Immune system

The immune system is a host defense system comprising many biological structures and processes within an organism that protects against disease.

For example, internal dysfunctions of the immune system can produce a variety of different diseases, including various forms of immunodeficiency, hypersensitivity, allergies and autoimmune disorders.

In humans, the blood–brain barrier, blood–cerebrospinal fluid barrier, and similar fluid–brain barriers separate the peripheral immune system from the neuroimmune system, which protects the brain.

The neuroimmune system is a system of structures and processes involving the biochemical and electrophysiological interactions between the nervous system and immune system which protect neurons from pathogens.

These mechanisms include phagocytosis, antimicrobial peptides called defensins, and the complement system.

The complement system is a part of the immune system that enhances (complements) the ability of antibodies and phagocytic cells to clear microbes and damaged cells from an organism, promote inflammation, and attack the pathogen's cell membrane.

To function properly, an immune system must detect a wide variety of agents, known as pathogens, from viruses to parasitic worms, and distinguish them from the organism's own healthy tissue.

The presented antigen can be either self or non-self, thus preventing an organism's immune system targeting its own cells.

This process of acquired immunity is the basis of vaccination.

The activation occurs through priming the immune system with an immunogen.

Immunodeficiency occurs when the immune system is less active than normal, resulting in recurring and life-threatening infections.

Immunodeficiency or immunocompromise is a state in which the immune system's ability to fight infectious disease and cancer is compromised or entirely absent.

These mechanisms include phagocytosis, antimicrobial peptides called defensins, and the complement system.

In a multicellular organism's immune system, phagocytosis is a major mechanism used to remove pathogens and cell debris.

Adaptive (or acquired) immunity creates immunological memory after an initial response to a specific pathogen, leading to an enhanced response to subsequent encounters with that same pathogen.

Immunological memory is the ability of the immune system to quickly and specifically recognize an antigen that the body has previously encountered and initiate a corresponding immune response.

In humans, immunodeficiency can either be the result of a genetic disease such as severe combined immunodeficiency, acquired conditions such as HIV/AIDS, or the use of immunosuppressive medication.

AIDS is a condition in humans in which progressive failure of the immune system allows life-threatening opportunistic infections and cancers to thrive.

In humans, immunodeficiency can either be the result of a genetic disease such as severe combined immunodeficiency, acquired conditions such as HIV/AIDS, or the use of immunosuppressive medication.

As the infection progresses, it interferes more with the immune system, increasing the risk of developing common infections such as tuberculosis, as well as other opportunistic infections, and tumors that rarely affect people who have uncompromised immune systems.

Disorders of the immune system can result in autoimmune diseases, inflammatory diseases and cancer.

A series of biochemical events propagates and matures the inflammatory response, involving the local vascular system, the immune system, and various cells within the injured tissue.

Common autoimmune diseases include Hashimoto's thyroiditis, rheumatoid arthritis, diabetes mellitus type 1, and systemic lupus erythematosus.

Systemic lupus erythematosus (SLE), also known simply as lupus, is an autoimmune disease in which the body's immune system mistakenly attacks healthy tissue in many parts of the body.

Even simple unicellular organisms such as bacteria possess a rudimentary immune system in the form of enzymes that protect against bacteriophage infections.

The vast majority of the bacteria in the body are rendered harmless by the protective effects of the immune system, though many are beneficial, particularly in the gut flora.

Immunology covers the study of all aspects of the immune system.

Immunology is a branch of biology that covers the study of immune systems in all organisms.

Common autoimmune diseases include Hashimoto's thyroiditis, rheumatoid arthritis, diabetes mellitus type 1, and systemic lupus erythematosus.

The underlying mechanism involves the body's immune system attacking the joints.

These mechanisms include phagocytosis, antimicrobial peptides called defensins, and the complement system.

Cells of the immune system contain these peptides to assist in killing phagocytosed bacteria, for example in neutrophil granulocytes and almost all epithelial cells.

This system does not confer long-lasting immunity against a pathogen.

Other components of the immune system adapt themselves to each new disease encountered and can generate pathogen-specific immunity.

This is passive immunity because the fetus does not actually make any memory cells or antibodies—it only borrows them.

Passive immunity can occur naturally, when maternal antibodies are transferred to the fetus through the placenta, and it can also be induced artificially, when high levels of antibodies specific to a pathogen or toxin (obtained from humans, horses, or other animals) are transferred to non-immune persons through blood products that contain antibodies, such as in immunoglobulin therapy or antiserum therapy.

Both subsystems use humoral immunity and cell-mediated immunity to perform their functions.

The study of the molecular and cellular components that form the immune system, including their function and interaction, is the central science of immunology.

The flushing action of tears and urine also mechanically expels pathogens, while mucus secreted by the respiratory and gastrointestinal tract serves to trap and entangle microorganisms.

Their functions include lubricating the eyes (basal tears), removing irritants (reflex tears), and aiding the immune system.

As a result of the symbiotic relationship between commensals and the immune system, the probability that pathogens will reach sufficient numbers to cause illness is reduced.

The immune system is a host defense system consisting of anatomical barriers, and physiological and cellular responses, which protect the host against harmful parasites while limiting inflammation by tolerating harmless symbionts.

The innate leukocytes include the phagocytes (macrophages, neutrophils, and dendritic cells), innate lymphoid cells, mast cells, eosinophils, basophils, and natural killer cells.

Macrophages (big eaters, from Greek μακρός (makrós) = large, φαγεῖν (phagein) = to eat ) are a type of white blood cell, of the immune system, that engulfs and digests cellular debris, foreign substances, microbes, cancer cells, and anything else that does not have the type of proteins specific to healthy body cells on its surface in a process called phagocytosis.

Breast milk or colostrum also contains antibodies that are transferred to the gut of the infant and protect against bacterial infections until the newborn can synthesize its own antibodies.

This helps to protect the newborn until its own immune system is functioning properly.

The flushing action of tears and urine also mechanically expels pathogens, while mucus secreted by the respiratory and gastrointestinal tract serves to trap and entangle microorganisms. However, as organisms cannot be completely sealed from their environments, other systems act to protect body openings such as the lungs, intestines, and the genitourinary tract.

The gastrointestinal tract contains trillions of microbes, with some 4,000 different strains of bacteria having diverse roles in maintenance of immune health and metabolism.

Disorders of the immune system can result in autoimmune diseases, inflammatory diseases and cancer.

The human immune system typically produces both T cells and B cells that are capable of being reactive with self-antigens, but these self-reactive cells are usually either killed prior to becoming active within the immune system, placed into a state of anergy (silently removed from their role within the immune system due to over-activation), or removed from their role within the immune system by regulatory cells.