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.

source: online medical dictionary:

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