**Abstract**

Approximately 80% of the pathogens that lead to deadly infections in humans choose mucosal tissue as the first site of infection. The mucosal surfaces of the body include the gastrointestinal tract, airways, oral cavity, and urogenital mucosa, which provide a large area conducive to the invasion and accumulation of many microorganisms and are of great importance in this regard. The large extent of mucus, as well as the accumulation of bacteria and countless foreign antigens in these areas, are the most important reasons for the importance of mucosal tissues. In addition to the myriad of symbiotic bacteria, large amounts of oral antigens (both pathogenic and non-pathogenic) enter a person's body daily and human mucosal tissues are exposed to these antigens. The function of the mucosal immune system is to distinguish pathogenic antigens from non-pathogenic ones. In this way, against a large number of oral antigens or co-tolerant microorganisms, and pathogenic antigens, a favorable (and even non-inflammatory, possible) immune response is produced. Mucosal tissue, as the largest lymphatic organ in the body, is home to 75% of the lymphocyte population and produces the highest amount of immunoglobulin. The amount of secreted IgA (slgA) produced daily by mucosal surfaces is much higher than the IgG produced in the bloodstream. A 70 kg person produces more than 3 grams of IgA per day, which is about 70–60% of the total antibodies produced in the body. The first embryonic organ in which immune system cells are located in the intestine. Some researchers consider this organ (and specifically mucosal lymph nodes) to be the source of the human immune system.

**Keywords:** mucosal immunology, mucosa associated lymphoid tissues, organized mucosal associated lymphoid tissue, diffuse mucosal associated lymphoid tissue, innate lymphoid cells, M cell, poly immunoglobulin receptor, mucosal vaccination
