**5. The immune system in GCA**

Although evidence regarding the genetic background of GCA and the possible influence of external factors, such as viruses and bacteria, have not elucidated disease pathogenesis, it is now well understood that the immune system plays a central role in the disease process. GCA is a complex systematic disorder and it is believed to represent the result of the breakdown of immunologic tolerance, resulting from interactions between the immune system and poorly defined components of the arterial wall.

A single triggering factor, initiating the inflammatory process, has not been yet identified. The initial insult may lead to a foreign-body giant cell attack on calcified internal elastic membrane in arteries and calcified atrophic parts of the medium layer of the aorta. [21] The prerequisite for a calcified artery explains why GCA almost exclusively occurs in older people.

Recent studies have raised the possibility that, in GCA, both the innate and the adaptive arms of the immune system are activated and may lead to vessel wall injury through, at least two, distinct pathophysiological mechanisms. [22]
