The dogma of the Immaculate Conception, as solemnly defined by Pope Pius IX in 1854, teaches that the most Blessed Virgin Mary, in the first instant of her conception, by a singular grace and privilege granted by Almighty God, in view of the merits of Jesus Christ, the Savior of the human race, was preserved free from all stain of original sin.
This means that Mary, through the merits of her Son and Savior, Jesus Christ, received a special grace so that she might become the spiritual mother of all who come to believe in her divine Son cf. Gen. 3:20; Jn. 19:26–27; Rev. 12:17). Mary’s Immaculate Conception should be seen as the way God wanted all of us to come into the world: in the state of sanctifying grace and free from original sin, just like Adam and Eve.
Scripture, not coincidentally, first teaches the doctrine of Mary’s Immaculate Conception in the Book of Genesis. Just after the sin of our first parents, God promised to send a Savior. Speaking to the serpent, God said, “I will put enmity between you and the woman, and between your seed and her seed; he shall bruise your head, and you shall bruise his heel” (Gen. 3:15). The serpent is Satan (cf. Jn. 8:44; Rev. 12:9), and the “seed of the woman” who would be sent to crush the devil is Jesus Christ. Therefore, the “woman” is Mary, His mother. It is significant that Jesus addresses His mother in the Gospels as “woman” (e.g., Jn. 2:4; 19:26–27). Mary shares in the victory of her Son over Satan, which includes His victory over sin and death. Because she is sinless and pure, there is indeed “enmity” (Gen. 3:15) or “complete opposition” between Mary and Satan.
At the Annunciation, Saint Gabriel the Archangel greets Mary with the words, “Hail, full of grace, the Lord is with you” (Lk. 1:28). The phrase “full of grace” is a translation of the Greek word kecharitōménē. This word conveys a sense of completion and perfection that was already present at the time of the Annunciation. Mary’s holiness was not only as complete as possible, but it extended over the whole of her life, from conception onward.
~ Taken from Faith facts: Answers to Catholic Questions, Vol 1.