Your Immune system keeps you healthy by fighting off infections, cancer, and disease. our immune system protects you from organisms that can enter your body, like bacteria and viruses. Your immune system also kills unhealthy body cells that can cause tumors, cancer, and illness. A strong immune system can fight off infections and illness. A weak immune system cannot fight off infections and illness. The immune system is made up of many cells, including B cells, antibodies, killer (CD8) cells, helper (CD4) cells, natural killer (NK) cells, and macrophages. The CD4 cell is responsible for telling most of these cells what to do and where to go in the body to fight infection and keep you healthy.