The immaculate conception is the Roman Catholic doctrine that Mary herself is without sin, that she was born without sin. The Bible does not teach the immaculate conception, it actually refutes it.
(Luke 1:46-47)"And Mary said, My soul magnifies the Lord, and my spirit rejoices in God my Savior," (ESV)
These verses come from what is known as the Magnificat (Luke 1:46-55). Mary refers to God as my Savior, and only sinners need a Savior. If Mary was without sin she would have no need for a Savior.
The Roman Catholic Church holds to the immaculate conception and other false doctrine because of it's rejection of Sola Scriptura (Scripture alone). Nowhere does Scripture teach that Mary was without sin. Mary found salvation by God's grace alone, through faith alone, in Christ alone.