« Understanding Primary Immunodeficiency (PI)

The Immune System

This network of organs, tissues, cells, and proteins fights against infection to keep your body strong and healthy.

Too many foreign invaders—germs or pathogens—can cause infections and make you sick.

Major Organs

The major organs and tissues of the immune system are the thymus, liver, bone marrow, tonsils, lymph nodes, spleen, and blood.

x Picture of the tonsils, one of the immune system’s first defenses


Tonsils are a pair of tissues rich in lymphocytes (immune system cells). And together they form one of the immune system’s first defense against infections.

x Picture of the thymus


The thymus controls the development of T lymphocytes (immune system cells). T lymphocytes are essential for protection against infections.

x Picture of lymph nodes

Lymph Nodes

You'll find them in several places, including your neck, armpits, and groin. Rich in B and T lymphocytes (immune system cells), lymph nodes contain lymphocytes that kill germs and pathogens that can make you sick.

x Picture of the spleen


The spleen contains white blood cells (immune system cells) that fight infection and disease. The spleen also functions as a filter, and destroys old and damaged blood cells.

x Picture of the liver


The liver is important in immune function. As blood passes through the liver, phagocytes (immune system cells) ingest bacteria, dead or dying cells, and harmful foreign particles.

x Picture of bone marrow

Bone Marrow

Bone marrow is the soft tissue located in the hollow centers of most bones. Healthy bone marrow contains stem cells that continuously produce red blood cells, white blood cells and platelets.

x Picture of blood cells


The circulatory system that carries cells and proteins of the immune system from one part of the body to another.

Learn more about blood cells »

Finding answers to FAQs

Discover answers to some of your most common questions about PI.

Read the FAQs