Janus kinase (JAK) inhibitors are a class of drugs that block the activity of Janus kinases, a family of enzymes involved in signaling pathways that regulate the activity of the immune system. JAK inhibitors are used to treat autoimmune diseases such as rheumatoid arthritis, psoriasis, and ulcerative colitis. They work by blocking the activity of JAK enzymes, which prevents them from activating the immune system and causing inflammation.