Hair loss in cancer starts when an individual starts on chemotherapy treatment. Chemotherapy, targets the cancerous cells which are usually fast dividing. Along with these cells, any other cells within the body that are fast dividing will also be targeted, and all cells in general. Hair is made by cells at the base of hair follicles found in the middle layer of your skin. These follicle cells are among the very fast dividing cells in your body and are thus affected by chemotherapy, resulting in falling off of hair. After treatment, these cells are regenerated and normal hair growth resumes.
Other rapidly dividing cells in the body include those in your bone marrow that make your red and white blood cells. It is for this reason that during chemotherapy, individuals have low immunity and are often secluded in order to prevent them from easily getting infections.