Re: Why aren't blood transfusions rejected by our immune systems?

Frank,

There is a misconception in your question. In fact, after blood transfusion 
from one unrelated individual there is rejection. 

In order to understand this we have to consider: 1. which are the 
components responsible to trigger a rejection, and 2. what are the 
differences between a heart transplant and a blood transfusion (in terms of 
what we expect from each of them).

Organ rejection, or rejection of cells between unrelated individuals is an 
immune phenomenon, mediated by the same entities that confer protection 
after vaccination. The immune system of an animal recognizes the presence 
of foreign or abnormal structures (antigens) and generates an immune 
response. Thus, a transplant immunizes you against the graft. This 
immunization process occurs due to the genetic differences between 
individuals (except for identical twins, there are not two persons exactly 
the same). 

There is a group of molecules called Histocompatibility molecules or 
antigens. These proteins are encoded by a group of genes called Major 
Histocompatibility Complex or MHC genes, and they are present in the 
surface of the majority of our cells. These molecules are extremely diverse 
(they have multiple allellic forms), and it is very hard to find two 
persons sharing the same type of them. 

The MHC (or HLA as they are called in humans) antigens can be divided in 
two kinds, based in their structure and function: Class I and Class II. In 
humans, Class I genes encode three products called HLA-A, HLA-B and HLA-C. 
Class II genes encode also three products called: HLA-DR, HLA-DQ and HLA-
DP. Thus, a human being can express up to 12 different HLA molecules per 
cell (6 from the mother and 6 from the father). Most of the immune 
responses that are responsible for transplant rejection are directed 
against differences in the MHC molecules.

In terms of the second consideration, what you want from an organ 
transplant is the permanent replacement of the function of the damaged 
organ (long term function). In the case of a blood transfusion you also 
pursuit the replacement of a function, but in this case the replacement is 
transient. After an intense bleeding, two functions important for survival 
are rapidly diminished: respiration and coagulation. This is due to the 
loss of red blood cells or erythrocytes, that carry oxygen to the tissues 
and eliminate carbon dioxide, and platelets that activate clothing and are 
main components of the blood cloth. These specialized components have a 
short life in the individual, and they can not self-replenish, they are 
derived from progenitor cells that live in the bone marrow. Thus in 
general, blood transfusions are meant to support the function of 
erythrocytes and platelets until the body is able to make their own. You 
will never find cells coming from the transfusion a couple of days after 
the transfusion.

At this point you will be saying: “this guy is not answering my question”. 
But here I go: There are three main types of cells in the blood: 
Leukocytes or white blood cells, these are nucleated cells that play 
various roles in immune defense and inflammation. 
Erythrocytes or red blood cells, these are small cells that do not have 
nuclei and contain molecules (hemoglobin) that function as carriers of 
oxygen and carbon dioxide. 
Platelets, that are fragments of a bone marrow precursor cell called 
megakaryocyte and that are important in coagulation. 
These cells, apart from their functions, are structurally different, and in 
view of the expression of MHC molecules: leukocytes express them in high 
levels, platelets express predominantly low levels of Class I MHC antigens, 
and red blood cells do not express them at all. Thus, a normal rejection 
process operates for leukocytes and platelets after a blood transfusion. In 
fact, people that have been affected by several blood transfusions have a 
high incidence of platelets and leukocyte rejection. Red blood cells, in 
contrast, do not elicit these kind of response. However they express a 
variety of glycoproteins that can be recognized by antibodies in the 
recipient. This is the basis of the ABO classification, and even when the 
mechanism is different, is also a kind of rejection.