Immune responses
July 2, 2005 2:08 AM   Subscribe

Why do our bodies reject new organs but not blood transfusions, assuming the blood transfusion is of the same blood type?
posted by IndigoRain to Health & Fitness (5 answers total)
Best answer: Googling "organ rejection blood transfusion" gives this. Essentially, the new blood is rejected after a couple of days, but since it's only supposed to stick around long enough for your body to make its own blood again, that's okay.
posted by chrismear at 3:15 AM on July 2, 2005

Best answer: A couple of reasons,

1. As mentioned in chrismear's link, the protein that's primarily responsible for tissue rejection, MHC Class I, is absent from or present at only low levels on erythrocytes, which make up the overwhelming bulk of the transfusion (ie, one ml of whole blood contains about 5 billion erythrocytes vs. 7 million of all the other cell types and platelets combined). The various leukocytes in the blood, which do express lots of MHC Class I and therefore represent an obvious target to the self-vs-nonself system in the recipient, aren't present in a large enough number to cause "rejection" of a transfusion - although they will be attacked by the recipient's immune system as foreign if the donor and recipient aren't sufficiently well-matched in their MHC Class I's. The vast majority of cells other than blood in the human body also produce lots of MHC I, and that's why *they* need to be matched when you do a transplantation.

The MHC I system, by the way, is completely separate from the ABO system, operates by a different mechanism, and is much more complex due to the huge number of MHC variants in the human population. And ABO typing applies just to erythrocytes while MHC I typing applies to just about every other cell type.

2. Transfusions are commonly (in the US at least) performed with leukocyte-depleted blood, which further reduces any host vs. graft response after transfusion.
posted by shoos at 4:53 AM on July 2, 2005

Response by poster: Thanks, both of you! I tried Googling before I posted but I guess I just wasn't getting the words right.
posted by IndigoRain at 8:41 PM on July 2, 2005

If you are interested in an alternate explanation to the self-vs-nonself system view of immunology look into the "danger theory".
posted by srboisvert at 11:22 AM on July 3, 2005

"Blood type" commonly refers to the ABO blood groups, but it's interesting to know that there are many other blood types that are rare.

Transfusion reactions are beginning to be more commonly recognized; serious, life-threatening reactions occur in 1 of 5000 matched transfusions, but less serious reactions may occur in as many as 1 in 250 (or more - they're underreported by physicians who, naturally, don't want to acknowledge wrongdoing in the litigious climate, especially when they think the end result was net harmless.) The reactions can be delayed, too.

But how harmless, and how delayed? An interesting study of progressive systemic sclerosis was published in 1999 or thereabouts. PSS is a lupus-like disease, in which a patient's immune system attacks self tissues; it affects only women (lupus' F:M ratio is about 10:1). Some investigators noticed most of their PSS patients had children, and assayed their bloodstreams for cells from their male offspring (easily identified in flow cytometry by the Y chromosome, and then further confirmed by MHC testing.)

The bottom line was that most of the PSS patients had such cells, and none of the controls, suggesting that the presence of non-self cells in the bloodstream caused a dangerous immune response.

Since learning of these sorts of things I have become much more reluctant to recommend transfusions, although neurologists don't often have any reason to transfuse anyway.

By the way, whole blood is almost never transfused any more. Packed red cells, fresh frozen plasma, and platelets can be transfused; but anyone who infused something containing leukocytes (white cells, present in whole blood) in 2005 would have to explain why this violation of standard-of-care had occurred.
posted by ikkyu2 at 11:47 AM on July 3, 2005 [1 favorite]

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