  Cancer is a term used to refer to a large set of diseases. The cancerous
cells grow and divide and, as a result, they form tumours that grow in size.
The immune system recognise the cancerous cells and attack them, though, it can
be weakened by the cancer. One type of cancer treatment is chemotherapy, which
uses drugs to kill cancer cells. Clinical, experimental, and theoretical
research has been developed to understand the dynamics of cancerous cells with
chemotherapy treatment, as well as the interaction between tumour growth and
immune system. We study a mathematical model that describes the cancer growth,
immune system response, and chemotherapeutic agents. The immune system is
composed of resting cells that are converted to hunting cells to combat the
cancer. In this work, we consider drug sensitive and resistant cancer cells. We
show that the tumour growth can be controlled not only by means of different
chemotherapy protocols, but also by the immune system that attacks both
sensitive and resistant cancer cells. Furthermore, for all considered
protocols, we demonstrate that the time delay from resting to hunting cells
plays a crucial role in the combat against cancer cells.
