1. Introduction
The innate immune system act as the first line of defense against bacterial infections, and its disorder results in the development of infectious, inflammatory and autoimmune diseases. Pattern recognition receptors (PRRs) are expressed outside or inside of innate immune cells, which can recognize pathogen-associated molecular patterns (PAMPs) conserved in different pathogens and trigger specific signaling pathways that regulate early host responses. Currently known types of PRRs include Toll-like receptors (TLRs) and C-type lectin receptors (CLRs) located on cell membrane, NOD-like receptors (NLRs), RIG-I-like receptors (RLRs) and AIM2-like receptors (ALRs) expressed in the cytoplasm[1,2]. Wherein the nucleotide-binding oligomerization domain 2 (NOD2) represents a well-characterized PRR of the NLR family that recognizes muramyl dipeptide (MDP) , activating signaling pathways and promoting cytokine expression[3]. Studies have shown that NOD2 plays an important role in the process of antimicrobial infection, and polymorphisms in coding genes of NOD2 are associated with susceptibility to leprosy and Crohn’s disease (CD)[4,5]. Since the discovery of NOD2, studies have provided insights into the structure of NOD2, signaling pathways regulated by NOD2 and roles of NOD2 in infection and maintenance of tissue environmental homeostasis, etc. In this review, we summarize the current information on the function and regulation of NOD2 in infectious diseases.