In the Metaphysics of Morals Kant states that conscience, moral feeling, love of human beings, and respect for oneself are "moral endowments" (moralische Beschaffenheiten) that serve as the foundations for the possibility of moral obligation in general. These moral endowments are not the result of social interaction, but are subjective preconditions that allow the subject to feel the force of the moral law, providing the receptivity to duty that is needed in order to be genuinely ethical. Kant surmises that these endowments arise as a direct result of the mind's awareness of the moral law--as a state arising from the effect the supremacy of the moral law has upon human consciousness--without which we would be "morally dead." Of these four moral endowments, conscience stands out as the most immediate expression of our moral heritage, compelling us to confront our self-image as rational free beings and take responsibility for our action. Moreover, unlike the other endowments, conscience is more than just a feeling, although it admittedly appears in this guise, for it adjudicates to what extent we have lived up to the moral law or fallen short of it. It is percisely this power of conscience to intervene, through either guilt or admonition, which places it in a position altogether different from that of the other moral endowments.
J. Howard, "Kant and the Moral Imputation: Conscience and the Riddle of the Given," American Catholic Philosophical Quarterly vol. 78 no. 4 (Fall 2004): 613-614.
UPDATE: I came across this interesting and related article on conscience (damîr) and Christian-Muslim Dialogue: Knowing by oneself, knowing with the other.