I can give simple accurate and easy answer of this question. Well, this network of tissues,cells,and organsfirst tries to keep out germs and bacterie,viruses,fungi,and parasites and then deals with them if they manage to get in. If it senses something in your body that could be bad for you,it triggersthe release of special cells.These travel to where the trouble is,attack the intruder, and help get rid of it. Basically thats the SHORTEST explanation i can give .