How do secretory IgA and blood IgA differ?

Secretory IgA (sIgA) and blood IgA (serum IgA) differ primarily in their functions and structures. sIgA is found in mucosal areas, such as the gut and respiratory tract, where it plays a crucial role in immune defense by preventing pathogen adherence and invasion. It is composed of two IgA monomers linked by a J chain and has a secretory component that protects it from degradation in harsh environments. In contrast, blood IgA is primarily found in circulation and is involved in systemic immune responses, reflecting different roles in the immune system.