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INNATE IMMUNITY
If a micro-organism manages to cross these barriers, it encounters defence mechanisms of the innate immunity (immunite non specifique/ immunità innata/ inmunidad inespecìficas) [E], [F], [ES], that are constitutive in the sense that they do not require prior activation by that specific foreign micro-organism. In contrast to adaptive immunity, which may take days or weeks after an initial infection, to have an effect, the innate immune system represents a faster-acting mechanism of defence. Innate immunity defends against pathogens by rapid responses coordinated through "innate" receptors that recognize a wide spectrum of conserved pathogenic components and through endogenous danger/alarm signals from distressed tissues triggered by pathogens attacking them. These responses to pathogens do not change and develop during the lifetime of an individual as for adaptive immunity. Plants and many lower animals do not possess an adaptive immune system, and rely instead only on their innate immunity.
Phagocytic cells (phagocytes/fagociti/fagocitos) [E], [ES]
A first response to pathogens entering the organism, is the activation of phagocytic cells, such as macrophages and neutrophils. Phagocytosis involves chemotaxis, in which phagocytic cells are attracted to micro-organisms by means of chemotactic substances such as microbial products, complement, damaged cells and white blood cell fragments. Chemotaxis is followed by adhesion, where the phagocytic cell sticks to the micro-organism. Adhesion is enhanced by opsonization, where proteins like opsonins are coated on the surface of the pathogen. This is followed by ingestion, in which the phagocytic cell extends projections, forming pseudopods that engulf the foreign organism. Finally, the micro-organism is digested by enzymes contained in lysosomes [ES]. Once phagocytic cells have done their job, they die and their "corpses", pockets of damaged tissue, form pus.
Anti-microbial proteins
In addition, anti-microbial proteins may be activated if a pathogen passes through the barrier offered by skin and mucosal surfaces. There are several classes of anti-microbial proteins, such as acute phase proteins, lysozyme, and the complement system.
The complement (complement/complemento/complemento), so-called because it “complements” and amplifies the action of antibodies, consists of a system of serum proteins that can be activated by antibody-antigen complexes or micro-organisms to undergo a cascade of proteolytic reactions, the end result of which is the assembly of membrane attack complexes. These complexes form holes in a micro-organism and thereby destroy it. At the same time, proteolytic fragments released during the activation process promote the defence response by dilating blood vessels and attracting phagocytic cells to sites of infection. The complement also enhances the ability of phagocytic cells to bind, ingest, and destroy the micro-organisms being attacked.
Fever (fiévre/Febbre/fiebre)
As well as important local effects, the cytokines (signalling proteins secreted by macrophages and lymphocytes) have long-range effects that contribute to host defence. One of these is the elevation of body temperature, which is generally beneficial to host defence as most pathogens grow better at lower temperatures. Moreover, adaptive immune responses are more intense at elevated temperatures.
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