Albert Barnes Commentary 1 Samuel 22:20

"And one of the sons of Ahimelech, the son of Ahitub, named Abiathar, escaped, and fled after David." — 1 Samuel 22:20 (ASV)

Abiathar - He may have remained at Nob to take care of the sanctuary when the other priests went to Saul, and so escaped. He continued as David’s faithful friend throughout his reign (1 Samuel 23:9; 1 Samuel 30:7; 2 Samuel 15:24, 29, 35).

Later in his life, however, he gave offense by taking Adonijah’s side against Solomon (1 Kings 1:7, 19, 42) and, as a consequence, was deprived of the high priesthood (1 Kings 2:26–27).

In Mark 2:26, he is spoken of as the High Priest who gave the showbread to David. Perhaps he was the instigator of this act of kindness to David; for this reason, as well as his constancy to David, he is mentioned by our Lord instead of Ahimelech.

It is also possible that as sagan to his father, he may have performed most of the priestly functions, just as Hophni and Phinehas did in the lifetime of Eli. Abiathar did not actually join David until he went to Keilah.