Fewer texts in the OT have been more important to the Christian church than Isaiah 7:14. To give a context let us go through a few ancient translations of Isaiah 7:14 in the Versions of the Old Testament.
Hebrew Masoretic of Isaiah 7:14
לָ֠כֵן יִתֵּ֨ן אֲדֹנָ֥י ה֛וּא לָכֶ֖ם אֹ֑ות הִנֵּ֣ה הָעַלְמָ֗ה הָרָה֙ וְיֹלֶ֣דֶת בֵּ֔ן וְקָרָ֥את שְׁמֹ֖ו עִמָּ֥נוּ אֵֽל׃
The Jews of the second century AD did not interpret העלמה as a virgin as St. Justin Martyr's dialogue with Trypho demonstrates. Therefore, the text probably should be read in a neutral way, the young woman will conceive and give birth to a son as intended by the surrounding context of that text without excluding a possibility that the young woman might be a virgin.
We can compare both MSS and DSS with LXX which differ from one another. There is no known manuscript or fragment of Isaiah 7:14 which use בתולה instead of העלמה. Perhaps the real issue should be whether or not the word בתולה, which always means virgin, was used in Isaiah for that purpose. The answer is, yes, it is used in Isaiah 62:5. LXX translated both indistinguishably with ἡ παρθένος, a virgin.
St. Jerome was the only Christian to argue from the Hebrew text, who concluded that the Hebrew העלמה should be read as virgo, if even in a periphrastic way. Jerome actually believed that the Hebrew העלמה meant abscondita “hidden.” Therefore the girl in Isaiah 7:14 was more than a virgin. She was a cloistered girl, which necessitates virginity.
Greek Septuagint of Isaiah 7:14
διὰ τοῦτο δώσει κύριος αὐτὸς ὑμῖν σημεῖον ἰδοὺ ἡ παρθένος ἐν γαστρὶ ἕξει καὶ τέξεται υἱόν καὶ καλέσεις τὸ ὄνομα αὐτοῦ Εμμανουηλ.
Its reading is found in Matthew 1:21 and it has become the proof text of the virgin birth of Christ for the Church among the Church Fathers. But to translate a Hebrew word העלמה the Greek Septuagint calls Dinah as ἡ παρθένος in Genesis 34:3 after being raped in a verse earlier.
The readings of Aquila and Theodotion, two of the Three Jewish revisers of the LXX in the first and second centuries clearly remove the idea of chaste woman from the text. Of course a young woman might incidentally be a virgin, but their usage of ἡ νεᾶνις renders the Hebrew העלמה and implies that the conception and the birth of the son will happen in the natural way.
How Christians explain that Isaiah 7:14 refers to a virgin? Can such passage be read textually as referring to a young cloistered maiden while maintaining that it necessitates virginity as suggested by Jerome because it's a sign to support a Christian doctrine of the virgin birth of Christ?1
1 I appreciate Justin’s argument from the Greek word σημειον in Dialogue 84. In essence he argues that it would not be a divine sign if the woman would give birth in the natural way. The sign is precisely that because the Messiah would be born in a supernatural way by a virgin. But Jews argue that the sign isn't alluding to a virgin birth.