PIP and breast cancer: In addition to cytokines like IL-2, IL-12, TNF-α, and IFN-γ, which have been reported to mediate the Th1 immune response [24], our group identified the prolactin inducible protein (PIP), an abundantly secreted protein in some human breast cancer cell lines [89,90], as an important immunoregulatory protein that regulates Th1 immune responses [91].