Abstract: Insects lack acquired immune system processed by vertebrates, and rely solely on developed innate immune system to defend against the infection from alien pathogens such as bacteria, fungi, or virus and so on. In this article, we review the progress in the molecular mechanisms of the development and action of the innate immune responses in insects, especially focusing on the roles and functioning mechanisms of some important immunerelated molecules. The innate immune response in insects is divided into humoral immunity and cellular immunity. They function together to kill and eliminate the pathogens via phagocytosis, nodulation, encapsulation, coagulation, and melanization. When the insects are infected by the alien pathogens, the pattern recognition proteins/receptor (PRPs) in insects would firstly recognize and combine with pathogen-associated molecular pattern (PAMPs) in the pathogens, and then initiate the activation and regulation of a series of innate immune response containing immune-related serine proteases and serine protease inhibitors, and finally trigger the production of the immune effectors, such as antimicrobial peptides, melanin and so on, to kill the alien pathogens. The antimicrobial peptides are a kind of cationic peptides with low molecular weight, and have a broad spectrum of antimicrobial activity. They are produced via various mechanisms based on the types of invading pathogens. There are two signal transduction pathways regulating the production of antimicrobial peptides: Toll pathway activated by fungi and most of gram-positive bacteria, and Imd pathway initiated by gram-negative bacteria. These two pathways are contributed to the expression of different antimicrobial peptides due to the activation of different transcription factors.

Key words: Insect, innate immune response, humoral immunity, cellular immunity, molecular mechanisms