Is cancer a defect of the immune system?
Cancer is uncontrolled division of abnormal cells. This is caused by mutations in a cell which causes a cell to divide more frequently and the lack of control by intracellular AND extracellular mechanisms. The extracellular mechanisms include growth inhibition by hormones, local factors, contact and also the immune system. 

These cancerous cells could be immune system cells and then, in that respect, the cancer could be considered an immune system disorder. Or the cells could be of different origin, but due to the lack of a functioning immune system (particularly NK cells), the cells may divide uncontrollably whereas otherwise they would've been tagged for immune or self destruction. Or a cancer may be caused by a virus; in this situation an immunocompromised patient is more susceptible to a cancer caused by this oncogenic virus. In these cases the risk of cancer is increased due to a compromised immune system. So cancer risk may be increased in these circumstances but cancer still is not an immune disorder.