Well, a common side effect seems to be a reduction in the process of cells that undergo rapid reproduction. That is the central theme of chemotherapy, trying to eliminate or at the very least, slow down cancer cells that are rapidly multiplying and taking over entire body systems. While the cancer cells are being slowed in their reproductive capabilities, so are other cells that reproduce quickly as a norm. The blood cells, both red and white, are produced by the marrow contained in the bones. The red blood cells are the superhighway for transporting everything from oxygen to nutrients to waste products in and out of every cell in the body. White blood cells are the disease fighters of the immune system, so a suppressed immunological response is the norm when someone is undergoing chemotherapy. Hair follicles also normally undergo a rapid rate of reproduction, so hair loss is a common side effect among these five items listed.