Immunoglobulin A is synthesized and secreted by plasma cells, a class of globulin with antibody activity or chemical structure similar to that of antibody molecules, and can be divided into two categories: serum type and secretory type. Serum immunoglobulin A accounts for about 10%-15% of the total immunoglobulin, while secretory immunoglobulin A is synthesized by lymphoid tissues widely distributed in the trachea, bronchus, esophagus, stomach, intestines, and genitourinary tract, and it has a certain immune function, which can fight against injury, inflammation, infection, tumor and other lesions in these areas. Increased immunoglobulin A is seen clinically in connective tissue diseases such as rheumatoid arthritis, cirrhosis of the liver, kidney disease and skin eczema. Decreased immunoglobulin A levels are seen in autoimmune diseases, recurrent respiratory infections, and immunodeficiency diseases.