Layered defenses that must know self from stranger
The immune system is a network that finds and fights viruses, bacteria, parasites, cancer cells and even wood splinters, while sparing the body’s own healthy tissue. Almost every living thing has one: bacteria carry enzymes against viruses, and ancient defences such as defensins and phagocytosis survive in modern plants and animals.
Defence comes in layers that grow more precise. Physical barriers keep most invaders out; anything that gets through meets the innate system, which reacts at once but in the same generic way to broad classes of threat and is found in all animals. Vertebrates add an adaptive system, switched on by the innate response, that tailors itself to each infection and remembers it. Both depend on telling self molecules from foreign ones, including antigens, the substances that bind immune receptors and set off a response.
The barriers are mechanical, chemical and biological. Skin, insect exoskeletons, egg shells and the waxy coating of leaves block entry, while coughing and sneezing expel irritants from the lungs. Skin and airways release antimicrobial peptides called β-defensins, saliva, tears and breast milk contain the enzyme lysozyme, and stomach acid attacks swallowed microbes. In the gut and genitourinary tract, harmless resident bacteria compete with pathogens for food and space and can shift the local pH or iron supply against them.
Innate cells spot invaders with pattern recognition receptors that detect molecules common to whole groups of microbes, or alarm signals from damaged cells; humans have ten toll-like receptors, first found in fruit flies. Neutrophils, which make up half to 60 percent of circulating white cells, are usually first to reach an infection. Macrophages and dendritic cells engulf microbes and digest them inside a phagolysosome, and dendritic cells pass fragments to T cells of the adaptive system. Natural killer cells target tumour or virus-infected cells whose surface MHC I markers have dropped.
Failures cut both ways. An underactive system brings repeated, dangerous infections, whether from inherited conditions like severe combined immunodeficiency, from HIV/AIDS or from immunosuppressive drugs; an overactive one attacks normal tissue, as in Hashimoto’s thyroiditis.
Source: Immune system
