Was looking for this answer. The adaptive immune system, particularly antibody mediated immunity is almost like very short term evolution that happens in the span of weeks inside of an organism. Antibodies are produced in B-cells virtually at-random through a process called VDJ recombination, in which the immune cells randomly combine various genes to produce antibodies with different paratopes (the regions of antibodies that recognize pathogens). When a B-cell receptor (which is essentially a membrane-bound antibody) recognizes a pathogen, that B-cell is then signaled to proliferate and mature into a plasma cell. These new progeny of the B-cells that recognized the pathogen undergo yet another “evolution” in which the antibody encoding region undergoes what is called somatic hypermutation, again a random process that either makes the antibody more or less specific, and in the latter former case gives feedback to the highly specific plasma cells to further proliferate and keep flooding the body with antibodies until no more pathogen is found to continue the process. This of course is only one part of it, there is also isotype switching, in which the specificity of the antibody remains the same, but the class of antibody changes, which allows it to interact with different arms of the immune system to eliminate pathogens through different channels. There is also T-cell immunity which I believe happens through somewhat similar mutation methods, except that antibodies are not directly involved. But I know much less about that side, so maybe someone else can talk about that.

Edit: a word