- White Blood Cells: responsible for attacking and destroying foreign invaders.
- Skin: the physical barrier that prevents most foreign invaders from ever entering the body.
- Mucous Membranes: line the respiratory, digestive, and urinary tracts, tapping foreign invaders and preventing them from entering the body.
- Lymphatic System: a network of vessels and nodes that help to remove foreign invaders from the body and help transport white blood cells and other immune cells to the sites of infection.
7 Obvious Signs That You Have a Weakened Immune System

