Immunity, Illness, and Infection

What is the immune system?

The immune system is the body’s defense against infection. A network of special cells, tissues, and organs work together to protect the body from a variety of “invaders” or germs. These germs or pathogens include bacteria, parasites, viruses, and fungi. In most cases, the body can defend itself from harmful attacks. Some people have weak immune systems and cannot fight off germs as well.

The first line of defense in the immune system is a shield that prevents germs from entering the body. The skin is the body’s main armor that acts as a physical barrier against attack. The linings of the respiratory and digestive tracts (mucous membranes) also stop the entry of harmful pathogens.

In this illustration neutrophil cells sense bacteria, squeeze through the blood vessel to the site of infection, and destroy the marked bacteria. — White blood cells, called neutrophils, patrol the body and travel through the blood and lymphatic system to look for and destroy germs or pathogens that cause disease.

What happens if a pathogen invader gets past the shield? The body responds with its next line of defense. Special cells of the immune system patrol the body and travel through the blood and lymphatic system to look for and destroy pathogens.

A foreign invader that causes a response in the immune system is called an antigen. Some immune cells act to attack any invading pathogen. Other cells are trained to recognize and remember specific pathogens. The immune system produces antibodies that lock onto specific antigens so that they can be destroyed. This is how immunizations or vaccines work to protect against certain diseases.

Prevent
Patrol
Alert
Destroy

Immune system resources

The Immune Platoon

Cancer patient wearing a face mask — Some patients may need to wear special masks to filter germs or dust and spores from the air.

Cancer and the immune system

Cancer and cancer treatments can weaken the immune system. This means that a child with cancer is often at higher risk for infection and illness. Someone with a weak immune system is said to be immunocompromised.

How cancer affects immunity

There are several ways that cancer can lower immunity:

The cancer or cancer treatment can reduce the number of immune cells that are available to fight infection.
Cancer treatments including radiation and certain medicines may weaken the skin or membranes lining the mouth and digestive tract.
Medical procedures and devices such as catheters provide a place where germs could enter the body.

Infection during cancer

Infections can be very dangerous for cancer patients with weak immune systems for several reasons.

Symptoms of infection can be hard to recognize. Usual signs of infection include redness, swelling, pain, and fever. In patients with low immunity, the only symptom of infection may be a fever.
The body may not respond quickly to infection. This can make an illness last longer because the immune system does not work as well.
Sometimes, an infection may spread quickly because there are not enough white blood cells to fight infection. People with weak immune systems need to seek medical care at any sign of infection.

White blood cells

White blood cells (leukocytes) are some of the most important cells of the immune system. White blood cells are made in bone marrow, and they travel through the lymphatic system throughout the body. Their main job is to fight infection and disease. A white blood cell count (WBC) is a test to measure the number of white blood cells in a patient’s blood. A low number of white blood cells puts a person at risk for infection.

This illustration shows a boy with organs of the lymphatic system labeled: Cervical nodes, lymph vessels, axillary nodes, inguinal nodes, spleen, thymus, and tonsils.. — The lymphatic system is a network of nodes, glands, and vessels that transports white blood cells through the body to fight infection.

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Reviewed: June 2018