Studies on immune responses in afferent and efferent lymph of lymph nodes draining the tissues have contributed to the concept that primary immune responses are generated in the lymph nodes with a delay before antigen-specific effector cells arrive at the affected tissue.
Lightning strikes the ground approximately 25 million times each year in the U.S. According to the NWS, the chance of an individual in the U.S. being killed or injured during a given year is one in 240,000. Assuming an average life-span of 80 years, a person's odds over their lifetime becomes one in 3000.
At any moment, there are as many as 1,800 thunderstorms in progress somewhere on earth and each is producing deadly lightning. Lightning detection systems in the U.S. see an average of 25 million strokes of lightning each year, from some 100,000 storms. It is estimated that the earth is struck by 100 lightning bolts every second. The clouds at the high levels of the thunderstorm are made of ice crystals.