“The T-lymphocytes, even though they were reactive against that very virus, were not able to kill virus-infected cells from another strain of mice.” In other words, the immune system was able to discern a cell that was self and had been infected from a cell that was not self. The immune system killed only the infected ones that were self. An individual’s elegant defense didn’t care simply about the infection; it cared about the infection when it attacked its own personal habitat. Italicized because it’s a key scientific insight.
