In the long process of evolution, under the pressure of pathogenic organisms, the body has adapted to produce two sets of immune systems, namely innate immunity and acquired or adaptive immunity. Natural immunity, or non-specific immunity, exists in all multicellular organisms, including a variety of effector cells and molecules, such as various granulocytes, monocytes/macrophages, dendritic cells (DC), NK cells and humoral bactericidal components such as complement, antimicrobial peptides and lysozyme. Acquired immunity, that is, specific immunity, did not appear until vertebrates. It is an antigen recognition cell produced by the rearrangement of somatic lg superfamily genes during individual development, including T and B lymphocytes.

Since Burnet [1] put forward the theory of clonal selection in the late 1950s, the acquired immune system has been extensively and deeply studied. Many significant progress and breakthroughs have been made in the structure, function and mechanism of cells, protein and genes involved in acquired immune response, while the research on natural immunity has been slow. However, the discovery of various natural immune recognition molecules and the preliminary elucidation of their structures and functions since the 1990s led to the "rise of natural immune research" in the late 1990s [2] and its "arrival of the Renaissance" [3]. This paper briefly introduces its core problem-"molecular pattern recognition" and its immunobiological significance.

1 natural immune recognition molecule and its "molecular pattern recognition function"

Types of natural immune recognition molecules 1. 1 natural immune recognition molecules are all protein encoded by germline genes, and they are divided into seven families according to their structural characteristics (see table 1), but some molecules such as classical pathway recognition molecule C 1q, bypass pathway recognition molecule C3 and peptidoglycan recognition protein have not been classified, and new natural immune recognition molecules are still in the process. Natural immune recognition molecules can also be divided into body fluid proteins circulating in plasma, endocytosis receptors expressed on the cell surface and signal receptors on the cell surface or in cells. According to the recognition method, it can be divided into CD 14, DEC-205, gelatin and other direct recognition molecules. And indirect recognition molecules (products of recognizing the reaction of natural immune system with pathogens) such as complement receptor and Toll receptor.