Immunoglobulin A. Molecular model of the structure of the antibody immunoglobulin A (IgA). This is the secretory dimeric form (sIgA), the main immunoglobulin found in secretions such as saliva, tears, and lung and intestinal fluids. This molecule plays a key role in the immunology of the mucosal system. The structure here consists of: the H-chain (blue), the L-chain (grey), the J-chain (dark green rectangle, centre), and the secretory component (red).