The expression and activation of certain receptors lead, in some primary tumors of the central nervous system such as glioma, to a reduction in the activity of natural killer (NK) cells, cytotoxic T lymphocytes (CTL), and the decreased proliferation of T cells; in this regard, the receptors most involved are HLA-G, HLA-E, and prostaglandin E receptor 2 (PTGER2) [73]. Here, PTGER2 is linked to glioma.