The three Es of cancer immunoediting: elimination, equilibrium, and escape. (a) After transformation of cells in a normal layer (diamond-shaped cells) into cancerous cells (with irregular shapes), attack by various different cell types of the immune system (indicated by round cells) may lead to elimination of the cancerous cells. (b) If elimination is unsuccessful, the immune system and the cancer can reach an equilibrium in which immune cells keep the cancer in check but cannot remove it completely. During the elimination phase, there is selection on the cancer cells, whose genomes are also unstable. This can lead to escape (c), in which mutated cancer cells become able to inhibit the immune system. The cancer can then grow unchecked. Figure modified from . CD4+, CD8+, CD4+CD25+ Treg, γδ and NKT cells are all types of T cell; Mφ cells are macrophages and NK cells are natural killer cells.