The skeleton also protects the soft sections of the body. The cranium's fused bones surround the brain, making it less sensitive to harm. The vertebrae encircle and protect the spinal cord, while the rib cage bones assist protect the thorax's heart and lungs. The skin is the only organ of the body that does not have any protective mechanism besides being exposed to the environment. The skin needs protection from heat and cold, chemicals, bacteria, and other substances in order to stay healthy. The immune system also plays a role in protecting the body's soft tissues. Immunoglobulins are proteins made by the bone marrow that help fight off bacteria and viruses that may cause infection or cancer. T-cells are part of the immune system that recognize infected cells as "foreign" and kill them. B-cells are responsible for producing immunoglobulins that bind to antigens (bacteria or viruses) so they can be destroyed by macrophages or other white blood cells. Dendritic cells are highly specialised antigen-presenting cells that interact with T-cells to trigger an immune response. Macrophages are large phagocytic cells that eat up foreign particles and dead cells that are unable to be disposed of by other cells of the immune system. The lymphatic system is important for removing waste from the body's muscles and tissues.