What happens when you put O+ blood in O- body

Question

What happens when you put O+ blood in a O- body?

I know O- can be put in everywhere but what happens, when you put a blood group in a body that would fit the group, but mismatches the rhesus factor.

I know it's not the smartest thing to do but what were the complications that could be expected gor doing it anyway — let's say in an emergency when nothing else is available.

Edit: As suggested in the comments, I should add this to it, for those who do not know about the different blood groups:

The Rh system

Red blood cells sometimes have another antigen, a protein known as the RhD antigen. If this is present, your blood group is RhD positive. If it's absent, your blood group is RhD negative. This means you can be 1 of 8 blood groups:

A RhD positive (A+)

A RhD negative (A-)

B RhD positive (B+)

B RhD negative (B-)

O RhD positive (O+)

O RhD negative (O-)

AB RhD positive (AB+)

AB RhD negative (AB-)

https://www.nhs.uk/conditions/blood-groups/

0- is compadible with ally blood groups, 0+ with all positive ones and so on.

Answer 1

Kinda interesting question IMHO, let me break it down a little.

First, let's tackle the ABO system. Here you are transfusing O into O, so that would be perfectly fine.

Secondly, the is the Rhesus factor. Rh+ erythrocytes have a special antigen on their surface, and if the recipient has antibodies against that antigen, that would cause problems (for comparison, as you had written in your question, Rh- blood is universally¹ transfusable since it doesn't have the antigen and therefore it doesn't matter if the recipient has antibodies or not).
Now we need to find out if our recipient has anti-Rh² antibodies. Unlike in the ABO system, Rh- people do not automatically express anti-Rh antibodies, unless they have been in contact with the antigen before.
This is is why, and here is the answer to your question, the very first transfusion in such a setup wouldn't normally cause any reaction, however it would stimulate the patient's body to produce antibodies, which means that any further exposure to Rh+ blood would cause Haemolytic Anemia.

Last but not least, while this answer covers your described situation, in real life you would also need to look at a few other compatibility factors.

Hope this was understandably written, here's a
reference article to some additional info primarily regarding Rh physiology.

¹ in regard to Rh system
² in reality there are other antigens in the Rh system rather then just D and d, so it is a bit more complicated - we'll leave it for now