A person’s RHD status refers to the presence or absence of a specific protein, known as the D antigen, on the surface of their red blood cells. This antigen is part of the complex Rhesus (Rh) blood group system. If an individual has the D antigen, they are considered RhD-positive; if they lack it, they are RhD-negative. This characteristic is a normal variation within the human population, much like the more widely known A, B, and O blood groups.
The Genetics of RHD Status
An individual’s RHD status is inherited from their parents through genes located on chromosome 1. The gene that codes for the D antigen, the RHD gene, is a dominant trait, meaning a person only needs to inherit one copy to be RhD-positive. Someone who is RhD-positive can have a genotype that is either homozygous (two RHD genes) or heterozygous (one RHD gene and one for its absence).
The inheritance patterns follow predictable outcomes based on the parents’ genetics. If both parents are RhD-negative, all of their children will be RhD-negative. Conversely, if two RhD-positive parents are both heterozygous, they carry a recessive gene for the absence of the D antigen, creating a 25% chance they can have an RhD-negative child.
When one parent is RhD-positive and the other is RhD-negative, the child’s status depends on the genotype of the positive parent. If the RhD-positive parent is homozygous (carrying two RHD genes), all of their children will be RhD-positive. If the RhD-positive parent is heterozygous, there is a 50% chance for each child to be RhD-positive and a 50% chance to be RhD-negative.
Significance for Male Health and Blood Transfusions
For a man, his RHD status has no direct impact on his personal physical health or any predisposition to specific diseases. The primary relevance of a male’s RHD status comes into play in medical contexts, particularly those involving blood.
In blood transfusions, matching RHD status is an important consideration for safety. An RhD-negative male should ideally receive RhD-negative blood. Introducing RhD-positive blood can cause his immune system to recognize the D antigen as a foreign substance and produce antibodies against it. An RhD-positive male, however, can safely receive either RhD-positive or RhD-negative blood because his system already recognizes the D antigen.
The status is also relevant for blood donation. Because RhD-negative blood lacks the D antigen, it can be safely given to other RhD-negative individuals without triggering an immune response.
The Male’s Role in Pregnancy and Rh Incompatibility
A man’s RHD status becomes particularly significant in the context of pregnancy when his partner is RhD-negative. If an RhD-positive man and an RhD-negative woman conceive, there is a possibility they will have an RhD-positive child. This scenario sets the stage for a condition known as Rh incompatibility.
During pregnancy or, more commonly, during childbirth, a small amount of the fetus’s RhD-positive blood can enter the mother’s bloodstream. Because the mother’s immune system does not have the D antigen, it identifies the fetal red blood cells as foreign. In response, her body may begin to produce antibodies against the D antigen in a process called sensitization.
This initial sensitization process typically does not harm the first RhD-positive baby. The risk arises in subsequent pregnancies with another RhD-positive fetus. The mother’s previously formed antibodies are small enough to cross the placenta and enter the fetal bloodstream. Once there, these antibodies attack the fetus’s red blood cells, causing them to break down in a condition called Hemolytic Disease of the Fetus and Newborn (HDFN).
To prevent this sequence of events, a preventative treatment is administered to the RhD-negative mother. An injection of Rh immune globulin (such as RhoGAM) is given during pregnancy and again shortly after delivery if the baby is confirmed to be RhD-positive. This injection prevents the mother’s immune system from producing its own antibodies against the D antigen, thereby protecting future pregnancies from the risks of HDFN.
Determining a Male’s RHD Status
The most direct way to determine RHD status is through a standard blood type test, which can be ordered by a physician during a routine check-up or for other medical reasons. This test analyzes a blood sample to identify the presence or absence of the D antigen on red blood cells, along with the ABO blood group.
Another common opportunity to learn one’s blood type is by donating blood. Blood collection centers determine the donor’s full blood type, including RHD status, as a standard part of their processing protocol.
Past medical records are another reliable source of this information. If a man has had surgery, received a blood transfusion, or had blood work done for other health reasons, his blood type is likely documented in his medical history. In the context of family planning, an obstetrician caring for a pregnant woman may ask for the father’s blood type to assess the risk of Rh incompatibility.