How Your Body Keeps You From Getting Sick

The immune system is critical for protecting against illness-causing organisms, such as viruses, bacteria and fungi, which are collectively known as pathogens. Without it, we would quickly become infected, sicken, and die.

Immune cells are constantly on the lookout for pathogens, patrolling the blood by moving around in the blood. Around half your blood volume is fluid known as plasma; the remainder is made up of millions of different types of cells.

The majority of cells are red blood cells that transport oxygen around. A minority are white blood cells, or immune cells that act as the body's army. The immune system produces many different types of these white blood cells, each with a specific role.

The Factory

All blood cells start their life in the bone marrow, which is the hollow part of the bones; every cell type originally comes from blood stem cells. While blood stem cells produce all of our blood cells, other types of stem cells in the bone marrow produce organs, the bone itself and so on.

Stem cells are able to replace themselves, but must also constantly produce new cells to replace old ones. Red blood cells are replaced after several months, while different immune cell lifespans range from days to years.

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