In your body, cells create a physical barrier to keep harmful things out. This barrier is usually found around the neck and nose. This barrier goes from hair to skin, skin to the mucus membrano or mucus. Some germs and bad cells get stuck in the mucus and than later you sneeze or cough it out.
The Immune System has a lot of parts, it is mostly built by cells. Cells that help, cells that destroy, cells that alert, all different cells. These parts and cells are phagocytes, the thymus, the spleen, antibodies, b cells, killer t cells, helper t cells, and t suppressor cells.