Germinal center

Inside a lymph node there are small round clusters of cells called follicles. When nothing is going on, the follicle sits quiet and looks uniform. When an antigen shows up, meaning any fragment of something foreign, such as a piece of a virus or a bacterium, that follicle switches on and a new zone sprouts inside it: the germinal center.

Under the microscope the germinal center looks split in two. One half stains dark and is packed with centroblasts, which are B lymphocytes multiplying at full speed; the other looks light and holds centrocytes. In there the cells are dividing at a brutal pace, roughly one division every six hours.

And while they divide, something happens that anywhere else in the body would be a disaster: the genes that build the antibody pile up copying errors that nobody corrects. That has a name, somatic hypermutation, and it happens precisely while the cells are multiplying. The other adjustment, class switching, takes place in the same spot, and it is what lets the lymphocyte make the different types of antibody and not only the first one it started with.

Two kinds of cells come out of this. Some become plasma cells and spend their time making antibodies; others stay on as memory cells, keeping a file on the visitor in case it comes back.