PDCD1 and inflammatory bowel disease: Determining the role of PD-1/PD-L1, PD-L2, indoleamine 2,3-dioxygenase (IDO), sialic acid-binding immunoglobulin-like lectins (Siglecs), and CTLA-4 in animal models of IBD, patients with IBD, as well as in CRC can provide insights into disease initiation and progression that may assist in identification of novel targets.