Lymphocytes help fight back. The body
has two kinds of these specialized blood cells.
B-lymphocytes produce
antibodies
. Antibodies
destroy the antigens and prevent them from
attacking healthy cells. Meanwhile, T-lymphocytes
invade cells that have already been infected. A few
T-lymphocytes stay around even after the germs
are gone. If the same kind of germ invades again,
the T-lymphocytes remember it. They fight the
infection before the person even feels sick. This is
known as immunity.
Vaccines imitate infections. Antigens in the
vaccine cause the white blood cells to respond
just like they would to an infection. T-lymphocytes
are left behind to fight the same germs in the
future. After a few doses of a vaccine, most people
are immune to the disease.
6
