Rare Blood Group Antigens: A Quick Reference for the Uncommon and Unforgettable

Most days in the blood bank, we’re juggling the usual suspects — ABO, Rh, Kell, Duffy, Kidd, MNS. But every now and then, an antibody pops up that doesn’t fit neatly into those boxes. That’s when the rare blood group systems come knocking: Diego, Cromer, Gerbich, and the rest of the “alphabet soup” that can send even seasoned transfusion medicine folks back to the reference books.

These antigens don’t make headlines in every transfusion service, but when they do appear, they can mean the difference between finding compatible blood and delaying critical care. And the truth is — most of us don’t carry all these details in our heads. Why would we? They’re rare. But rare doesn’t mean unimportant.

That’s why I put together a quick reference guide: something you can pull up on service, during board prep, or when you just need to remind yourself whether anti-Chido is clinically significant (spoiler: it’s not).

What’s inside the guide

Each system is broken down into:

• Antigens – high or low prevalence, defined where possible

• Genetics – the molecular basis (when known)

• Prevalence – who has it, and how often

• Antibody type – IgG vs IgM, reactivity patterns

• Clinical significance – HTRs, HDFN, or mostly nuisance

• Notes – testing pearls (like Sdᵃ being inhibited by guinea pig urine, or LWa being destroyed by DTT)

  
  

I’ve included the systems that come up just enough to trip people up: Diego, Cromer, Gerbich, Chido/Rogers, Landsteiner–Wiener, Dombrock, Bennett–Goodspeed, Colton, Cartwright, Sid, and FORS. And for those who like to think ahead, there’s also a short overview of the 700 and 901 series (the collections and high-incidence buckets where tomorrow’s systems often begin).

Why it matters

• For learners: Rare systems are high-yield board material, especially in transfusion medicine and immunohematology.

• For practice: Even if you never see anti-Wra in your blood bank career, you’ll eventually run into an antibody that doesn’t “fit” — and knowing how to interpret rarity vs significance is the key to safe transfusion.

• For curiosity: These systems are windows into red cell biology — from aquaporins to complement regulators to acetylcholinesterase. They remind us just how much the red cell carries beyond hemoglobin.

  
  

Download the guide

I’ve made the reference sheet available as a PDF download so you can keep it in your binder, your desktop, or your teaching slides:

Rare_Blood_Group_Antigens_Quick_Guide.pdf

Download PDF • 156KB

Final thought

Blood group systems may be rare, but patients with rare antibodies are not imaginary. They show up in our hospitals, on our call nights, and in our transfusion reactions. Having a quick way to orient yourself can be the difference between feeling stuck and moving forward with confidence.

This guide is my way of sharing a tool I wish I had earlier — compact, high-yield, and practical. I hope it helps you, your trainees, or your colleagues the next time an “uncommon and unforgettable” antibody shows up on your bench.