innateimmune

Innate immunity, also called the innate immune system, is the body's immediate, non-specific defense against pathogens. Present from birth, it responds rapidly and broadly to a wide range of microbes, without requiring prior exposure. It functions as the first line of defense and also helps shape the subsequent adaptive immune response.

Key components include physical and chemical barriers such as intact skin, mucous membranes, acidic environments, and antimicrobial substances (defensins, lysozyme, lactoferrin). Cellular defenses consist of phagocytes (neutrophils and macrophages), dendritic cells, and natural killer cells. Soluble factors include the complement system and a network of cytokines and chemokines that promote inflammation and coordinate cellular responses.

Pathogen detection relies on pattern recognition receptors (PRRs) that identify conserved microbial structures called pathogen-associated molecular patterns (PAMPs) and danger signals known as damage-associated molecular patterns (DAMPs). Prominent PRR families are Toll-like receptors, NOD-like receptors, and RIG-I-like receptors. Activation triggers phagocytosis, production of reactive oxygen species, release of inflammatory mediators, and recruitment of additional immune cells.

The inflammatory response is a hallmark of innate immunity, helping to contain infection and initiate tissue repair. The complement system, a cascade of plasma proteins, enhances opsonization, promotes lysis of pathogens, and attracts immune cells.

In addition to its protective role, innate immunity interacts with the adaptive immune system by presenting antigens and providing cytokine signals that guide specific adaptive responses. Some innate mechanisms can also exhibit trained or memory-like features after certain exposures.