This is an excellent explanation. To provide some context, though, a hemoglobin of 6.6 is not all that low. It’s certainly low enough that the person should get transfused a unit of blood (usually indicated if the value is below 7). However, the real issue is figuring WHY it got low in the first place. Is the patient’s own immune system attacking their blood (autoimmune)? Is there a bleed somewhere (like the gastrointestinal tract)? Is there cancer in the bone marrow leaving (less likely if only the red cells are low but not the other blood counts)? Is there an iron deficiency for some reason? Figuring out the cause of the anemia will be important to knowing how to treat and the actual prognosis.

Kidd antigens: A red cell has a ton of molecules on the cell surface that helps it do what a cell gotta do. One such molecule is an ion transporter (helps urea travel from one side of the cell membrane to the other. Almost all people have this protein, but at some point there was a mutation, so about 50% of people have “version a” while about 50% of people have “version b” and another 25% have both “Jka” and “Jkb”. These are the main Kidd antigens (Jka and Jkb are abbreviations). So, really there’s nothing special about having a Kidd antigen on your red cell. The problem happens if you’re missing one (ie type Jkb only) and get exposed to some blood from a Jka person. There’s a chance your immune system notices the difference and forms anti-Jka antibodies. Once the hospital knows you have formed this antibody, they’ll do their best to give you Jka-negative blood to prevent the transfusion from being destroyed by your immune system.

Just as an aside, there are tons of different blood group system (types of molecules on the red blood cell surface that can vary between one person and another). It’s impractical to “match” a blood unit up front to each persons molecules except for the ABO and Rh+/- everyone’s talked about. So we only special-select blood if there’s a known antibody, with the exception of sickle cell patients.

One caveat: while Rh is a protein, A and B are carbohydrate antigens.

For the most part, you can’t tell under the microscope a person’s blood type. However, some antigens, if they’re missing entirely, result in misshapen cells. For example, if missing all Rh antigens (not just the D that gives +/- as mentioned above) you get a shape called “stomatocyte”, where instead of a pale round area in the middle, it looks more like a closed mouth.

The difference is how deep the dirty part gets pushed into your tissue. The Clostridium tetani bacterium hates oxygen (air), but is happy like a bug in a rug when introduced to the anaerobic Petri dish that is your tissues under the skin. Doesn’t have to be a nail; getting stabbed with anything that was exposed to dirt could be a problem