The immune system protects the body from harmful substances, germs and cell changes that could make us ill. It includes specialized cells and proteins, such as T-cells and B cells, as well as organs such as the thymus, spleen and lymph nodes.
The first line of defense is called innate immunity. It is made up of soluble proteins (complement proteins, defensins and ficolins) and phagocytic cells that recognize and destroy invading microorganisms or toxic substances. This mechanism relies on structural features of the pathogen or toxin that distinguish it from normal mammalian tissue and cells.
When the innate response fails to eliminate an invading pathogen or toxin, other components of the immune system take over. They include inflammatory cells such as macrophages and dendritic cells that engulf invading microorganisms, and lymphocytes including T cells and B cells that produce antibodies. These antibodies bind to specific markers, or antigens, on the surface of pathogens, neutralizing them or marking them for attack and destruction by other immune system cells.
After the initial contact with an antigen, a small number of B and T cells survive and multiply to provide long-lived memory cells that can respond more quickly and robustly to that particular antigen on a second encounter, even decades later. This process is called immune memory, and it is essential to the ability of the adaptive immune response to recognize and eliminate pathogens or toxins without damaging healthy mammalian tissues. Failure of the immune system to do so leads to autoimmunity, a broad class of diseases that affects multiple organ systems.