The brain is criss-crossed by a network of small blood vessels. Through these blood vessels, the so-called capillaries, the brain is supplied with nutrients, oxygen and other substances. The walls of these blood vessels form the blood-brain barrier. Therefore, the common belief that the blood-brain barrier is a "shell" covering the brain is a misconception.

The vessel walls of the capillaries, which form the blood-brain barrier, demonstrate a characteristic, distinguishing them from the other blood vessels in the body. Between adjacent cells of vessel walls (so-called capillary endothelial cells) exist solid connections. They are formed by special protein complexes known as “tight junctions”. These junctions prevent the passage of substances from cell to cell. Substances that have to pass from the blood to the brain or from the brain to the blood, cannot pass the cells, but have to be fed through the vascular wall cells via special transport systems. This hereby controlled process allows a selective transfer of substances between nerve cells and blood and protects the nerve cells from penetration of harmful substances.

If the permeability of capillary walls is increased, it is theoretically possible for unwanted substances to transgress, which could lead to potential harm of the nerve cells in this area. In this context, one should not forget that even under physiological conditions (meaning within the scope of everyday life) the permeability of the blood-brain barrier is subject to temporary fluctuations.

Scientifically undisputed is the fact that the permeability of the blood-brain barrier increases with effective temperature increases in the brain. This happens during fever, but also with temperature increases due to high frequency electromagnetic fields (HF-EMF) with intensities significantly above the legal limit values. Whether the blood-brain barrier can be damaged also below this limit values through high frequency electromagnetic fields of the mobile telephone system is subject to research.