The part of the immune system being described is the germinal center reaction. The 1 million times faster mutation rate only occurs for B-cells dividing in these structures and it's caused by some kind of chemical signaling (the name has slipped my mind). Importantly, it only effects the mutation rate of the thing that codes for the antibody the B-cell expresses, but it has been observed that in this part the mutation is pretty much random.
On average, by the way, there is about 1 point mutation in the part coding for antibodies for every single division in the germinal center. But since the particular mutation is random, I'm not sure I like describing this as the B-cell editing its own genome. It makes it more mysterious than necessary.
I should probably turn this part of my PhD thesis into a set of blog posts once I've defended..."affinity maturation" is a weird and wonderful process.
On average, by the way, there is about 1 point mutation in the part coding for antibodies for every single division in the germinal center. But since the particular mutation is random, I'm not sure I like describing this as the B-cell editing its own genome. It makes it more mysterious than necessary.
I should probably turn this part of my PhD thesis into a set of blog posts once I've defended..."affinity maturation" is a weird and wonderful process.