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The immune system is a network of biological systems that protects an organism from diseases.

Many species have two major subsystems of the immune system. The innate immune system provides a preconfigured response to broad groups of situations and stimuli. The adaptive immune system provides a tailored response to each stimulus by learning to recognize molecules it has previously encountered. Nearly all organisms have some kind of immune system. Bacteria have a rudimentary immune system in the form of enzymes that protect against viral infections. These mechanisms include phagocytosis, antimicrobial peptides called defensins, RNA interference, and the complement system. Dysfunction of the immune system can cause autoimmune diseases, inflammatory diseases and cancer. Immunodeficiency occurs when the immune system is less active than normal, resulting in recurring and life-threatening infections. Autoimmunity results from a hyperactive immune system attacking normal tissues as if they were foreign organisms. Immunology covers the study of all aspects of the immune system.

The immune system protects its host from infection with layered defenses of increasing specificity. If a pathogen breaches these barriers, the innate immune system provides an immediate, but non-specific response. If pathogens successfully evade the innate response, vertebrates possess a second layer of protection, the adaptive immune system, which is activated by the innate response. Here, the immune system adapts its response during an infection to improve its recognition of the pathogen. This improved response is then retained after the pathogen has been eliminated, in the form of an immunological memory, and allows the adaptive immune system to mount faster and stronger attacks each time this pathogen is encountered. Both innate and adaptive immunity depend on the ability of the immune system to distinguish between self and non-self molecules.

The innate immune system is the dominant system of host defense in most organisms, and the only one in plants.

Cells in the innate immune system use pattern recognition receptors to recognize molecular structures that are produced by pathogens. Cells in the innate immune system have pattern recognition receptors that detect internal infection or cell damage.

Dendritic cells serve as a link between the bodily tissues and the innate and adaptive immune systems, as they present antigens to T cells, one of the key cell types of the adaptive immune system. Natural killer cells (NK cells) are lymphocytes and a component of the innate immune system that does not directly attack invading microbes.

Inflammation is one of the first responses of the immune system to infection.

The adaptive immune system evolved in early vertebrates and allows for a stronger immune response as well as immunological memory, where each pathogen is "remembered" by a signature antigen.

The cells of the adaptive immune system are special types of leukocytes, called lymphocytes. B cells are involved in the humoral immune response, whereas T cells are involved in cell-mediated immune response.

Helper T cells regulate both the innate and adaptive immune responses and help determine which immune responses the body makes to a particular pathogen.

The immune system is affected by sleep and rest, and sleep deprivation is detrimental to immune function.

The immune response can be manipulated to suppress unwanted responses resulting from autoimmunity, allergy, and transplant rejection, and to stimulate protective responses against pathogens that largely elude the immune system (see immunization) or cancer.

To the immune system, these antigens appear foreign, and their presence causes immune cells to attack the transformed tumor cells. The main response of the immune system to tumors is to destroy the abnormal cells using killer T cells, sometimes with the assistance of helper T cells.

Some drugs can cause a neutralizing immune response, meaning that the immune system produces neutralizing antibodies that counteract the action of the drugs, particularly if the drugs are administered repeatedly, or in larger doses.

Some bacteria form biofilms to protect themselves from the cells and proteins of the immune system.

Quick Facts

• Dysfunction of the immune system can cause autoimmune diseases, inflammatory diseases and cancer.
• Bacteria have a rudimentary immune system in the form of enzymes that protect against viral infections.
• The adaptive immune system provides a tailored response to each stimulus by learning to recognize molecules it has previously encountered.
• Many species have two major subsystems of the immune system.
• Dendritic cells serve as a link between the bodily tissues and the innate and adaptive immune systems, as they present antigens to T cells, one of the key cell types of the adaptive immune system.

Source material: Wikipedia - "Immune system". Adapted and summarized for DiscoverScroll. Original contributors are credited through the linked Wikipedia article. Read original on Wikipedia. CC BY-SA 4.0. Changes were made from the original.