The Rh blood group system is spectacularly complex, while at the same time enormously important. Beginners trying to understand the system are often really confused by all the terminology that blood bankers use to describe the five main Rh antigens. We say things like, “well, the most likely genotype is R1R1, obviously” when it just isn’t obvious at all when you are trying to figure things out! Further, the genetics of this system have been variously understood and explained over the years, and it is very easy to get lost in the jungle of phrases and haplotypes and buzzwords!
“But, BBGuy,” you cry, “surely you can make this easy for us!” My friends, the answer is “no, I’m just not THAT good (and don’t call me “Shirley”)! But, I can make it easier!” This post is designed to help you get started on the right track, and is the first of two on this topic.
The Rh blood group system is a complex set of red blood cell surface proteins that most likely function to maintain the integrity of the cell membrane. As of May 2016, we know of at least fifty Rh antigens according to the International Society for Blood Transfusion, but we spend most of our time worrying about only five of them. Those five (which we call D, C, c, E, and e) account for the vast majority of clinical and laboratory issues in this blood group, and for most of the confusion that people learning the terminology experience!
The Deal with D and d
Before we get rolling, you must understand this: The D antigen is the main Rh antigen. The presence or absence of D defines a person as “Rh positive” or “Rh negative,” an extremely important distinction that establishes a person’s basic blood type (in conjunction with their ABO type). Commonly, blood bankers will use the term “d” (“little d”) to indicate the lack of the D antigen (in other words, we will describe someone who is Rh negative either as “D-negative” or when writing it, as “d”). Little d is a silly way to say or write it, because there is no little d antigen; little d is merely a slang term to indicate that there is no D.
Years ago, based on the theories of Fisher and Race in England, blood bankers believed that the five main antigens were inherited as a combination of alleles from three different genes: One D gene, one gene for C or c (called “C/c” which coded for either C or c, not both), and one gene for E or e (called “E/e” and likewise coding for either E or e). Each child received a group of Rh genes from each parent (a so-called “haplotype“), and the combination of alleles inherited from both parents resulted in the child’s final Rh phenotype. We now know that this theory was wrong (more on this in a minute), but the names for the five main antigens (D, C, c, E, and e) stuck around.
Meanwhile, another famous blood banker, Dr. Alexander Wiener, was developing his own theory and shorthand terminology scheme. Most of what he proposed has been discredited, but he did develop something that we still use today. Dr. Wiener gave shorthand names to the eight possible haplotypes resulting from the possible combinations of the alleles of those three genes mentioned above. Those names are blood bank gospel, and you must memorize them if you are going to be involved in blood banking! Plus, virtually anyone writing test questions covering the Rh system will expect you to know them cold! Here is the modified Wiener terminology for the eight Rh haplotypes:
|D-positive Haplotypes||D-negative Haplotypes|
|R1: DCe||r': dCe|
|R2: DcE||r": dcE|
|R0: Dce||r: dce|
|Rz: DCE||ry: dCE|
Looks Complicated! How Do I Memorize This?
Don’t worry; I’m not going to stop there! There are ways to break this down to make it more manageable. First, look closely at the table above and notice that all the D-positive combinations (haplotypes) in the left column have the a capital “R” as the letter in the shorthand (and start off with a capital “D” in the list of three antigens). Conversely, it’s easy to see that all of the haplotype that start with a small “r” lack D (remember, all of these have a “little d” at the start of the list of three antigens in the haplotype, but there is no “d,” only the absence of “D”). So, the main part of your work is already done! You know that any haplotype that starts with “R” has a “D,” and any that starts with an “r” does not have D (so you write that as “d”). You just have to figure out the combination of C/c and E/e that makes up the rest of the list of three antigens in the haplotype. Look at the image below: You’ve already completed the D/d position by looking at the haplotype. You’ve got two positions left to fill (the C/c and E/e positions):
Good news! The number, letter, or symbol after the “R” or “r” guides you on how to fill in the boxes above. Here’s how to do it:
Rule 1: The number 1 or ‘ (single prime) after the R or r, respectively, tells you to capitalize the first of the two remaining openings, the C/c position, and results in a “Ce” combination.
For example, for R1, the first position, C/c, is capitalized and the second, E/e, is not, like so:
The same rule applies to r’, but the haplotype lacks D (written as “d”):
Rule 2: The number 2 or ” (double prime) after the R or r, respectively, tells you to capitalize the second open position, the E/e, and results in a “cE” combination.
For example, for R2, the second position, E/e, is capitalized and the first, C/c, is not, like so:
Obviously, the same rule applies to r”, but this haplotype also lacks D:
Rule 3: 0 or nothing after the R or r, respectively, tells you that neither of the two open positions is capitalized, and results in a “ce” combination.
