What is the Purpose of Our Immune System (and What Does It Do)?

A Definition of the Immune System

The immune system is a complex system of cells, tissues, and organs whose purpose is to protect the body against infection. The National Institute of Allergy and Infectious Diseases reports that scientists continue to investigate exactly how the immune system attacks invaders like microbes, infected cells, and tumors yet leaves healthy tissues alone. Doctors and scientists seek to learn more about how the immune system protects us from disease so that they can work toward developing new plans for preventing and treating infectious and immune-related diseases. To date, scientists have learned a great deal about the immune system, including its relationship to stress, cancer, and autism.

Scientists also have found that one purpose of the immune system may help to encourage the growth of good bacteria in the gut, which “helps fight off harmful invaders, particularly in our intestinal tract.” Dr. June Round, senior study author and assistant professor of pathology at the University of Utah School of Medicine said her work “highlights that the immune system shapes the composition of bacterial communities in the intestine. This interaction is important because it’s becoming more…clear that resident microbes are very important for our health.” While Round’s study focused on mice, a human study found that gut microbiota interacts with both the innate and immune systems.

Purpose of the Immune System: How it Works

What's the purpose of the immune system? The immune system is divided into two parts: the innate system, or the defenses you are born with, and immunity, which develops as you grow.  The innate system is in various places in your body. Skin is the body’s first line of defense, as it prevents pathogens from entering the body. The lining of the mouth and nose includes mucous membranes that produce the mucus that traps bacteria and other pathogens, gastric acid produced by the stomach helps to kill off bacteria in food, and saliva washes pathogens off the teeth to reduce the bacteria and pathogens in the mouth.

Immunity takes over when the bacteria and pathogens make their way through the innate system’s first line of defense. The immune system’s line of defenses are in the blood, either as white blood cells or as chemicals released by the cells and tissues. The specialized different types of white blood cells in the bloodstream are called neutrophils, lymphocytes, eosinophils, monocytes, and basophils. These white blood cells move throughout the bloodstream reacting to various types of infection caused by the invading bacteria, viruses, or pathogens.

Lymphocytes, B Cells, and T Cells

Lymphocytes are especially important to the immune system, as they perform a variety of tasks to help protect the body. Lymphocytes attack viruses and other pathogens, but they also produce antibodies that destroy bacteria. Lymphocytes are divided into two types of cells: B cells and T cells. Both B cells and T cells develop immunity to certain types of bacteria and viruses. Bone marrow contains stem cells that create both B and T cells, but B cells mature in the bone marrow while T cells mature in the thymus. Another difference between B cells and T cells is the way in which they work in the immune system.

B cells produce antibodies, a special type of protein that attacks antigens. Antigens are chemicals in molecules that antibodies respond to because they are foreign. B cells have the ability to recognize millions of different antigens, even those that had never before entered the body and man-made molecules. B cells bind to antigens and then transform into plasma cells that make antibodies to fight specific antigens. Other B cells don’t transform into plasma; rather, they become memory B cells that respond even more aggressively when the same antigen invades the body again.

T-cells and the immune system's purposeT cells also directly attack invaders, though they cannot recognize antigens without the aid of other cells. Some T cells bind to antigens, but they require a second signal to activate. As the T cells become activated, they grow and begin to divide. The divided T cells attack the pathogens and release chemicals to destroy them. Like B cells, other T cells form memory cells, allowing the body to respond more quickly when the same antigens make it past the innate system.

How the Immune System Distinguishes Between Invaders and the Body’s Own Cells

The lymphatic system also plays a part on the body’s defense against infection, and lymph nodes are an important part of the system. Lymph nodes, found in lymph vessels, contain B and T cells that defend against pathogens that enter the lymph through the bloodstream. The thymus, a lymphatic organ, is another important part of the lymph system. The thymus is responsible for instructing white blood cells how to recognize the body’s own cells so that they can distinguish between invading pathogens and the body’s cells. Another part of the lymphatic system, the spleen helps filter blood and contains white pulp. White pulp is specialized tissue that contains white blood cells that attack pathogens as those in the lymph nodes do.

Images via Flickr by NIAID and NASA’s Marshall Space Flight Center