**2. Sinonasal immune system**

The sinonasal mucosal immune system is the primary barrier in contrary to pathogens, consist of innate and adaptive immunological components, called Nasal-Associated Lymphoid Tissues (NALT) which is included in Mucosa-Associated Lymphoid Tissue (MALT) [1–9]. Human NALT consist of lymphoid tissue, formed by lymphocyte-containing follicles in junction with Follicle-Associated Epithelium (FAE) and High Endothelial Venules (HEVs), located in the nasal mucosa and pharyngeal tonsils (adenoids). NALT play a function as a barrier and as an induction site for immunological reactions, which Immunoglobulin (Ig) A has a pivotal role. The Common Mucosal Immune System (CMIS) connects NALT inductive sites with effector areas of the nasal mucosa, for the formation of antigen-specific T Helper 1 (TH1) and Th2 cell, Cytotoxic T Lymphocyte (CTL), B lymphocyte (such as plasma cells and memory cells), and Innate Lymphoid Cells (ILCs) [3, 10–13]. With other immunological cells like dendritic cells, Microfold (M) cells, and macrophages; nasal epithelial cells and goblet cells involve in initiating local and systemic immune response in contrary to pathogens [1, 12].

**Figure 1.** *Nasal innate immune system [6].*
