Antigenicity Causes Immune Reactions of Blood
When blood transfusions from one person to another were first attempted, immediate or delayed agglutination and hemolysis of the red blood cells often occurred, resulting in typical transfusion reactions that frequently led to death. Soon it was discovered that the bloods of different people have different antigenic and immune properties so that antibodies in the plasma of one blood will react with antigens on the surfaces of the red cells of another blood type. If proper precautions are taken, one can determine ahead of time whether the antibodies and antigens present in the donor and recipient bloods will cause a transfusion reaction.
Multiplicity of Antigens in the Blood Cells
At least 30 commonly occurring antigens and hundreds of other rare antigens, each of which can at times cause antigen-antibody reactions, have been found on the surfaces of the cell membranes of human blood cells. Most of the antigens are weak and therefore are of importance principally for studying the inheritance of genes to establish parentage.
Two particular types of antigens are much more likely than the others to cause blood transfusion reactions. They are the O-A-B system of antigens and the Rh system.
O-A-B Blood Types
A and B Antigens—Agglutinogens
Two antigens—type A and type B—occur on the surfaces of the red blood cells in a large proportion of human beings. It is these antigens (also called agglutinogens because they often cause blood cell agglutination) that cause most blood transfusion reactions. Because of the way these agglutinogens are inherited, people may have neither of them on their cells, they may have one, or they may have both simultaneously.
Major O-A-B Blood Types
In transfusing blood from one person to another, the bloods of donors and recipients are normally classified into four major O-A-B blood types, as shown in Table 35-1, depending on the presence or absence of the two agglutinogens, the A and B agglutinogens. When neither A nor B agglutinogen is present, the blood is type O. When only type A agglutinogen is present, the blood is type A. When only type B agglutinogen is present, the blood is type B. When both A and B agglutinogens are present, the blood is type AB.
Table 35-1 Blood Types with Their Genotypes and Their Constituent Agglutinogens and Agglutinins
Genetic Determination of the Agglutinogens
Two genes, one on each of two paired chromosomes, determine the O-A-B blood type. These genes can be any one of three types but only one type on each of the two chromosomes: type O, type A, or type B. The type O gene is either functionless or almost functionless, so it causes no significant type O agglutinogen on the cells. Conversely, the type A and type B genes do cause strong agglutinogens on the cells.
The six possible combinations of genes, as shown in Table 35-1, are OO, OA, OB, AA, BB, and AB. These combinations of genes are known as the genotypes, and each person is one of the six genotypes.
One can also observe from Table 35-1 that a person with genotype OO produces no agglutinogens, and therefore the blood type is O. A person with genotype OA or AA produces type A agglutinogens and therefore has blood type A. Genotypes OB and BB give type B blood, and genotype AB gives type AB blood.
Relative Frequencies of the Different Blood Types
The prevalence of the different blood types among one group of persons studied was approximately:
O 47%
A 41%
B 9%
AB 3%
It is obvious from these percentages that the O and A genes occur frequently, whereas the B gene is infrequent.
Agglutinins
When type A agglutinogen is not present in a person’s red blood cells, antibodies known as anti-A agglutinins develop in the plasma. Also, when type B agglutinogen is not present in the red blood cells, antibodies known as anti-B agglutinins develop in the plasma.
Thus, referring once again to Table 35-1, note that type O blood, although containing no agglutinogens, does contain both anti-A and anti-B agglutinins; type A blood contains type A agglutinogens and anti-B agglutinins; type B blood contains type B agglutinogens and anti-A agglutinins. Finally, type AB blood contains both A and B agglutinogens but no agglutinins.
Titer of the Agglutinins at Different Ages
Immediately after birth, the quantity of agglutinins in the plasma is almost zero. Two to 8 months after birth, an infant begins to produce agglutinins—anti-A agglutinins when type A agglutinogens are not present in the cells, and anti-B agglutinins when type B agglutinogens are not in the cells. Figure 35-1 shows the changing titers of the anti-A and anti-B agglutinins at different ages. A maximum titer is usually reached at 8 to 10 years of age, and this gradually declines throughout the remaining years of life.
Figure 35-1 Average titers of anti-A and anti-B agglutinins in the plasmas of people with different blood types.
Origin of Agglutinins in the Plasma
The agglutinins are gamma globulins, as are almost all antibodies, and they are produced by the same bone marrow and lymph gland cells that produce antibodies to any other antigens. Most of them are IgM and IgG immunoglobulin molecules.
But why are these agglutinins produced in people who do not have the respective agglutinogens in their red blood cells? The answer to this is that small amounts of type A and B antigens enter the body in food, in bacteria, and in other ways, and these substances initiate the development of the anti-A and anti-B agglutinins.
For instance, infusion of group A antigen into a recipient having a non-A blood type causes a typical immune response with formation of greater quantities of anti-A agglutinins than ever. Also, the neonate has few, if any, agglutinins, showing that agglutinin formation occurs almost entirely after birth.
