerative colitis. It works by blocking specific white blood cells (alpha 4-beta7 lymphocytes) from migrating to areas of inflammation in the gastrointestinal tract. Previous studies have shown that patients who have previous exposure to another type of medication for Crohn's disease and Ulcerative colitis (anti-TNF medications) do not respond to vedolizumab as well as those who have never been exposed. This study will investigate biologic markers in the blood and tissue the help identify possible reasons for this difference.

Patients will be enrolled in the study when they are selected to initiate vedolizumab therapy. Vedolizumab is an intravenous infusion given at increasing intervals. The first infusion of vedolizumab will be considered baseline. Patients may be enrolled at baseline or at colonoscopy prior to the first scheduled infusion of vedolizumab (pre-treatment visit). The standard induction protocol for vedolizumab consists of infusions at week 0 (baseline), week 2, and week 6. Vedolizumab, as per standard-of-care, is then continued at an interval of every 8 weeks following induction therapy. Vedolizumab treatment will be prescribed as per standard practice and will be prescribed regardless of participation in this study. Patients will not receive medication for the sole purpose of this research. Treatment decisions including timing of wash out period between anti-TNf and vedolizumab will be made by the subject's physician and not per the research protocol. There will not be a protocolized washout period for patients previously receiving anti-TNF therapy because treatment will be standard of care and per the directing physician.

Standard of care laboratory tests will be performed by the clinical laboratory at Mount Sinai Hospital. All other measurements will be completed at Prometheus Therapeutics & Diagnostics in San Diego, California or at a Takeda identified lab for vedolizumab concentrations and antibodies.

Inclusion Criteria:

• Male and female adults with a diagnosis of inflammatory bowel disease
• Patients who will be started on vedolizumab therapy as clinically indicated and in discussion with their treating physician will be eligible. Patients will not be started on vedolizumab for the sole purpose of this study

Exclusion Criteria:

-Patients with prior exposure to vedolizumab or natalizumab

Male and female adults with a diagnosis of inflammatory bowel disease who will be started on vedolizumab therapy as clinically indicated and in discussion with their treating physician will be eligible. Patients will not be started on vedolizumab for the sole purpose of this study