Understanding the Immune System - How It Works

The immune system is made up of cells and organs that protect your body from outside invaders such as bacteria, viruses, fungi, and parasites (types of germs) that can cause infection and disease. The immune system also gets rid of abnormal pre-cancerous cells and cancerous cells that are growing out of control. When it works correctly, it fights off infection and keeps you healthy. However, when it does not work correctly, germs and other abnormal cells in the body can more easily cause disease.

Key Organs of the Immune System

The first line of defense against germs is your skin, the single largest organ of the body. It provides a physical barrier that keeps bacteria and viruses from entering the body. Viruses such as HIV cannot get through normal, healthy, unbroken skin. HIV can, however, get into the body through unbroken mucous membranes, which are the moist membranes of the vagina (birth canal), rectum ('butt'), and urethra (tube that brings urine out of the body).

The internal parts of your immune system take care of germs that do get inside the body. The white blood cells that defend the body from invaders and get rid of possibly dangerous abnormal cells begin their lives in the bone marrow. Once they leave the bone marrow, they travel to the lymph organs, which serve as a home base for mature white blood cells. There, the white blood cells await instruction to go out and fight infection.

Lymph organs are spread throughout the body and include the lymph nodes, thymus, spleen, appendix, tonsils and adenoids, and clumps of tissue in the small intestine known as Peyer's patches. Lymph nodes are located in the neck, armpits, abdomen, and groin. Each lymph node contains cells ready to fight invaders. The lymphatic vessels connect the lymph nodes and carry lymph, which is a clear fluid that "bathes" the body's tissues and helps to clean out invaders or germs.

The spleen is an important organ for a healthy immune system. It is about the size of a fist, and it is located in the upper left of the abdomen ("belly"). One of its key roles is to filter blood and to identify and get rid of worn-out white

Key Cells of the Immune System

Some key cells of the immune system are:

Dendritic cells and macrophages

T cells

B cells

Dendritic Cells and Macrophages

Dendritic cells are found mostly in the skin and mucous membranes that protect the openings of the body (e.g., nose, mouth, and throat). These cells capture and carry invaders to the lymph nodes or spleen. Macrophages (their name comes from Latin and means "big eaters") protect different organs, including the intestines, lungs, liver, and brain. Like dendritic cells, macrophages capture and carry invaders to the lymph organs.

These two types of white blood cells are known as scavengers. They engulf (eat) foreign invaders, break them apart, and display pieces of the germs—known as antigens (from antibody-generating) —on their surfaces. The body can then make antibodies to that specific germ, which helps the body get rid of that invader faster and remember it in the future. These cells also produce chemical messengers (known as cytokines) that instruct other immune cells to go into action.