Ten, if a study in this week's issue of Science is to be believed. The study introduces a new way of getting a global history of all the viruses a person's immune system has had the pleasure of knowing. The technique has some significant limitations, but it still has the potential to provide new perspectives on how the human immune system functions.
First, the technique, which its creators are calling "VirScan." It relies on the fact that, after a person's immune system mounts an attack against a pathogen, a small collection of B cells, called memory B cells, continue to produce antibodies that recognize the invader. These allow the immune system to mobilize rapidly if the same pathogen is ever encountered again. But the memory B cells also allow us to study the antibodies they produce.
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