Keratin and pus escape and a foreign body reaction results in a microabscess, similar to perforating folliculitis. The purulent material subsequently tracks within the presacral subcutaneous tissue, producing acute and chronic pilonidal abscesses, as well as potential laterally displaced, epithelialized tracts. The conversion from a microabscess to the burrowing infection defines pilonidal disease and is thought to be caused by the same forces that create the pits. It was once thought that every pilonidal lesion contained a nest of hair. This does not, however, diminish its importance in the pathophysiology of pilonidal disease.
Hair has three distinct roles.
The first role is played by the distended hair follicle. It is not shed like its normal counterparts but can remain in the follicle during its expansion. The hair contributes to local inflammation once the follicle ruptures and even perforates the posterior wall of the follicle, enhancing microabscess formation. Hairs from other parts of the body can also affect local disease. Free hairs act as secondary invaders, entering the follicle through its open mouth and contributing to the foreign body reaction.