(Science: immunology) major class of immunoglobulin of external secretions in mammals, also found in serum and body fluids such as tears and saliva and in the respiratory, reproductive, urinary and gastrointestinal tracts.
immunoglobulin a protects the bodys mucosal surfaces from infection and in secretions, it is found as a dimer (400 kD) joined by a short j chain and linked to a secretory portion or transport piece. In serum found as a monomer (170 kD).
immunoglobulin a is the main mechanism for providing local immunity against infections in the gut or respiratory tract and may act by reducing the binding between an immunoglobulin a coated micro organism and a host epithelial cell. Present in human colostrum but not transferred across the placenta. Have heavy chains.