The immune systemThe immune system is made up of processes and structures within an organism that help protect it from external attacks. The immune system is made up of cells, tissues and organs; such as lymphocytes, lymph nodes, phagocytes and many others. For the immune system to function properly, it must first learn to discriminate between its own cells and foreign cells. Cells marked "self" should be left intact. Foreign cells should be identified as “non-self” and cause the immune system to start defending the body. Any microbe or foreign antigen, an antigen is any substance that causes an immune response, will activate the immune system. To prevent an autoimmune disease, a disease in which the body attacks its own cells, the cells must first undergo central tolerance; the ability of the immune system to recognize self-cells and not attack them. There are two types: the first, central tolerance and the second, peripheral tolerance. Central tolerance is acquired when the cell first develops in the bone marrow; the immature immune cell must learn not to attack its own body cells. If the ...