Fig. 1

Distribution of the mucosal immune system in the body. The mucosal immune system is distributed throughout the body primarily in the respiratory, gastrointestinal, and urogenital tracts. The respiratory tract includes, in ascending order, the nasal cavity, oral cavity, airway, and lungs. The airway is divided into the trachea, bronchi, and bronchioles according to branching of the airway. The lungs comprise mainly the alveolus and pulmonary interstitium. The gastrointestinal tract, particularly the intestine, is dotted with lymph nodes, and the mucosal immune response is the most active immune response in the gastrointestinal tract. The uterus, bladder, and vagina comprise the female urogenital tract. In addition, the mucosal immune system is distributed in other important regions, such as the conjunctiva, middle ear, and breast. The most important mucosal immune component, mucosa-associated lymphoid tissue can be divided according to the organs they occupy, and the susceptible organs for SARS-CoV-2 widely exist throughout the body