What is The Immune System?

“Cells” are the basic building blocks of all animals, plants and microorganisms (bacteria, fungi etc.). Each organ in our body is made of cells, both the cells that define that organ (e.g. nerve cells in brain, muscle cells in heart) but also the cells that support organ function, like blood vessels and immune cells. The immune system protects us from infection, controls abnormal cells from growing and helps repair and rebuild damage that occurs in the body.

Our immune system is a complex network of organs, cells and molecules.  Our immune system is made up of physical barriers and several types of immune cell and organs. Physical barriers (e.g. the oily layer on top of our skin, the mucus lining of the nose, lungs, gut and stomach acid) are the most basic part of the immune system because they make it hard for bugs to get into our bodies.

The cells that contribute to the immune system are mostly derived from the bone marrow. Bone marrow is the soft material in the centre of large bones. It acts like a factory where “stem” cells produce an ongoing supply of blood cells:

  • red blood cells to carry oxygen
  • platelets to repair leaky blood vessels and
  • white blood cells.

White blood cells (or immune cells) fight infection and detect and eliminate abnormal cells from your body. There are many different types of white blood immune cells that have evolved to do different jobs.

For example some white blood immune cells are well adapted to fight bacterial infections (e.g. neutrophils) whereas other immune cells are better at fighting parasites like worms (e.g. eosinophils). There is more information about what these cells do in the answer to Question 2.

Immune cells are moved around the body in the bloodstream but also in a second network of vessels called the lymphatic system. The lymphatic system is a much more delicate set of pipes, but runs along the same pathways as blood vessels. Lymphatic vessels connect the immune system via a series of lymph glands (or nodes) and lymphatic organs.

If white blood immune cells are soldiers then these lymph nodes and organs are like army barracks. Lymph organs are shown in the diagram above – and include the spleen, tonsils, lymph nodes, but immune cells also congregate in special areas in different organs (e.g. in Peyer’s patches in the gut). Together, these locations make up the lymphatic system.

Immune cells constantly circulate through the body, coming to rest in these lymphatic organs where they can share information about infections and abnormal cells so that they are ready to fight an infection the next time they meet it. This is where cells of the immune system learn about vaccinations (e.g. diphtheria and tetanus) and the reason why you experience “swollen glands” in your neck when you suffer a cold.