CD4 and cholangiocarcinoma: Heterogeneous recruitment of various immune cells, i.e., CD8+, CD4+, natural killer (NK) cells, tumor-associated macrophages (TAMs), cancer-associated fibroblasts (CAFs), and myeloid-derived suppressor cells (MDSCs) constitute an integral part of the tumor microenvironment (TME) of cholangiocarcinoma (CCA) [1].