How Does the Immune System Work and What Are Its Functions?

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The human immune system is comprised of special proteins, organs, cells and tissues. It is responsible for protecting the human body against microorganisms and germs. This body system is a complex network that communicates to fight foreign invaders and protect humans from getting sick. It also plays a role in healing humans from illness and injury. So, how does the immune system work?

T and B Cells

Most of the cells in the immune system are white blood cells. Lymphocytes are a major type and are majorly classified as B cells and T cells. B cells’ primary role is producing antibodies. An antibody is responsible for binding to an antigen and then marking it as needing to be destroyed by other cells in the immune system. T cells play a critical role in coordinating immune response and helping to destroy infected cells. Neutrophils and macrophages are other types of white blood cells.

Neutrophils and Macrophages

These are two types of white blood cells that circulate throughout the blood and analyze and search the body for foreign invaders. When they find bacteria, or another foreign antigen, they swallow them up and destroy them. They destroy them by creating reactive oxygen intermediate molecules and other toxic molecules.

MHC and Co-Stimulatory Molecules

All cell surfaces have MHC molecules. These molecules play a major role in the body’s team of defenses. For example, when bacteria infects a cell, this molecule will then bind to an antigen and display it on the surface of the cell. After displayed, a matching T-cell receptor recognizes it. Communication then begins. Once the T-cell receptor and MHC interact, as well as the co-stimulatory molecules, several things can happen, including T-cell tolerance, activation or death.

Cytokines and Chemokines

The secretion of chemokines and cytokines is one way co-stimulatory molecules interact and a way for T-cells to respond after interacting with a T-cell receptor or MHC. Cytokines are proteins. They may cause the growth, activation or death of surrounding immune system cells. Chemokines are small cytokine molecules. They attract immune system cells.


In answering, how does the immune system work, it is important to learn about antibodies. B cells play a role in eliminating foreign antigens, but it needs the help of T cell signals to do this. Through communication and working together with a cytokine, an antibody is produced to target the invader.

When It All Comes Together

When all of the above are working together, the immune system is working. An antigen enters the body and is then detected by the various immune cells. They respond and produce antibodies after triggering B lymphocytes. These antigens are then locked onto by specialized proteins. Once an antigen is produced, it remains in the body. This builds immunity. For example, once a person has battled chickenpox, he or she will maintain the antibodies that were used to fight it and this prevents the person from developing chicken pox again, in most cases.

This is also how immunizations and vaccines work. Once a person receives it, it triggers the body to produce the necessary antibodies to prevent that particular illness.


KidsHealth from Nemours. (2011). Immune System. Retrieved on March 15, 2011 from KidsHealth from Nemours:

University of Hartford. (2001). Immune System. Retrieved on March 15, 2011 from the University of Hartford: