Summary: The macrophage is a cell of immune system and its ability to fight invaders can inspire us to build more effective defense techniques.

About Neutrophils

The neutrophil is also a phagocytic (eating) cell like the macrophage (as we discussed in the previous blog post). While macrophages reside in the tissues exposed to the outer world, neutrophils can go anywhere they are needed. They are transported by the blood in arteries and veins. In fact, the blood is a perfect vehicle as blood vessels do not leave a single cell out when they deliver food.

They exit the blood and head over to the battle scene in minutes after they are called. Like macrophages, they use strong chemicals to destroy their prey after taking them in. In addition, neutrophils are also allowed to pour those chemicals out and turn the surrounding tissues into a "toxic soup". On one hand, this creates a lethal environment for invaders. On the other hand, it causes a damage normal tissues. In fact, this collateral damage can explain why neutrophils have a short lifetime (i.e., about five days). In contrast, macrophages live for months.

Exiting Blood

Normally, neutrophils are flowing along with the blood in the blood vessels at a high speed. At any point, macrophages encountering with an invader can produce signal alarms to call neutrophils to the battle scene. To have neutrophils stop and exit the blood in order to be delivered to the battle scene, adhesion molecules are used.

An adhesion molecule called selectin is produced and positioned on the surface of endothelial cells (cells lying on the inner wall of the vessel), when they receive the alarm signal from the macrophage. This molecule adheres to another adhesion molecule, SLIG (selectin ligand) that is found on the surface of neutrophils. The interaction between these adhesion molecules causes the neutrophil to slow down and start to roll along the inner surface of the blood vessel (see the picture below).

Slowed neutrophil
(taken "How the immunity system works")

When the slowed down neutrophil receives the alarm signal, it starts to express an adhesion molecule, integrin, on its own surface. Once integrin appears on the surface of the neutrophil, it interacts with its adhesion partner, ICAM, which always is expressed on the surface of endothelial cells of the vessel. This interaction is strong enough to stop the neutrophil (see the picture below).

Stopped neutrophil
(taken "How the immunity system works")

Once the neutrophil is stopped, chemoattractants -- chemicals that attract needed molecules to the battle site -- make the neutrophil exit the endothelial cell and travel to the battle scene. Chemoattractants include C5a -- a complement system protein, which you might remember from this post -- and formyl methionine (f-met) which is a fragment of bacterial proteins. These chemoattractants help the neutrophil find the invader (see the picture below).

Exiting blood to follow the "scent" of f-met and C5a
(taken from "How the immunity system works")

Lessons for Cybersecurity

A vehicle like blood would be a perfect match for sharing the cyber intelligence. Even though there are already some protocols for sharing it, they are not as effective and fast as the blood. When we get to lymph nodes, you will see another inspiring system for cyber intelligence sharing.