Random Analogies

"Each T lymphocyte, or T cell, is like a specially trained detective. The T cell examines the evidence that is exposed by the APC. When specific T cells come into contact with the ragweed pollen fragment on the APC and recognize it as foreign, an army of specialized T cells called "helper" cells (actually TH2 cells) is activated, thus releasing chemicals (cytokines) that stimulate B lymphocytes. B lymphocytes produce IgE antibodies that bind to the allergens (such as the pollen fragment). "

"Lithotripsy is like getting jabbed in the back with an unsharpened pencil once per second for an hour. You'll be bruised and a little sore. You also might have fragments that didn't get fully fractured that can pass, but will take time (and pain) to do so."

...-3,-2,-1,0,1,2,3...

An example of symmetry.