The immune system is a network of biological mechanisms that guards an organism against disease. It recognises and responds to a wide range of pathogens, including viruses and parasitic worms, as well as cancer cells and things like wood splinters, separating them from the organism’s own healthy tissue. Many species have two major immune system subsystems. The innate immune system has a predetermined reaction to a wide range of events and stimuli. By learning to recognise chemicals it has previously seen, the adaptive immune system provides a personalised response to each stimulus. They rely on molecules and cells to carry out their tasks.
Almost every organism has some form of immune system. Bacteria have a primitive immune system in the form of enzymes that defend them from virus infections. Other fundamental immunological systems arose in ancient plants and animals and continue to exist in their present descendants. Phagocytosis, antimicrobial peptides known as defensins, and the complement system are examples of these systems. Jawed vertebrates, including humans, have more advanced defence mechanisms, including the ability to adapt to more efficiently recognise infections. Adaptive (or acquired) immunity generates an immunological memory, which leads to a better response to future encounters with the same pathogen. Vaccination is based on this process of acquired immunity.
Immune system dysfunction can lead to autoimmune illnesses, inflammatory diseases, and cancer. Immunodeficiency occurs when the immune system is less active than it should be, resulting in repeated and even fatal infections. Immunodeficiency in humans can be caused by a hereditary disease like severe combined immunodeficiency, acquired disorders like HIV/AIDS, or the use of immunosuppressive medicine. Autoimmunity occurs when the immune system overreacts and attacks normal tissues as if they were foreign organisms. Hashimoto’s thyroiditis, rheumatoid arthritis, type 1 diabetes, and systemic lupus erythematosus are all examples of autoimmune disorders. Immunology is the study of the immune system as a whole.