Abstract
This case illustrates the complex interactions of the immune responses after vaccination and highlights their potential connections to various autoimmune conditions. A 22-year-old man with quiescent ulcerative colitis (UC) presented with abdominal pain, rectal bleeding, and thrombocytopenia 7 days after receiving the third coronavirus disease 2019 mRNA vaccination. Laboratory data confirmed the diagnosis of immune thrombocytopenia. High-dose intravenous immunoglobulin administration boosted the patient's platelet count. Simultaneously, colonoscopy revealed that his UC had relapsed. Although salazosulfapyridine briefly improved his symptoms, his stool frequency worsened one week later. The patient also developed pyoderma gangrenosum. Subsequent treatment with infliximab notably improved both pyoderma gangrenosum and UC.