I am very fond of food and eat just about everything. Maybe that’s why I’m so fascinated by the immune system’s big eaters?

The immune system can be split up in various ways. An important distinction is between the cells that react immediately to an external danger and the cells that almost has an academic approach, which analyzes the situation, the pros and cons and spend several days before they go into action. The big eater, or macrophage, belong to the cells that react immediately.

The macrophages are found all around the body, in the bloodstream and in the tissues. They act as the body’s garbage workers and removes dead cells, cell debris and microbes. The macrophages The eating process is called phagocytosis.

Macrophage eating and digesting a bacterium shown in five steps. L = lysosome

But how does this eating actually occur? Macrophages are like most cells, mainly round and apparently symmetrical. There is no opening into the cell anywhere on the surface of the cell membrane. But attached to the cell membrane the macrophage has different proteins that function as recipient molecules or receptors.

When any of these proteins bind to bacteria, the cell membrane in this area soon form a large pocket that the bacterium is drawn into. Finally the bacterium is entirely enclosed by membrane on all sides, and the pocket is ligated so the connection with the surface is broken. Now the bacterium is located inside the macrophage, albeit encased in a layer of membrane.

The actual digestive process now requires that the bacterium is exposed to acid, bleach (peroxide) and enzymes so it is killed and digested. To this end, the bacterial bag is fused with a small cell body called the lysosome and which contains digestive enzymes. Thus the bacterium is killed and broken down into its building blocks, that is into fatty acids, sugars and amino acids.

Clearly, mcrophages and bacteria are enemies. Therefore, many bacteria have developed strategies to avoid the macrophages’ relentless garbage removal service. Some bacteria are resistant towards acid or destroy the bleach and are therefore not killed inside the macrophage. A well known bacterium which behave like this is the tuberculosis baccillus. This acid-resistant bacterium can live for many years in the body, encased within in macrophages.

Blog by Anne Spurkland, published 14th November 2015
Originally published in Norwegian, 16.9.2012