Skin cyst. A skin cyst is a fluid-filled lump just underneath the skin. It's common and harmless, and may disappear ...SkiptomaincontentHomeHealthAtoZBacktoHealthAtoZSkincystAskincystisafluid-filledlumpjustunderneaththeskin.It'scommonandharmless,andmaydisappearwithouttreatment.Itcanbedifficulttotellwhetheralumpisacystorsomethingelsethatmightneedtreatment.YoushouldthereforeseeaGPifyouhaveanysortoflumpsoitcanbeproperlydiagnosed.Cystsaresometimesconfusedwith boilsorskinabscesses.Boilsandabscessesarepainfulcollectionsofpusthatarecausedbybacterialinfections.Acystmaygoontobecomeaboilorabscess.WhatacystlookslikeAskincystisaround,dome-shapedlump.It'syelloworwhite,oftenwithasmalldarkplugthroughwhichyoumightbeabletosqueezeoutpus.Cystscanrangeinsizefromsmallerthanapeatoafewcentimetresacross.Theygrowslowly.Skincystsdonotusuallyhurt,butcanbecometender,soreandrediftheybecomeinfected.Foul-smellingpuscomingoutofthecystisanothersignofinfection.TypesofskincystEpidermoidcysts(oneofthemaintypes)arecommonlyfoundontheface, neck, chest,shouldersorskinaroundthe genitals.Theyaffectyoungandmiddle-agedadults,andareparticularlycommoninpeoplewithacne.Theydonotusuallyruninfamilies.Cyststhatformaroundhairfolliclesareknownaspilarcysts.They'reoftenfoundonthescalp.Pilarcyststypically affectmiddle-agedadults,mostlywomen.Unlikeepidermoidcysts,theyruninfamilies.Acystthatformsontheeyelidiscalledachalazionormeibomiancyst.Whydocystsform?Someofthecellsinthetoplayerofskinproducekeratin,aproteinthatgivesskinitsstrengthandflexibility.Normally,thesecellsmoveup tothesurfaceoftheskinastheystarttodiesotheycanbeshed.Butthecellssometimesmovedeeperintotheskinandmultiply,formingasac.Theysecretekeratinintothemiddleofthesac,whichformsa thick,yellowpaste.Thiscanoozeoutofthecystifit'sburst.Anyonecandevelopaskincyst,butyou'remorelikelytohaveoneifyou'vebeenthroughpuberty,youhaveahistoryofacne,oryou'veinjuredtheskin (forexample,ifyou'vedamagedahairfollicle).Skincystsarenotcontagious.WhatyoucandoifyouhaveaskincystCystsareusu