Immune System: Function & Responses

Instructor: Kelly Robson

Kelly has taught High School Science and Applied Communications. She holds an Education Specialist Degree in Ed. Leadership.

The immune system is the body's defense against invaders that could do it harm. This lesson covers the different parts and their functions in protecting the body against infections.

Function of the Body's Defense System

The immune system is the body's defense mechanism that protects the body from intruders. It is made up of a network of cells, tissues, and organs. The word 'immunity' in this case means protection against disease. That is exactly what the immune system is supposed to give you.

There are many different types of invaders that can harm the body. Most of the invaders are little microbes, such as bacteria, parasites, fungus, and viruses. All of these can cause infections inside the body. The more technical name for the invaders - or at least the ones that cause the immune system to move into action - is antigens.

The immune system is extraordinary. It will attack these annoying little antigens by locking them into their sights, sending for back up, and chewing them up until they are gone. The immune system is so smart that it can recognize a 'self' cell and a 'non-self' cell. Basically, the immune system knows which cells belong to you ('self cells') and which ones do not ('non-self cells'). Antigens are the non-self cells that trigger the immune system into action. These antigens can be viruses, even a molecule of a virus, or tissues and cells from another person. This reaction is why a person may reject an organ transplant.

When all is working properly, the immune system leaves the healthy cells and tissues in the body alone. When the immune system is not working well, it may attack the self cells; however, when the immune system is not working properly, it can cause harm to the healthy parts, causing disorders such as allergic diseases, arthritis, and certain forms of diabetes. This malfunctioning is known as autoimmune disease.

Sometimes, the antigens are actually harmless. This is the case with stuff like ragweed or pollen. Both are harmless to the body; however, they may trigger the immune system into action. These types of harmless antigens are known as allergens.

Special Parts & Special Roles

The immune system is a huge communications network made up of cells, tissues, and organs that are placed all throughout the body. Such placement is beneficial because it cuts down on travel time. Let's look more closely at some of these special components and their roles in protecting the body.


There are millions of cells throughout the body that are just waiting to jump on these antigens and destroy them. These cells are called leukocytes, or white blood cells. Leukocytes are divided into two categories: phagocytes and lymphocytes.

A Single Human Lymphocyte Cell

Phagocytes are the cells that destroy the antigens. They are the ones that chew up the antigens until they are gone. The most common type of phagocytes is the neutrophil, which fights bacteria. A doctor can draw blood and, if there is a high number of neutriphil cells, determine that the body probably has a bacterial infection.

Lymphocytes are the immune system's intelligence gathering cells and front line of defense. There are two types of lymphocytes. B lymphocytes, or B cells, travel around the body and tag the antigens. They send signals to the T lymphocytes, or T cells. The T cells then begin to destroy the antigens the B cells identified. The T cells get back-up from the phagocytes, which finish off the antigens.

Organs & Vessels

The organs of the immune system are called lymphoid organs. This is because they are home to the lymphocytes, or white blood cells. Bone marrow is soft tissue that is in the hollow center of the bones. It is the ultimate source of all blood cells, including the lymphocytes. The spleen is a gathering place for all of the cells.

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