To be immune to an illness means that your body has the ability to effectively recognize and fight off that specific illness due to the presence of antibodies or memory cells. This immunity can come from previous exposure to the illness (natural immunity) or from vaccination (artificial immunity).
In other words, when you are immune, your body has already developed a defense mechanism against the pathogen that causes the illness, which allows it to respond quickly and effectively if you are exposed to that pathogen again.
So, the correct statement would be: "Your body already has antibodies for a certain illness." The first statement, "Your body doesn't know how to fight the illness," describes a state of susceptibility, not immunity.