Secretory Immunoglobulin A (SIgA) is the predominant antibody found on mucosal surfaces, including the gastrointestinal, respiratory, and genitourinary tracts, as well as in secretions such as saliva, tears, and colostrum. Structurally, SIgA is a polypeptide complex composed of two IgA monomers, a joining (J) chain, and a secretory component, which protects it from enzymatic degradation and enables it to function in harsh environments like the gut. SIgA serves as the first line of immune defense by binding to pathogens and toxins, preventing their attachment and invasion of epithelial cells, a process known as immune exclusion. It also helps maintain mucosal barrier integrity and regulates the composition of the microbiota by interacting with commensal bacteria.