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I just remembered another biological analogy by Steve Burbeck. The bloodstream is responsible for both nourishing and cleansing. Helper T-cells in particular are responsible for sending out alerts when they identifies things they consider harmful. This triggers other T-cells to arrive and take care of the problem. Occasionally, helper T-cells malfunction, but that's also a problem that the other T-cells will take care of in a similar fashion.
The analogy to blogs is left as an exercise to the reader.