I’m new to this discussion and board, but felt the need to comment.
First, at the moment of conception changes take place to the Mother’s body that are instantaneous, irrefutable, and everlasting. That is true for “normal” conceptions. How much more so would that be the case with Mary and Jesus?
Second, immunoglobulins (antibodies, etc) are primary products of white blood cells. Therefore, there is an exchange of blood prior to delivery. The blood in question just isn’t the red blood we’re used to thinking about.
Third, when the ECF’s wrote of Mary being sinless they wouldn’t have specified that she was conceived that way. At that time, conception was definitely not fully understood. Human eggs were not thought of and sperm cells had not been visualized yet. Most people thought that the man’s seed contained fully formed humans that merely needed to be incubated by the woman. The female component of reproduction was not fully known or understood.
Fourth, even if Scripture and the ECF’s were completely silent on Mary’s sinless nature, we have thousands of references, studies, and critiques of her ever sinless nature. As knowledge progressed, more research was done into various areas of the Faith. The ECF’s don’t speak against IVF or gunning down innocents because those two concepts (artificial conception and guns) were not yet in the minds of man. Although Scripture and Tradition were inspired by God, He couldn’t overload man’s feeble mind with thing that had yet to be discovered. The story of the Incarnation and Eucharist was already stretching the minds of these holy men. If people of the day waked away from Christ because He told them “my flesh is real food, and my blood is true drink. Unless you eat of the Son of Man you shall not have life within you” (I’m paraphrasing, sorry I’m no bible scholar), wouldn’t they really have thought He (and His followers later) were nuts for talking about His mother’s state of sin(lessness) at her conception? How much more would talk of conception, antibodies, etc have confused the ECF’s?
Fifth, in an effort to be parsimonious, wouldn’t it be most logical to believe that the moment of Mary’s conception, while performed in the “normal” way, she was “destined” together NT Ark of the Covenant and therefore when God created her soul, He did so without staining it with original sin? It complicates matters to assign Mary’s soul original sin at conception, but wipe it clean at a later date. The most simple (simplest) answer is usually correct… The most simple answer is that God knew Mary at her conception and knew she was to be the Mother of His Son; so He preserved her from original sin when He breathed life into her.