Primary line of defense? First? I don't know what sort of vocab youre using, so I can't give an exact answer, but I know that the epidermis (skin) is our largest organ and our first line of defense against infection and disease. When we get cuts, it makes more vulnerable parts of our body become open to things that our skin usually takes on and handles just fine. That's why our body is supposed to clot- to seal off the wound and stop bacteria from getting in.