uhm, something more on differentiating the three lines of defence: first line is the physical barrier between the internals of an organisms( eg, skin, cilia, mucuous, etc etc), second line is more cell mediated (ie, phagocytes, inflamation is more or less a response that allow extra number of whiteblood cells and phagocytes to be active around wounded region, and nutrient for cell to replenish damaged cells) and third line is highly specific, cell mediated (cytotoxic t cells) or bloodstream(? darn, can't remember the exact word) by antibodies...