The innate intelligence of the body at all times uses the immune system to survey the chemical markers of every molecule and cell in our body. Because the marks on our body cells differ from those that brand foreign substances, the immune system can pick out intruders and avoid the reckless execution of friends. If the system recognizes a mark as self, it will usually respect the substance; if it detects a foreign mark, it will launch an attack to destroy the invader.
Any substance that trigger such an attack is called an antigen. Viruses, parasites, fungi and bacteria can act as antigens. So can blood cells or tissue from another human being and altered self-components, including cancer cells or cells infected by a virus. Even seemingly innocuous substances, such as ragweed pollen, mildew, animal hairs, or house dust, can provoke a full blown attack.
Amazing isn’t it?