How does your body build a custom defense against germs it has never met?

By making a vast number of different guesses in advance. Unlike the innate immune system, which is pre-programmed for broad categories of pathogen, the adaptive immune system is highly specific to each pathogen the body has met, and that specificity comes from a genetic shuffle. A process called V(D)J recombination randomly picks one variable, one diversity and one joining gene segment and discards the rest, so a small number of gene segments generates a vast number of different receptors, each one carried by an individual lymphocyte. Somatic hypermutation, an accelerated random mutation of antibody genes, adds even more new variants. When a lymphocyte meets its matching antigen it is activated, and because the shuffle is an irreversible change in its DNA, all of its offspring inherit the same receptor, including the memory B and T cells behind long-lived immunity.

Those lymphocytes come in two kinds: B cells, which secrete antibodies that bind a foreign antigen and inactivate it, and T cells, which carry out a cell-mediated response. The human body holds about 2 trillion of them, roughly as much mass as the brain or liver, yet only 2% are in the bloodstream at any moment; the rest move through tissues and the lymphatic system, and about 1 to 2% of the whole pool recirculates every hour, raising the odds that each cell meets the pathogen it reacts to.

The first time, all this takes 4 to 7 days to mount a significant response. Afterwards, memory makes the next response stronger, sometimes for life: someone who recovers from measles is protected against it for good, though not every infection works that way, as chickenpox shows. That memory is the basis of vaccination. Even the system's name has a human story: the immunologist Robert Good first called it "adaptive" in 1964, while describing antibody responses in frogs, and explained only that he preferred the word.