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The offspring of two types of B cell clones (red and yellow) are present in several germinal centers (numbered) in a human lymph node (blue).
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Germinal centers (GCs)—the sites of antibody development—are not exclusive niches for rookie B cells. They are also open to experienced old-timers, according to Bende et al. (page 2655).
GCs are temporary lymph node structures that form during an immune response to help newly activated B cells better recognize an antigen. As they proliferate, the B cells' antibody-encoding DNA is mutated. The environment of the GC somehow helps select the best new clones, which then differentiate into either memory B cells or cells that secrete high-affinity antibodies.
This transition route from low to high affinity within GCs was thought to be used exclusively by B cells that encounter antigen for the first time. But Bende and colleagues now find that the GCs are hospitable to returning memory B cells as well.
The group dissected GCs out of human lymph nodes and sequenced the antibody-encoding RNA of the resident B cells. A single type of B cell clone was present in several GCs, suggesting that B cells traffic between GCs. Several GCs also contained the offspring of memory B cells that had undergone further mutation. The memory cells might have returned to the GCs for additional improvements upon a second encounter with an antigen.
GCs are known cancer hotspots, as the mutating B cells are vulnerable to chromosomal translocations and other cancer-causing events. These high cancer rates are probably due in part to the repeated visits from memory B cells during recall responses. 
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