To continue - there are two main types of drugs - first small molecules (aspirin, speed, phenacetin etc). These molecules are generally too small for the immune system to see and dont have enough "docking" sites for the immune molecules to latch on to. Many modern drugs are big (e.g. proteins such as artificial insulin) and they are quite likely to generate immune responses if they dont look like what the body expects. They have lots of molecular sites for the immune molecules to grab and recognize. Often these molecules are "pegylated" by the chemists, surrounded by nonimmunogenic polymers that sheild the large molecule from the immune system.