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The immune system is a host defense system comprising many biological structures and processes within an organism that protects against disease.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 immune system protects organisms from infection with layered defenses of increasing specificity.In simple terms, physical barriers prevent pathogens such as bacteria and viruses from entering the organism.In many species, the immune system can be classified into subsystems, such as the innate immune system versus the adaptive immune system, or humoral immunity versus cell-mediated immunity.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.Immunodeficiency occurs when the immune system is less active than normal, resulting in recurring and life-threatening infections.

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.

In contrast, autoimmunity results from a hyperactive immune system attacking normal tissues as if they were foreign organisms.

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

Other basic immune mechanisms evolved in ancient eukaryotes and remain in their modern descendants, such as plants and invertebrates.

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

One class of non-self molecules are called antigens (short for antibody generators) and are defined as substances that bind to specific immune receptors and elicit an immune response.

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