RhD positive refers to the presence of the rhesus D (RhD) antigen on the surface of red blood cells. People who have the RhD antigen are RhD positive, while those without it are RhD negative. In the UK, around 85% of the population are RhD positive. RhD status is determined by genetics and depends on how many copies of the RhD antigen a person has inherited from their parents. RhD negative individuals can develop antibodies against RhD positive blood if they are exposed to it, which can cause problems during pregnancy if the baby is RhD positive and the mothers immune system attacks the babys red blood cells. Anti-D injections can be given to prevent sensitization in RhD negative women carrying RhD positive babies.