The Immune System protects the body from worms, germs, and other agents of harm. The immune system has three lines of defense. Only pathogens that are able to get through all three lines of defense can harm the body.
The skin and nose are the first line of defense in the immune system. But, since the kid has a cut, it is immediately broken. This causes the body to inflame and try to close the cut with scabs.
As the bacteria enters the body through the cut, it starts to infect red blood cells. This alerts Interferons that “warn” neighboring cells. Later white blood cells are released to kill off the remaining bacteria.