System sounds very cold and clinical. I'm not sure that a person would say that about themselves, but they might.
I also like DJ's suggestion of essence, but it almost has a feeling of talking about his soul instead of his actual body.
Another suggestion would be to say... It's preventing germs from entering my person as we speak. I agree with Mrs. Mosley that saying body is probaly best. The suggestion I had of 'very being' is a tiny bit on the dramatic side.
Hey, your vocabulary is pretty darn good if you ask me.
