Your immune system faces a remarkable challenge: it must defend you against viruses and other pathogens it has never seen before. It solves this problem by learning. Instead of storing a fixed list of answers, it tries new approaches and remembers the ones that worked in the past. In this lecture, I will describe how the body generates an enormous repertoire of antibodies, each with a different molecular “shape,” and how infection or vaccination triggers a surprising process in which some immune cells deliberately mutate their genes to generate novel antibodies. These cells undergo cycles of mutation and competition within your lymph nodes (hence the swelling!), to generate improved variants. The result is better antibodies that bind more effectively to their targets. This is Darwinian evolution, but accelerated to the timescale of days and occurring inside your own body. I will discuss how biophysical models can help us understand this learning process, why it matters for vaccines, how failures of immune learning contribute to autoimmunity, and why immune learning might become less effective with age.