Immunity

In a medical sense, immunity is a state of having sufficient biological defenses to avoid infection, disease, or other unwanted biological invasion.

The immune system is the collection of organs and tissues involved in the adaptive defense of a body against foreign biological material. It may be broken down into the adaptive immune system, composed of four lymphoid organs thymus,  lymph nodes,  spleen and submucosal lymphoid nodules and the group motile cells that are involved in the body's defense against foreign bodies. The term may also be used to refer to the totality of a body's defense systems, encompassing both the adaptive immune system and other passive defenses, such as the skin.

In multicellular organisms, the immune system is an organ system that acts as a defense against foreign pathogens (such as viruses, bacteria, parasites), some  poisons, as well as cancer. Components of the immune system also function in the return of extracellular fluid to the blood.

Bacteria and monocellular organisms have an "immune system" (under the broader of the two definitions above) designed to combat bacteriophages (viruses that infect bacteria). They do this by simultaneously expressing restriction enzymes that cut DNA at certain sequences, and enzymes that protect their own DNA from this enzyme by methylating the same sequence. Therefore, the bacterium's DNA will not be damaged by the first enzyme because of the presence of the second enzyme. However, when a bacteriophage attempts to infect this bacterium, the viral DNA has not been protected, and gets degraded by the first enzyme. While study of the bacterial immune system provides useful insights into immunology, the remainder of this article will focus on higher organisms' immune systems, particularly the human immune system.