See How Tough Your Immune System is With "Blood Wars"
Thanks to a new art/science exhibition called "Blood Wars," you can find out whose dad has the toughest immune system once and for all. The brainchild of artist Kathy High, "Blood Wars" pits white blood cells from two different people against each other. From the article: "In order to create the blood duel, High gets a phlebotomist to take blood samples from two different people. She then separates the white blood cells from the rest of the blood and stains them using different colors. They are then placed in a Petri dish and their interactions are filmed under a microscope using time-lapse microscopy. The cellular 'winner' of each round will go onto fight another participant."
Geeze, what kind of geeks read slashdot these days?
File under 'M' for 'Manic ranting'
It doesn't quite work that way, but the answer to your question is that the database is stored in the DNA of immune cells.
Essentially, the newborn cells of the adaptive immune system (B and T-cells) undergo rearrangements of their DNA to produce a incredibly wide variety of receptors.
Then, they go through a selection process - if they react strongly with self, they die (negative selection). After a few more maturation and selection steps, the surviving immune cells are sent throughout the body.
If one of them later binds strongly to something (which is presumably foreign) in the right context, they activate. They trigger an immune response and proliferate. A subset of these daughter cells become essentially immortal - outlasting the immune response they fought in, but ready to quickly mobilize should that foreign substance be encountered again.
So, the memory cells are the hardware, but the rearranged antigen receptor gene they harbor is the information they need to work.