The answer is IgA.
IgA is the immunoglobulin that consists of a dimer or a tetramer, a J-chain polypeptide, and a polypeptide chain called the secretory component. IgA is the principal antibody class in the secretions that bathe the mucosal surfaces, acts as an important first line of defense. It is also an important serum immunoglobulin, mediates a variety of protective functions through interaction with specific receptors and immune mediators.
