Basics of the Immune System

Contributor: Hannah Brooks. Lesson ID: 12696

Do you know there are wars going on inside you all the time? Not the "Should I eat that third piece of pie?" battles, but fights against germs, dust, and things that can hurt you and make you sick!


Life Science

learning style
personality style
Grade Level
Middle School (6-8), High School (9-12)
Lesson Type
Dig Deeper

Lesson Plan - Get It!


How do soccer players defend the goal from the opposing team? Did you know you have little tiny "goalies" in your body?

In sports, defenders work to protect the goal or the end zone.

These players may not be the offensive stars on the team, but they play an important role in helping the team succeed. Your immune system operates behind the scenes to keep you healthy and growing! Like a sports team, the immune system is made up of different components that depend on each other to be successful. There is no end goal in immunity, but there is a constant cycle to ensure that the individual is able to fight off pathogens.

human immune system

The immune system has three layers of defense that operate to protect you.

The first line of defense is the physical barriers to prevent entry into the body. This includes your skin, nails, hair, and chemicals in your stomach.

  • Have you ever wondered what purpose the hair in your nose serves?

Well, it prevents harmful materials from entering your body!

There are also special cells that line your nasal passages to protect against pathogen invaders. These cells have cilia, or microscopic hairs, that can move invaders towards body exits instead of towards internal organs and tissues.

nasal mucosa

If an invader makes it past all of these physical barriers, it is attacked by the inflammatory response, or the second layer of defense. In the inflammatory response, white blood cells and fluid are released to the area of infection. These special blood cells, called "phagocytes," begin attacking the pathogen invader by consuming the invading cell and breaking it down.


Have you ever had an infected splinter or cut? They turn red and can become swollen. This reaction is because of the buildup of fluid and blood cells from the inflammatory response!

Sometimes, pathogens are not destroyed by the second line of defense, and are attacked by the immune response, or third line of defense. The immune response relies on a variety of cells to identify and destroy pathogens. There are immune cells that respond to specific pathogen threats — these cells are called "lymphocytes."


Notice how the lymphocyte has basic cell organelles and a textured cell membrane. This cell membrane has receptors that allow the blood cell to identify and respond to pathogens. There are two types of lymphocytes that operate during the immune response: T-cells and B-cells. T-cells use receptors to identify pathogens based on the antigens found on the invader surface. Once the pathogen has been identified, B-cells can use antibodies to mark the pathogen for destruction. Antibodies are structures that are created by B-cells and are able to bind to pathogen invaders' antigens. Antibodies work as signals or markers the body uses to identify which cells to destroy.


These immune system cells are constantly at work in your body, even if you are totally unaware! They are identifying and attacking invaders each second of the day. When an invader enters the body and infects an area, it can make you sick.

Interested in learning more about your immune system? Check out The Immune System Explained I - Bacterial Infection (Kurzgesagt — In a Nutshell, below) for an overview! Discuss what you learned with a parent or teacher!


When you start to feel sick, your immune system is already working hard to help you heal. The three lines of defense limit the number of infections that an individual contracts, so it is important to keep all three functioning!

In the Got It? section, you will learn more about the complexity of the immune response.

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