▁compared ▁to ▁a ▁few ▁million ▁white ▁blood ▁cells ▁and ▁a ▁billion ▁red ▁blood ▁cells . ▁Therefore , ▁the ▁major ▁challenge ▁for ▁C TC ▁research ers ▁is ▁the ▁prev ail ing ▁difficulty ▁of ▁C TC ▁pur ification ▁that ▁allows ▁the ▁mole cular ▁character ization ▁of ▁C TC s . ▁Several ▁methods ▁have ▁been ▁developed ▁to ▁isol ate ▁C TC s ▁in ▁the ▁peri pher al ▁blood ▁and ▁essentially ▁fall ▁into ▁two ▁categories : ▁bi ological ▁methods ▁and ▁physical ▁methods , ▁as ▁well ▁as ▁hy brid ▁methods ▁that ▁combine ▁both ▁strateg ies . ▁Te chni ques ▁may ▁also ▁be ▁class ified ▁based ▁on ▁whether ▁they ▁select ▁C TC s ▁for ▁isol ation ▁( pos itive ▁selection ) ▁or ▁whether ▁they ▁exclude ▁all ▁blood ▁cells ▁( negative ▁selection ). ▁ ▁Bi ological ▁methods ▁Bi ological ▁methods ▁isol ate ▁cells ▁based ▁on ▁highly ▁specific ▁ant igen ▁binding , ▁most ▁commonly ▁by ▁mon oc l onal ▁ant ib od ies ▁for ▁positive ▁selection . ▁Ant ib od ies ▁against ▁tum or ▁specific ▁bi om ark ers ▁including ▁Ep C AM , ▁H ER 2 ▁and ▁P SA ▁have ▁been ▁used . ▁The ▁most ▁common ▁technique ▁is ▁magnetic ▁nan op article - based ▁separation ▁( imm un om agnet ic ▁ass ay ) ▁as ▁used ▁in ▁Cell Search ▁or ▁MAC S . ▁Other ▁techniques ▁under ▁research ▁include ▁micro fl uid ic ▁separation ▁and ▁combination ▁of ▁imm un om agnet ic ▁ass ay ▁and ▁micro fl uid ic ▁separation . ▁As ▁the ▁development ▁of ▁micro fab ric ation ▁technology ,