In the Constitution Ineffabilis Deus of 8 December, 1854, Pope Pius IX declared that the Blessed Virgin Mary “in the first instance of her conception, by a singular privilege and grace granted by God, in view of the merits of Jesus Christ, the Savior of the human race, was preserved exempt from all stain of original sin.”
The doctrine of the Immaculate Conception affirms that Mary was born free of original sin. The teaching is not explicitly mentioned in the Bible, yet there is enough biblical evidence to show that the belief in the Immaculate Conception is perfectly biblical.
Luke 1, 26-29 — In the sixth month the angel Gabriel was sent from God to a city of Galilee named Nazareth, to a virgin betrothed to a man whose name was Joseph, of the house of David; and the virgin’s name was Mary. And he came to her and said, “Hail, full of grace, the LORD is with you!” But she was greatly troubled at the saying, and considered in her mind what sort of greeting this might be.
We must pay attention to the expression “full of grace” from the Greek κεχαριτωμένη, kecharitomene. From that expression we learn that Mary received from God all the necessary grace for her salvation. The source of this truth recorded in the Gospel of Luke, is an angel who is delivering a message directly from God. What does it mean to be full of grace? When measuring any other thing, such as a fluid, we say that a vessel is full of water because it cannot take any more water in, or because it has all the water needed to attain a certain level of completeness. In the same way, Our Blessed Mother was informed of her unique standing before God by the angel: she was given all of the divine grace necessary to achieve her mission as the New Eve. These graces were received by Mary in advance of the redemptory sacrifice of Christ firstly because, to fulfill her role as the New Eve she had to be of the seed of the original Eve, (Genesis 3, 1-20) and secondly, because that particular condition tied her to the chain of original sin (1 Corinthians 15, 22). In order to be the Bride of God the Holy Spirit and the Mother of God the Son, Mary had to be made a perfect daughter of God the Father. These graces are then given to Mary by God out of His desire to redeem mankind. In Mary we see fulfilled in advance, the perfection that obedient mankind will receive as a result of Jesus’ sacrifice on the Cross.
The expression κεχαριτωμένη complements and fulfill many prophetic utterances registered in Scripture. We find her in the Wisdom of Proverbs 8, Ecclesiasticus 24, and the mysterious canticle of the Song of Songs chapter 4, which says in verse 7: “You are all fair, my love; there is no flaw in you.” These texts have been used for centuries in liturgy and prayer, to praise the Mother of God but they alone do not constitute proof of her Immaculate Conception.
Here are some comments on the use of the term kecharitomene and the Immaculate Conception:
“…Luke 1:28 uses the perfect passive participle kecharitomene. The perfect stem of a Greek verb denotes ‘continuance of a completed action’; ‘completed action with permanent result is denoted by the perfect stem.’ On morphological grounds, therefore, it is correct to paraphrase kecharitomene as ‘completely, perfectly, enduringly endowed with grace.'” 
“Since Mary would not have been a worthy mother of God if she had ever sinned, we assert without qualification that Mary never committed a sinful act, fatal or non-fatal: ‘You are wholly beautiful, my love, and without blemish.’ Christ is the source of grace, author of it as God and instrument of it as man, and, since Mary was closest to Christ in giving him his human nature, she rightly received from Him fullness of grace: grace in such abundance as to bring her closest in grace to its Author, receiving into herself the One who was full of every grace, and, by giving birth to Him, bringing grace to all.” 
When Mary accepted to be the Bride of the Holy Spirit, she became the Ark of the New Covenant. Thus the ancient ark was a prophetic model, prefiguring Mary. The ark had a place in the Holy of Holies in the innermost sanctuary of the Tabernacle. This was a Most Holy Place, so sacred that no ordinary person could enter in it. The Holy of Holies was God’s special dwelling place in the midst of His people. A thick curtain, made of fine linen and blue, purple and scarlet yarn, embroidered with the figures of Cherubim, separated the Holy of Holies from the Holy Place. The Cherubim, are spirits who serve and praise God constantly, guarding the throne of Most High. That veil was the last limit, hiding the Holiness of God from the view of sinful men. Entering the Holy of Holies meant to enter into God’s very presence. Only the high priest could past through the veil. The High Priest, a type of Christ Himself, God’s chosen mediator with His people, could only enter this sacred space on Yom Kippur, the Day of Atonement. The Holy of Holies was there to teach Israelites the holiness of God. Something they could not fail to take very seriously (Habakkuk 1, 13). The veil was there to insure that no man could enter into God’s holy presence. Even when the high priest accessed the Holy of Holies, he had to clean himself ceremonially. He had to dress with special clothing, bring the incensory to let the smoke fill the room, thus covering his eyes from a direct view of God. Most importantly he had to bring the blood of sacrifices with him, to make atonement for his sins and for those of his people. The liturgy of the Day of Atonement was there to point at the necessity of a perfect priest, the one we now have in Christ, and to show the Israelites the awesome sanctity of God (Hebrews 9, 7; 9, 24-26; 6, 19-20; 10,19-22).
How much more sanctity, purity and innocence would be required from the woman in whom the very life of God was going to dwell for nine months! God could not have required that kind of holiness from the model and later, fail to require it from the very fulfillment! Such model could not be more holy than the reality it represented!
Indeed, the doctrine of the Immaculate Conception explains how God prepared a perfect vessel to receive the Son of God in the mystery of the Incarnation.
 A Greek Grammar of the New Testament and Other Early Christian Literature: A revision of F. Blass and A. Debrunner “Grammatik des neutestamentlichen Griechisch”. The University of Chicago Press, 1961.
 Refuting the Attack on Mary: A Defense of Marian Doctrine by Father Mateo, publ. Catholic Answers, El Cajon, California, 1993, 1999.
 Thomas Aquinas, Summa Theologica IIIa:27.4-5
 See the chapter Mary is the Ark of the New Covenant.