Hopefully, this is becoming obvious, so the image below shows both R0 (on top) and r (below):
OK, here’s a weird little tidbit: If you look at the “0” in the R0 haplotype, you will notice that it looks like a “zero” rather than the letter “O,” right? It’s usually written that way (it definitely is in the AABB Technical Manual), but blood bankers always say it like the letter O. In other words, you would pronounce “R0” as “R Oh” rather than “R zero.” Blood bankers do weird stuff…
Rule 4: Any letter after the R or r tells you that both of the two open positions are capitalized, and results in a “CE” combination.
Here are Rz and ry:
Memorize That, But Here’s Reality
After years of further research, it is now clear that these five antigens are the result of the actions of two genes on chromosome 1 rather than three (Update: This was brilliantly and correctly proposed by Dr. Patricia Tippett, as kindly pointed out in the comment section below by Malcolm Needs). As was thought before, the D antigen actually is the product of the actions of a single gene, but that gene is now known as RHD. On the other hand, instead of a single gene controlling expression of C/c antigens and another gene regulating E/e antigens, both sets of antigens are derived from the actions of a separate, single gene, known as RHCE. The four possible alleles of the RHCE gene are RHce, RHcE, RHCe, and RHCE (note that the technically correct way to write the names of the alleles is as follows: RHCE*ce, RHCE*cE, etc., but everyone knows what you mean when you write them as I did above). Each parent contributes one RHD and one RHCE allele to make up a child’s Rh type (expressed as the presence or absence of the main five Rh antigens above). While the old theories were wrong, the basic idea of haplotypes is still ok to use, as the RHD and RHCE alleles do tend to be inherited as a group (in other words, they are “linked” and it is reasonable to consider them as one group).
NOTE: This terminology can be confusing, especially with RHCE. Remember, the gene is known as RHCE, but that doesn’t mean that all RHCE genes encode C and E antigens. Rather, four possible alleles (alternate forms of the gene) can occur at the RHCE locus on chromosome 1. Only one of those alleles is inherited from each parent. For example, a person could inherit an RHCe allele at the RHCE gene site. See the table below.
|RHD Gene Possibilities||RHCE Gene Possibilities|
|RHD allele||RHce allele|
|Deleted RHD allele||RHcE allele|
|Mutated RHD allele||RHCe allele|
Now that we know more about Rh genetics, we can modify the first table above to better describe the Wiener haplotype terminology. See the following:
|D-positive Haplotypes||D-negative Haplotypes|
|R1: RHD + RHCe alleles (DCe)||r': Absent/mutated RHD + RHCe alleles (dCe)|
|R2: RHD + RHcE alleles (DcE)||r": Absent/mutated RHD + RHcE alleles (dcE)|
|R0: RHD + RHce alleles (Dce)||r: Absent/mutated RHD + RHce alleles (dce)|
|Rz: RHD + RHCE alleles (DCE)||ry: Absent/mutated RHD + RHCE alleles (dCE)|
Note that each child inherits only one copy of chromosome 1 from each parent. In the example above, the child received the R1 from mom, and the R2 from dad. The child’s final Rh phenotype is determined by the combination of genes he received from his parents. Note that the alleles are “codominant,” meaning that all of the inherited alleles lead to expression of the coded antigens. In this case, all C/c/E/e combinations are expressed (in other words, if a person has one RHCe and one RHcE allele, all four antigens are expressed). This is shown in the simple Punnett square below showing the genotypes and phenotypes of the offspring of an R1R0 parent and an rr parent:
The table below outlines the frequencies of the various haplotypes in various races (with the most common haplotype for each race in red and the second most common in blue):
As you would expect from the presence of two genes in the Rh system, two separate proteins carry these Rh antigens, with one protein (called “RHD”) carrying D and another (called “RHCE”) carrying the combination of C/c and E/e (as well as the rest of the Rh antigens outside of the first five). The proteins are very similar in appearance (and likely, in function), and both appear to be dependent on the presence of another protein known as “Rh-related Antigen” (or “RhAG”) for expression.
So, What Now?
If this is your first exposure to this stuff, take a little while and make sure that you can generate the list of the 8 possible haplotype designations. As I mentioned, it is crucial that you know them. You should also learn at least the two most common Rh haplotypes in each race listed on the table above. In a future post, I will outline how we can actually use those combinations to solve problems, and how they are assessed commonly on examinations.
If this post looks familiar to long-time BBGuy.org readers, it’s because it is a heavily modified and (hopefully) clarified version of a post from 2011.