21 The beginning of signs under Moses was blood and water; and the last of all Jesus' signs was the same. First, Moses changed the river into blood; and Jesus at the last gave forth from His side water with blood. This was perhaps on account of the two speeches, his who judged Him, and theirs who cried out against Him; or because of the believers and the unbelievers. For Pilate said, I am innocent and washed his hands in water; they who cried out against Him said, His blood be upon us: there came therefore these two out of His side; the water, perhaps, for him who judged Him; but for them that shouted against Him the blood.
And again it is to be understood in another way; the blood for the Jews, and the water for the Christians: for upon them as plotters came the condemnation from the blood; but to you who now believest, the salvation which is by water. For nothing has been done without a meaning. Our fathers who have written comments have given another reason of this matter. For since in the Gospels the power of salutary Baptism is twofold, one which is granted by means of water to the illuminated, and a second to holy martyrs, in persecutions, through their own blood, there came out of that saving Side blood and water, to confirm the grace of the confession made for Christ, whether in baptism, or on occasions of martyrdom.
There is another reason also for mentioning the Side. The woman, who was formed from the side, led the way to sin; but Jesus who came to bestow the grace of pardon on men and women alike, was pierced in the side for women, that He might undo the sin.
Source: Catechetical Lectures (New Advent)