Reduced ability to produce reflex-evoked neurogenic inflammation, a sign of decreased defense against COVID-19 infection?

Background: A histamine skin prick test (SPT) generally evokes a wheal and a flare. The present study was initiated by an observation that histamine did not evoke a flare around a wheal in the skin of an 86-year-old man. Could that be of relevance to the findings that old men are prone to a more severe COVD-19 infection with a higher mortality than young ones?

Materials And Methods: Histamine SPT was performed on the forearm of six old men, all above the age of 80. The skin reactions were photographed from above and from the side. The photographs taken from above were treated in a computer with LYYN, a program that increases color differences. With the help of ImageJ (NIH), the size relation between flare and wheal was calculated. On the photographs, taken as side views, areas, heights, and diameters of wheals were measured. Controls consisted of three groups of younger people.

Results: Among the old men, no or only a small flare was seen. All the controls had prominent flares. Histamine SPT evoked small wheals in the group of old men as compared to young men.

Conclusion: Reduced neurogenic inflammation evoked by histamine from mast cells in blood and tissue may reduce the defense against COVID-19 infection.