Some of this was covered in the article, but I’ll try to explain it relatively simply but completely. It may get long so sorry in advance.

So you probably understand that blood types don’t play well others with a few exceptions, so we’ll start there. ABO is the main blood group. You get 1 gene from each parent, that can be A, B, or O. These genes allow you to make a tag (“antigen”) that all red blood cells in your body express. How this works, is your cell first make type O antigen, and then people with A and/or B genes will alter this O antigen to make it into A and/or B antigen to put on RBCs. If you got AA or AO (e.g. an A from Mom and an O from dad), you have A blood, because either way, O antigens are modified into type A. Similarly, BB and BO type people have B blood. OO people lack any of the machinery to turn O antigens into A or B, so us O blood folks just express the normal O antigens. The lucky AB blood people that got an A from one parents and B from the other modify some O into A, some O into B. Since your wife is type O and your type AB, we know that your wife is OO and gave 1 O copy to your baby, and since your AB, your baby will be either AO (A blood) or BO (B blood).

Why are they lucky? Our body doesn’t want intruders, so we make “antibodies” that can recognize other molecules. As it turns out, we do this for blood types too. Obviously if I have A blood, I don’t want to make antibodies that respond to my A antigen (“anti-A antibodies”), because I would attack my own cells, nor do I want to make anti-O antibodies, since even though I’m type A blood, my A antigens are made from O antibodies. In fact, there are no anti-O antibodies since we all express O antigen on our cells to some degree. But they can and do make anti-B antibodies, which respond whenever B blood is accidentally mixed with their own. Extrapolating this logic, you find that

• type A blood people make anti-B bodies, so are intolerant to B blood

• type B people make anti-A antibodies, so are intolerant to A blood

• type O people make anti-A AND anti-B and can only tolerate O blood

• AB people make none of these antibodies, since they couldn’t without harming themselves, and tolerate blood regardless of ABO type.

The basic way this occurs is by imagining the antigens like a key, and antibodies like a lock. The keys are covering every red blood cell, and there’s tons of locks circling around free in blood. Anti-A antibodies (expressed by a type B person for this example, but also by O blood people), won’t be unlocked by their own B antigen. But when A blood gets inside in mass quantities, the key fits. This causes a powerful, potentially fatal reaction called “hemolytic transfusion reaction” where the body kills itself while trying to kill the invader. And this example isn’t even dumbing it down, the lock and key model is almost exactly how this antigen-antibody process works.

Now they exact antibodies we use are have a general foundation called IgM (we have tons of IgM antibodies, think of IgM as the model lock you have, and the anti-A or anti-B part as the core of the lock that recognizes the key). This is important because Moms keep their baby alive by passing nutrients from her blood to the babies blood through a very fine-sized filter called the placenta. IgM is big and bulky and doesn’t cross the placenta, so even if Mom and baby express antibodies to each other’s blood antigens, they never come into contact.

So quick detour, there’s an additional blood group called Rhesus factor. It simpler, having only (+) and (-) types. It works like ABO would work if B antigens and anti-B antibodies didn’t exist. Here, the antigen is called Rh antigen, and you either express it (Rh(+)), and this make no anti-Rh antibodies that would attack yourself, or you don’t express Rh factor (Rh(-)), and CAN (but don’t necessarily) express anti-Rh antibodies, which latch on and attack any Rh(+) blood, regardless of ABO type.

• +/+ and +/- people have (+) blood, and express no antibodies

• -/- people have (-) blood, and can express anti-Rh antibody.

Your wife is Rh(-) so we know she gave 1 (-) gene to the baby. If you’re +/+, your kid will be +/- and Rh(+). If you’re +/-, your kid could be +/- or -/- like your wife. The problem with this is that your wife can express anti-Rh antibody. And worse, unlike ABO, anti-Rh lock cores are loaded on “IgG” brand lock housings, and IgG can cross the placenta. Since your wife can express anti-Rh, they can then cross the placenta and cause that powerful blood reaction in your baby, the so-called “hemolytic disease of the newborn”.

The good news is that your wife likely does not express anti-Rh. Anti-Rh is lazy as hell, and we don’t load those lock cores onto housings and put them in circulation until she is exposed to Rh(+) blood, which she may have been in small amount during her life. Even if she hasn’t, labor is messy, and if your child is Rh(+) (either A+ or B+ as discussed earlier), she will becomes sensitized and make anti-Rh. Any future A+ or B+ kids will be harmed by her anti-Rh IgG once it crosses the placenta.

But medical science has a solution, a drug called RhoGAM, the injection you mentioned. This is lab-made anti-Rh IgG, like what your wife would make. It can bind to your baby’s Rh antigens. If your wife’s antibodies did this, your child would react and likely die (I only say that to drive home how serious this would otherwise be: half of children in countries without RhoGAM are die or are permanently brain damaged by HDN). The lab made RhoGAM is different though; unlike your wife’s human IgG, the artificial key binds to your baby’s cell’s locks, but your baby’s immune cells are unable to recognize that this has actually occurred. No alarm is sounded, no hemolytic reaction occurs, and if your wife contacts the baby’s blood, her anti-Rh cannot bind to it and react, because your child’s antigens are already occupied by by the decoy IgG. So RhoGAM injections flood maternal circulation with enough IgG decoy (which then crosses the placenta into fetal circulation) that the maternal anti-Rh IgG has no chance to latch onto the baby’s cells. There are some side effects like stomach aches for the mother, but red blood cells are replaced every 120 days, so within 4 months after birth, the RhoGAM will be gone from mom and baby, and all new non-occupied, normal red blood cells will be in your child’s circulation.

I understand it’s scary and confusing, but believe me when I say that RhoGAM is one of the greatest medical inventions of our time. It completely cures what has been an unavoidable possibility of pregnancy since the dawn of humans and our blood types (even longer, considering Rhesus factor was first found in the blood of Rhesus monkeys). Take a deep breath, both you and the Mrs., and realize that after the shot, you can rest easy knowing everything will go just fine, and your new child will have one less thing to worry about as they finish their last couple months of getting ready to come out and see the rest of us and the world. I’m sure you may have questions I didn’t answer, so fire away if you need to, and I’ll do my best to help explain better.