How Vaccines Work: An Immunological Primer

Vaccines prevent disease. They do so by causing you to develop immunity against an infectious organism before it gets a chance to infect you and cause disease. In essence, vaccines teach your immune system how to recognize infectious organisms and fight them off much faster. Without vaccination, exposure to an infectious organism is like being thrown into the deep end of a pool without water wings where your immune system thrashes around desperately trying to save you. But vaccines are like the shallow kiddie pool where you touch the bottom and your immune system gets acquainted with the water so that it will have a better idea how to handle it later on when you can't touch the bottom. Vaccines do this by exposing the immune system to dead, weakened, or pieces of infectious organisms, usually along with a signal molecule called an adjuvant, so that it can learn that the molecules associated with the infectious organism are bad and should be attacked if they're ever seen again. The adjuvant acts like a wake-up call to the immune system to make sure it takes a look and does what it's supposed to.

Vaccines are education for the immune system.

The human body has 2 different immune systems: the innate immune system and the adaptive immune system, and they talk to each other quite a bit. The innate immune system is the infantry of the immune system; it reacts rapidly to invading pathogens and can destroy most of them, but it can have a hard time telling exactly who to kill and when and how much firepower to use. Meanwhile, the adaptive immune system is more like a laser-guided cruise missile from a fighter jet that very specifically takes out invading pathogens and tags the enemies that the innate immune system should kill off. However, just like it takes time for jets to get to where they need to go, the adaptive immune system takes several days to warm up and start fighting.

The innate immune system is scattered throughout the body in circulating white blood cells, in the mucous membranes, in the skin, and in the gut; all places where invading pathogens are likely to try to enter. The adaptive immune system, when there's no infection present, generally hangs out in the bone marrow, thymus, and lymph nodes; but when there is an infection some of them will go out to the site of infection to join the battle while others will stay behind and direct subsequent reinforcements.

In an infection where there is no prior vaccination present, say S. aureus in a pimple, the innate immune system quickly (in minutes) recognizes many of the molecular patterns that are generally associated with all bacteria, including S. aureus. These patterns are signals for the innate immune system to kill and also go talk to the adaptive immune system. So the innate system goes to work blasting away as well as it can at the S. aureus, but it's good at hiding so the innate system doesn't see all of it and S. aureus is able to establish an infection.

So meanwhile, the innate immune system has gone off to the adaptive immune system and handed it the information it has gathered about the infection. The adaptive system examines it and then begins to multiply and specialize. Some of the multiplying cells get out to the site of infection and join the battle. Other cells make antibody against the S. aureus, which is then secreted into the blood and binds the bacteria, lighting them up brightly and saying "Destroy this now!" to the innate and adaptive system cells that are already there.

Once the infection is cleared, many cells of the adaptive immune system specialize again and become memory cells that wait out in the body, able to specifically recognize the same infection they just fought off. This means that if S. aureus tries to come back, the memory cells will see it very specifically and very quickly and they can then jump-start the rest of the adaptive immune system and mount a very rapid lethal attack against it.

The problem is that many infections are strong enough to kill, or do serious and permanent damage to, a human before the adaptive immune system is strong enough to fight it off. Although the innate immune system starts fighting quickly, it takes the adaptive immune system about 4 days to start responding very much and a full 7 days to really get into swing. Memory immune cells can bypass that waiting period and kick both immune systems into battle mode right away.

Vaccines are designed to skip the infection part and go directly to teaching the adaptive immune system to recognize the infectious organism and then form the memory immune cells that will kill the pathogen before it even has a chance to cause infection.

This is why vaccines are effective, useful, and necessary. They also happen to be quite safe*.



NOTE: This was largely inspired by Abel Pharmboy's guest post over at Dr. Isis' place. I left a long comment there and then realized that it wouldn't do very well for me to be a hypocrite. If you feel I've left anything out, or if there's anything you want to know more about please let me know. I tried to keep it simple because even the basics of how the immune system functions could keep me busy with posts for the rest of the year.

*Bears, however, are something you should worry about. Bears are crazy dangerous.