How Do Vaccines Work?

The Immune System

Analogies are powerful. A good analogy is useful for illustrating a point. But sometimes people overextend or read too much into analogies, often ending up far from the truth. When talking about the immune system, a lot of people use words like “weak” or “strong.” It is fine to use these descriptions that make the immune system sound like a muscle, but we should not take this analogy too far. A muscle atrophies if not used; the immune system does not. A muscle becomes exhausted by overuse; the immune system does not.

It is more useful, and more accurate, to compare the body’s immune system to a loyal, intelligent, and powerful watchdog that ejects intruders from a home. The intruders, chiefly viruses and bacteria, enter the body through the nose, mouth, or broken skin. A healthy immune system recognizes the intruders as they begin to multiply in the lungs, bowels, or blood and destroys them.

A sound immune system distinguishes what belongs (healthy cells) from what doesn’t (viruses and bacteria that invade the body from outside, or cancer cells that develop inside the body). A faulty immune system either allows invaders to wander unchecked through the body, as in the case of immunosuppressive diseases like AIDS, or attacks the body’s own healthy cells, as in autoimmune diseases like multiple sclerosis and scleroderma.

The body fights intruders by two means, active and passive immunity. Both forms of immunity are necessary.

Passive immunity

Passive immunity is protection by antibodies that a person receives from another human or animal. The most important form of passive immunity is the transfer of maternal antibodies during pregnancy. Some of the mother’s antibodies, which pass to the developing fetus, can remain active in the baby for about a year after birth to protect the child from diseases such as measles and chickenpox.

Physicians use another form of passive means of immunity when they provide susceptible patients with concentrated antibodies that have been collected from donated blood. A person with a weakened immune system who has been exposed to chickenpox virus might receive an injection of chickenpox antibodies. A traveler preparing to leave for a country where hepatitis A virus is common may receive an injection of hepatitis A antibodies. A person who is bitten by a snake may even receive antibodies to the snake venom concentrated from the plasma of horses.

Passive immunity gives only short-term protection. When the antibodies transferred in this way lose their potency, the immune system is vulnerable once again.

Active immunity

The active immune system is composed of two interacting and overlapping systems.

The nonspecific immune system consists of specialized cells, such as macrophages (literally “big eaters”), neutrophils, and natural killer cells as well as chemicals that send signals to cells. The nonspecific system acts within minutes or hours after viruses or bacteria infect the body. Its response to a second or third invasion of any particular germ is no greater or quicker than its response the first time.

The specific immune system , upon which vaccination depends, consists primarily of B lymphocytes, T lymphocytes, and antigen-presenting cells. The specific system takes several days and sometimes much longer to respond the first time a particular germ invades, but it responds more quickly and more powerfully the next time that germ invades. It does this by creating B and T memory cells , which stand ready for the next infection of the same antigen.

Some B lymphocytes become plasma cells, which in turn produce antibodies . These complex proteins circulate within the blood and lymph streams, attach to invading antigens, and mark them for destruction by other immune cells. Antibodies are quite specific: The antibodies for measles virus, for instance, will not interact with rubella virus (German measles), but they will interact with the measles vaccinevirus.

Vaccines and the active immune system

Vaccines are designed to work with the active immune system. When vaccines enter the body (most must be injected), the active immune system gears up for them, producing antibodies and memory cells. Because the vaccines resemble the corresponding germs (for example, the surface of the measles vaccine virus is very similar to the surface of the real measles virus), the system produces antibodies and memory cells that work against both the vaccine virus and, more importantly, against the actual virus. So when the real measles virus invades a vaccinated child, his immune system immediately identifies the virus and eliminates it before the child becomes ill.

This information is excerpted from the book Vaccinating Your Child: Questions and Answers for the Concerned Parent (Peachtree Publishers, Ltd., 2000). The book’s authors are Dr. Sharon G. Humiston, a pediatrician and clinical researcher at the CDC and the University of Rochester, and Cynthia Good, an award-winning journalist and host of the television show “Good for Parents”.