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.
The incidence of individual lightning strikes in any particular place is highly variable, but lightning does have an underlying spatial distribution. Previously, the frequency of lightning over the entire surface of the Earth was estimated to be 100 times a second. In 1997 NASA and National Space Development Agency (NASDA) of Japan launched the first Lightning Imaging Sensor (LIS)-equipped satellite to detect and record lightning.