Myeloid and Lymphoid components
The immune system basically divides into the myeloid and the lymphoid component. The myeloid system is the evolutionary older one and provides a fast but rather unspecific immune surveillance of invading pathogens and of derailed own cells. It also calls in cells of the lymphoid system to launch a more specific immune attack.
The lymphoid, also called adaptive immune system provides the „precision weapons“. Specifically Natural Killer (NK) cells and T cells are needed to keep growing tumors effectively under control.
In the microenvironment of solid tumors, cells of both systems can adopt pro- as well as anti-cancer roles.
In highly inflamed, so-called „hot“ tumors, myeloid-derived suppressor cells (MDSCs) skew towards pro-tumorigenic M2 macrophages and towards cancer permissive regulatory T cells (Tregs). An efficient anti-tumor immune response would require M1 macrophages, active, non-exhausted T and NK cells. An acidic, high lactate-containing tumor environment favors the pro-tumorigenic immune cells and suppresses the immune attack on the tumor.