Innate Immunity

Innate immunity is the congenital resistance of the organism to pathogenic infection, and is the first line of anti-defense of the host to intruders, including natural physiological barriers (e.g., skin and mucosal barriers, blood–brain barrier, placental barrier, blood–testis barrier, etc.) and the innate immune response. This anti-defense mechanism exists from lower to higher organisms (including humans). The most direct killing and clearance of pathogenic microorganisms by innate immunity is achieved through the secretion of innate immune molecules and phagocytes in the organism. Innate lymphoid cells (ILCs) are important cells distributed at the skin and mucosal barriers to receive neural/endocrine signals to regulate immunity and play an important role in immune regulation, tissue repair, and maintenance of local lymphoid tissue stability. Among innate immune molecules, defensin plays a very important role. Defensins are a variety of small-molecule polypeptides secreted by the mucous membranes of the respiratory, digestive, and genitourinary tracts, and by the epithelial cells of the skin. These small-molecule polypeptides directly kill bacteria, fungi, or enveloped viruses by disrupting the membrane structure of pathogens. In addition, once the pathogen invades the organism, the complement in the body fluid can directly recognize the bacterial components, which activates the complement system and generates a cascade reaction to assemble a membrane attack complex and directly kill the pathogenic bacteria.