DEAR DR. ROACH: I had two blood transfusions in 2012 during an almost yearlong bout with an out of control infection that ended up in a fused knee. In a recent antibody screen, I was identified as having anti-jkb. The information on this that I was able to find on the internet was incomprehensible to me. In simple terms, what does this mean? Am I able to donate blood? Can I receive donated blood? — S.L.

ANSWER: Most people know about the ABO blood types and the Rh factors that are the primary considerations that govern how we can transfuse blood, but there are many other blood antigens that can also cause transfusion reactions. “Antigen” is a general term for a substance than can provoke an immune response, usually a protein, sometimes connected with sugars. One of the families of antigens that may cause a transfusion reaction is called the Kidd blood group system. It’s named after the first patient in whom this was found.

Antibodies to these antigens can cause transfused blood to be destroyed. This is called a hemolytic — literally, “blood-breaking” — transfusion reaction. Worse, Kidd antibodies (anti-jka, anti-jkb and anti-jk3) can be difficult to find. The level in the blood goes down to very low, sometimes undetectable levels after transfusion, but the antibodies are still capable of causing a transfusion reaction. The transfusion reaction in this case is typically delayed, not immediate.

For this reason, it would be worthwhile for people with anti-Kidd antibodies to wear a medical alert bracelet in case of emergency transfusion need.

People like you, with anti-jk antibodies, may still donate blood, but the blood bank must give this blood only to people who lack the antigen.

DEAR DR. ROACH: I am a healthy and active 65-year-old male. I had hepatitis B as a teenager. I then had hepatitis C, which was treated and now has been non-detected for five years.

Will I ever be able to give blood? Before being treated for hepatitis C, doctors told me to stay away from shellfish, especially raw oysters. Is it OK to eat them now? My liver functions are normal. — D.B.

ANSWER: At the present time, most blood banks, such as the American Red Cross, do not accept blood donations from anyone with any history of hepatitis C, even those who are apparently cured, as you are.

Raw oysters, and some other raw shellfish, may carry bacteria called vibrio vulnificus. This species is in the same family as cholera, and is a problem in people with liver disease. Cirrhosis is the major concern, but people with fatty liver, alcohol use without known liver disease and liver disease due to hereditary hemochromatosis — a genetic disease leading to iron overload — are at risk, as are people with other chronic illness, such as diabetes and rheumatoid arthritis, though at a much lower level of risk.

Check with your doctor. If you have no liver disease now, and normal lab tests are a hopeful sign, you might be able to eat them, though it would certainly be safest not to.

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Dr. Roach regrets that he is unable to answer individual letters, but will incorporate them in the column whenever possible. Readers may email questions to ToYourGoodHealth@med.cornell.edu or send mail to 628 Virginia Dr., Orlando, FL 32803.