This is tutorial talk, in which I will introduce the main components of the immune system in the context of cancer. I will introduce the different phenotypes of macrophages, the four classes of CD4+ T cells, and the cytotoxic T cells (CTL) or CD8+ T cell. As will be explained, the tumor may be recognized by macrophages and dendritic cells, and these cells will then activate 'effective' T cells to kill cancer cells. However, the tumor can fight back against the immune system, and in fact it can even use the system to its own advantage, by "educating" macrophages so that they will actually enhance tumor growth by increasing VEGF production. Another factor that works in favor of a tumor are the T regulatory cells, enhanced by the tumor, which inhibit the activities of the effective T cells.