The second is called cellular innate immunity, which is accomplished by cells called phagocytes that ingest or eat the pathogens and by natural killer cells that destroy certain types of cancer cells. Innate immunity is nonspecific, meaning it is not directed against any particular invaders, but against any pathogens that attack the body. An additional and more sophisticated system of defense is called adaptive immunity that has the ability to recognize and destroy specific pathogens. This type of defensive reaction is called the immune response. Any substance that is capable of generating this type of immune response is called an antigen or immunogen.
Antigens are toxins or enzymes that the immune systems considers foreign.
Immune responses directed against antigens are called antigen specific. Specificity is one of the properties that distinguish adaptive immunity from innate immunity. Adaptive immunity works with innate immunity to provide the body with a heightened resistance to parasites and other intruders. Adaptive immunity is also responsible for allergic reactions and for rejecting transplanted tissue, which it mistakes for a foreign invader. Lymphocytes are a class of white blood cells that are the principal components of the adaptive immune system.