Table 3 Processes and players in antitumour immunity

Apart from eradicating pathogens, an important function of the immune system is to remove cells that have the potential to become malignant. Abnormal processes in cells, such as the processes underlying CIN, will often lead to a change in antigens presented on the cells’ surface. This change can alert cells of the immune system, which will eliminate the abnormal cell; a process often termed immunosurveillance [210]. Two important types of immune cells that mediate responses against (potential) tumour cells are Natural Killer (NK) cells and cytotoxic T-cells. NK cells are cytotoxic lymphocytes that recognise and eliminate abnormal cells without the need for prior sensitisation; and are therefore part of the innate immune system [211]. NK cells are able to recognise cells with lowered or absent expression of MHC class I molecules and/or the expression of the stimulatory NKG2D receptor which is often expressed as a result of cellular stress [211]. Another important component of antitumour immunity are cytotoxic T-cells (CD8 T-cells). CD8 cells eliminate cells by recognition of specific MHC class I molecules, but have to be activated first with the help of (professional) Antigen Presenting Cells (APC) e.g. dendritic cells [212]. The interplay between cancer and the immune system comprises many more different players and factors, which is beyond the scope of this review, and has been reviewed elsewhere [212]. However, this anti-tumour immune control in itself can also sculpt the tumour population into becoming less immunogenic; a process known as immunoediting [210, 213]. Immunoediting can be seen as a process where the selective pressure of the immune system leads the tumour to eventually become less immunogenic. As a result, many cancers become effective at evading, or repressing the immune system.