CD4 and biliary tract cancer: The immunophenotypes of regulatory T cells, including CD28 on CD4+, CD4 on CD39+ resting Treg, CD4 on CD39+ resting Treg, and CD39+ resting Treg % resting Treg, CD25 on CD39+ activated Treg, exhibit a significant causal relationship with biliary tract cancer, serving as protective factors against its development.