The immune system’s immediate, non-specific first response to infection or injury — the rapid reaction that buys time while the adaptive system prepares a targeted attack.Mediated by neutrophils, macrophages, dendritic cells, and NK cells via pattern recognition receptors (Toll-like receptors, NOD receptors) that detect conserved molecular patterns of pathogens. Antimicrobial peptides (LL-37, defensins) are innate immune effectors that directly kill bacteria and recruit immune cells. Distinct from the adaptive immune response, which requires days to develop but produces highly specific and long-lasting immunity.