protective slime (their immune system pretty much)
Sorry but that is not true, the immune system is the part of the body that controls where the white blood cells are sent, the white blood cells then surround the antigen and engulf it, they then produce an antibody which will be sent out to any part of the body should this antigen enter the body again. The slime coat simply is a protective barrier that acts as a physical barrier to stop things such as antigens and particles touching the scales.
The immune system and slime coating are two very different things.