Imagine walking around with 10 billion different keys in your pocket. You could unlock 10 billion different locks. This is what your immune system can do. It can potentially protect us against 10 billion different infections, even though it has never seen the infective bug before. It has the plans to make 10 billion different antibodies and it can make these very rapidly when the right lock (virus, bacterium, tetanus toxin, pollen grain etc) comes along to fit the key on the exact one of our white blood cells. Australian scientist Frank Macfarlane Burnet explained how this works in Clonal Selection Theory of Acquired Immunity, which won him the Nobel Prize in 1960.
As soon as the lock is in its key, that specific white blood cells divides and divides and divides into a clone of thousands of identical cells and each cell can produce millions of specific antibodies to fit that specific lock. So in a few days of the infection we have protective antibodies circulating in our blood to neutralise the bug or the toxin.
The only downside is if the ineffective virus, bacterium or toxin can act faster than our immune system, causing death or permanent damage before our defence system has achieved full production of the correct antibodies. That is why we are vaccinated against the nasty organisms so that we already have our specific clones of white cells ready to go into faster production.