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
Scientists produce many kinds of vaccines Live vaccines contain a weakened form of a virus or bacteria The weakened germs cannot reproduce easily They will not make a person sick Scientists also make inactivated vaccines The germs in these vaccines are killed with heat or chemicals For this reason inactivated vaccines are easier to store and handle HOW A VACCINE WORKS Vaccine Germs Y Antibodies Y co Dead or weakened germs in the vaccine enter a person's body The body responds to the vaccine as it would to an infection Antibodies remain in the body to fight the same germs in the future
CHAPTER 2 PRO VACCINES PREVENT DEADLY DISEASES Vaccines prevent illnesses that are dangerous or even deadly For instance smallpox killed nearly 300 million people in the 1900s But through the years millions of people received the smallpox vaccine Fewer people got the disease By 1980 smallpox had disappeared Measles mumps tetanus and polio were once common diseases too But because of vaccines these illnesses are rare in most countries Some vaccines are for serious diseases while others are for common illnesses such as the flu
