How the Human Immune System Works

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  • 0:02 immune System
  • 0:41 Phagocytes
  • 1:18 Lymphocytes
  • 3:00 Immunizations
  • 4:04 Lesson Summary
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Lesson Transcript
Instructor: Rebecca Gillaspy

Dr. Gillaspy has taught health science at University of Phoenix and Ashford University and has a degree from Palmer College of Chiropractic.

Your immune system protects you against microorganisms that could make you sick. It is made up of an army of white blood cells that work together to recognize and destroy the bad guys. Learn how your immune system and immunizations work.

Immune System

The United States of America has a military force that protects our homeland from invaders. Your body has a similar force called your immune system, which is a collection of tissues and cells that protect against germs and other invading microorganisms. When the bad guys like bacteria, viruses, microbes, toxins and parasites try to get in, your immune system deploys its huge army of soldiers, or defending cells to keep you safe from infection and disease known as your white blood cells. Let's take a look at these soldiers and how they work together to defend your body.


I mentioned that your immune system is made up of a huge army, and I was not kidding. Your body makes millions of white blood cells every day in places like your bone marrow, which is the soft tissue inside your bones.

You have different types of white blood cells. Some called phagocytes are cells that eat up and destroy other cells. In fact, the prefix 'phago' means 'eating', and the suffix 'cytes' means 'cells'; so, these guys are literally eating cells, and if you are a bacteria or other invader, you do not want to be found by these guys, because you will be eaten alive.


Phagocytes are always on patrol going out on seek-and-destroy missions aimed at foreign invaders. These guys aren't really specific, and their orders are to shoot first and ask questions later. This makes them different from another type of white blood cell called lymphocytes. Lymphocytes are white blood cells that recognize and deactivate specific foreign substances. Whereas phagocytes go out and kill anything they don't recognize, lymphocytes are more like trained assassins. They learn about the bad guys the first time they see them and then remember them if they ever try to invade again.

Here's how lymphocytes work. Sometimes your body is invaded by a bacteria, virus, or other foreign substance that induces an immune response. We call this an antigen. It's detected, and your immune system is called into action. One of the first things to happen is specific lymphocytes called B lymphocytes, or simply B Cells, make antibodies. You want antibodies to form because they are specialized protein molecules that are designed to lock onto and disable specific antigens. Antibodies are good at their job and handily apprehended the bad guys (a.k.a. antigens). But destruction of the antigens requires a different type of white blood cell, which we call T lymphocytes, or T Cells. There are different subsets of T cells, but some known as the killer T cells destroy the antigens that have been marked by the antibodies.


This saves the day and saves you from getting infected, but the good news is not over yet. Not only is the antigen destroyed, but the antibodies that recognize that specific antigen continue to circulate around your body. So, if that same antigen ever tries to sneak inside your body again, your new antibodies will deny it access. This is why you only get some childhood sicknesses, like chickenpox, one time. Your body made antibodies when the virus first came in, and then it's ready to defend against a second attack.

This also explains how immunizations work. When you go to your doctor to get your immunizations, what you are getting is injections of weakened microorganisms that stimulate your immune system to produce antibodies. These shots don't contain enough of the antigen to make you sick, but they do allow the antibodies to grow inside of you. That way, if you ever come in contact with the actual antigen, your body will be ready and able to fight it off.

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