IDO2 and infection: 4 Immunomodulatory factors: Some immunomodulatory factors have been found to be highly expressed in the caput epididymis, such as indoleamine 2,3-dioxygenase, activin A, and Bin1b (Li et al., 2001), which not only can promote the tolerance of the epididymis to sperm antigens, but also can resist pathogen infection (Wijayarathna and Hedger, 2019).