How do we get so many types of antibodies to fight disease?

At one time many thought that our immune cells would encounter an invader and then make unique antibodies that would destroy the invader. This turned out not to be the case. We know of no molecular mechanism that could create such molecules de novo. However, it turned out that we from our genes we make millions of different immune cells each making one type of antibody. The ones that react with our tissues are killed off at an early stage. The fraction of the cells that don’t react with us are continually testing chemicals in their environment to see if there is something there that they can bind with. Usually these chemicals are pieces of bacteria or viruses that are trying to invade us. If binding occurs then this clone of cells expands dramatically so that there are enough of them to fight off the invader. Some cells make antibodies that float around in the blood and another type of cells attack the invader directly. So we don’t encounter a chemical and then create an antibody that will bind to it but amplify a clone of cells that already have such a reactive antibody on their surface. The antibodies come from our genes and are not made de novo in response to a specific chemical.

The whole process is much more complicated that my simplistic description but basically that is how it works..