Toll-like receptors.

There are two major arms of the immune system: the innate immune response and the adaptive immune response. Innate immunity is the first line of defense against microbes and serves to limit infection within the early hours after exposure to a pathogen. It is classically associated with the recognition of pathogens by phagocytic cells via specific receptor recognition molecules or through complement fixation. Essential components of the innate immune response include neutrophils, natural killer cells, natural killer T cells, mast cells, complement, and antimicrobial peptides. Innate immune activation via pattern recognition receptors results in a specific expression of co-stimulatory molecules and cytokines. This inflammatory milieu shapes the subsequent adaptive response, which involves B cell activation and T cell-mediated recognition of foreign antigens presented on major compatibility complexes (MHC) I and II on the cell surface of antigen-presenting cells (APCs). Activated B and T lymphocytes then undergo clonal expansion to provide an antigen-specific immune response.