To understand what it means to be Rh negative, you really need to understand a little bit about genetics and how we inherit our characteristics from our parents- and a little bit about blood types in general.
Blood is a mix of cells and plasma that carries oxygen and nutrients to every cell and organ in the body. Your blood also carries waste away from your cells and organs. Science is still making new discoveries about the function of blood, blood types and inheritance. It's all fairly recent science. It wasn't until the 1901 that Karl Landsteiner discovered the existence of a blood "type". Since then, we have made tremendous leaps and bounds in acquiring knowledge about blood types, and that trend is continuing.
It's important to know that everyone has a specific blood type. Your specific blood type is made up of many different systems. Currently, there are 30 human blood group systems that are recognized by the scientific community, and with the rapid changes that are occurring in medical science there may be more discoveries at any time. Although there are numerous blood groups, for the purpose of this article, we are only going to talk about two most common- the A-B-O system, and the Rhesus System.
You inherit your blood type from both of your parents through the action of alleles. An allele is simply the genetic material for different possible expressions of a particular gene. There are a vast number of alleles that make up the genetic code for your body. Your specific genetic code is called a genotype. The result of the genetic coding ie- red hair, eye color, blood type etc. is called your phenotype.
Each parent contributes an allele to develop each part of the body of the child, including the blood system. For example, for the A-B-O system, each parent will contribute either an A allele, a B allele, or an O allele. Each parent will also contribute one allele that will determine whether the baby will be born rh positive, or rh negative. The combination of these alleles produce a persons unique blood type.
Every child then has two alleles that will determine their A-B-O blood type, and two alleles that determine Rhesus status. Possible combinations of the A,B, and O alleles in a child are OO, OA, OB, AA, BB, and AB. We can further simplify blood types because some alleles are recessive, and some are dominant. The A and B alleles are co-dominant, while the O allele is recessive. Recessive alleles do not express themselves if there is a dominant allele present. For example, for a person to have type O blood, both parents must have contributed the type O allele: if an A or a B allele is present, it will dominate.
As you can see in the chart on the chart below, if a person inherited an A allele from one parent, and an O allele from the other parent, the person would have a genotype of OA, but would be considered to be a type A blood type because the O is recessive and the A protein would assert itself.
Why do the A and B alleles dominate?
They dominate very simply because they have protein on their blood cells that a type O does not. Consider the following example.
The vast majority of the time your blood type is going to be either type A, type B, type AB or type O. Type O is also called type NULL, or ZERO in some parts of the world. Additionally, all of these specific types will be classified as either being Rhesus (D) positive or negative.
Remember, there are other blood types out there, but for the purpose of this discussion we’re going to stick with just the basics. Check out this video for a really great visual explanation of how we inherit our blood types.
The following is a chart for informational purposes only that shows some of the combinations that can result in certain blood types. (Inheritance and Alleles n.d.)
Parents blood type
Possible child blood type
Not possible type
A and A
A and B
A, O, B, AB
A and AB
A, B, and AB
A and O
B and B
B and AB
A, B, AB,
AB and AB
A, B, AB
AB and O
O and O
A, B, AB
Each blood type is different, with its own identifying characteristics or “antigens”, and each blood type can be identified by its unique protein (antigen) pattern.
An antigen is a fancy word for anything that can be recognized by a body as either SELF or NOT SELF or, very simply, anything that can trigger an immune response. The word comes from the term “antibody generator”. When discussing blood types, proteins are often called antigens.
If your red blood cells have the “A” protein, your body identifies the A antigen as SELF, and you have type A blood.
If your red blood cells have the the B protein, your body identifies the B antigen as SELF, and you have type B blood.
If your red blood cells have both the A and B proteins, you body identifies both the A and the B antigen as SELF, and you have type AB blood.
If your red blood cells have NO “A or B” proteins, your body does not have any A or B antigens, then you have type O (NULL or ZERO) blood.
Most blood types do not mix well with each other because of the unique protein structure of each type. Any protein or antigen that is not considered SELF by the body will be removed by antibodies that the blood will produce until it has rid itself of the foreign proteins. Antibodies are very simply the body sending out the troops to get rid of the “foreign” invaders.
For example, type A blood does not recognize type B proteins (antigens) as SELF, so it will produce antibodies to fight off any type B proteins. You cannot mix type A blood with type B, or even AB blood because a type A blood type will never recognize that B protein as SELF.
By the same token, type B blood cannot receive type A or AB blood, because it will never recognize the A protein as SELF.
Type AB blood will recognize both A and B proteins, therefore it will not attack any other blood type. For this reason, type AB+ blood is known as the universal recipient.
Type O blood does not have any A or B proteins, so it would treat any other blood type as an invader, because all of the other blood types have either an A or a B protein, or both.
At one time type O blood was considered to be the universal donor except for rare blood types, but because of new understanding of blood types this is no longer the case. As of this writing, the ultra rare Bombay blood type is the true universal donor.
We need to add one more factor into this growing equation. Your blood is identified using a combination of the A-B-O system and the Rhesus system.