- Innate immunity represents the body’s first line of defense against pathogens, consisting of both physical barriers and cellular responses that react quickly but non-specifically to threats. This ancient defense system is present from birth and does not require prior exposure to pathogens to function effectively.
- Physical and chemical barriers form the initial defense line. The skin provides a robust physical barrier, while mucous membranes line body cavities and produce protective secretions. These barriers are enhanced by chemical defenses such as the acidic pH of the stomach, lysozyme in tears and saliva, and antimicrobial peptides produced by epithelial cells. The microbiome, consisting of beneficial bacteria on skin and mucosal surfaces, also helps prevent pathogen colonization.
- Cellular components of innate immunity include various white blood cells that recognize and respond to pathogens. Neutrophils, the most abundant type, quickly migrate to infection sites and engulf pathogens. Macrophages not only phagocytose pathogens but also release signaling molecules that coordinate immune responses. Natural killer cells identify and destroy virus-infected or cancerous cells. Dendritic cells bridge innate and adaptive immunity by presenting antigens to T cells.
- Pattern recognition receptors (PRRs) on immune cells recognize pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs). These include Toll-like receptors, NOD-like receptors, and RIG-I-like receptors. When activated, these receptors trigger inflammatory responses and antimicrobial mechanisms. The complement system, a cascade of plasma proteins, marks pathogens for destruction and enhances inflammatory responses.
- Inflammation is a crucial innate immune response characterized by redness, swelling, heat, and pain. This process increases blood flow to infected areas, allowing immune cells better access. Inflammatory mediators like cytokines and chemokines recruit additional immune cells and help coordinate the overall immune response. While essential for defense, excessive inflammation can cause tissue damage and contribute to various diseases.