Agglutination Process in Transfusion Reactions
When bloods are mismatched so that anti-A or anti-B plasma agglutinins are mixed with red blood cells that contain A or B agglutinogens, respectively, the red cells agglutinate as a result of the agglutinins’ attaching themselves to the red blood cells. Because the agglutinins have 2 binding sites (IgG type) or 10 binding sites (IgM type), a single agglutinin can attach to two or more red blood cells at the same time, thereby causing the cells to be bound together by the agglutinin. This causes the cells to clump, which is the process of “agglutination.” Then these clumps plug small blood vessels throughout the circulatory system. During ensuing hours to days, either physical distortion of the cells or attack by phagocytic white blood cells destroys the membranes of the agglutinated cells, releasing hemoglobin into the plasma, which is called “hemolysis” of the red blood cells.
Acute Hemolysis Occurs in Some Transfusion Reactions
Sometimes, when recipient and donor bloods are mismatched, immediate hemolysis of red cells occurs in the circulating blood. In this case, the antibodies cause lysis of the red blood cells by activating the complement system, which releases proteolytic enzymes (the lytic complex) that rupture the cell membranes, as described in Chapter 34. Immediate intravascular hemolysis is far less common than agglutination followed by delayed hemolysis, because not only does there have to be a high titer of antibodies for lysis to occur, but also a different type of antibody seems to be required, mainly the IgM antibodies; these antibodies are called hemolysins.
Blood Typing
Before giving a transfusion to a person, it is necessary to determine the blood type of the recipient’s blood and the blood type of the donor blood so that the bloods can be appropriately matched. This is called blood typing and blood matching, and these are performed in the following way: The red blood cells are first separated from the plasma and diluted with saline. One portion is then mixed with anti-A agglutinin and another portion with anti-B agglutinin. After several minutes, the mixtures are observed under a microscope. If the red blood cells have become clumped—that is, “agglutinated”—one knows that an antibody-antigen reaction has resulted.
Table 35-2 lists the presence (+) or absence (−) of agglutination of the four types of red blood cells. Type O red blood cells have no agglutinogens and therefore do not react with either the anti-A or the anti-B agglutinins. Type A blood has A agglutinogens and therefore agglutinates with anti-A agglutinins. Type B blood has B agglutinogens and agglutinates with anti-B agglutinins. Type AB blood has both A and B agglutinogens and agglutinates with both types of agglutinins.
Table 35-2 Blood Typing, Showing Agglutination of Cells of the Different Blood Types with Anti-A or Anti-B Agglutinins in the Sera
|
Sera |
||
|
Red Blood Cell Types |
Anti-A |
Anti-B |
|
O |
− |
− |
|
A |
+ |
− |
|
B |
− |
+ |
|
AB |
+ |
+ |
Rh Blood Types
Along with the O-A-B blood type system, the Rh blood type system is also important when transfusing blood. The major difference between the O-A-B system and the Rh system is the following: In the O-A-B system, the plasma agglutinins responsible for causing transfusion reactions develop spontaneously, whereas in the Rh system, spontaneous agglutinins almost never occur. Instead, the person must first be massively exposed to an Rh antigen, such as by transfusion of blood containing the Rh antigen, before enough agglutinins to cause a significant transfusion reaction will develop.
Rh Antigens—“Rh-Positive” and “Rh-Negative” People
There are six common types of Rh antigens, each of which is called an Rh factor. These types are designated C, D, E, c, d, and e. A person who has a C antigen does not have the c antigen, but the person missing the C antigen always has the c antigen. The same is true for the D-d and E-e antigens. Also, because of the manner of inheritance of these factors, each person has one of each of the three pairs of antigens.
The type D antigen is widely prevalent in the population and considerably more antigenic than the other Rh antigens. Anyone who has this type of antigen is said to be Rh positive, whereas a person who does not have type D antigen is said to be Rh negative. However, it must be noted that even in Rh-negative people, some of the other Rh antigens can still cause transfusion reactions, although the reactions are usually much milder.
About 85 percent of all white people are Rh positive and 15 percent, Rh negative. In American blacks, the percentage of Rh-positives is about 95 percent, whereas in African blacks, it is virtually 100 percent.
Rh Immune Response
Formation of Anti-Rh Agglutinins
When red blood cells containing Rh factor are injected into a person whose blood does not contain the Rh factor—that is, into an Rh-negative person—anti-Rh agglutinins develop slowly, reaching maximum concentration of agglutinins about 2 to 4 months later. This immune response occurs to a much greater extent in some people than in others. With multiple exposures to the Rh factor, an Rh-negative person eventually becomes strongly “sensitized” to Rh factor.
Characteristics of Rh Transfusion Reactions
If an Rh-negative person has never before been exposed to Rh-positive blood, transfusion of Rh-positive blood into that person will likely cause no immediate reaction. However, anti-Rh antibodies can develop in sufficient quantities during the next 2 to 4 weeks to cause agglutination of those transfused cells that are still circulating in the blood. These cells are then hemolyzed by the tissue macrophage system. Thus, a delayed transfusion reaction occurs, although it is usually mild. On subsequent transfusion of Rh-positive blood into the same person, who is now already immunized against the Rh factor, the transfusion reaction is greatly enhanced and can be immediate and as severe as a transfusion reaction caused by mismatched type A or B blood.
Erythroblastosis Fetalis (“Hemolytic Disease of the Newborn”)
Erythroblastosis fetalis is a disease of the fetus and newborn child characterized by agglutination and phagocytosis of the fetus’s red blood cells. In most instances of erythroblastosis fetalis, the mother is Rh negative and the father Rh positive. The baby has inherited the Rh-positive antigen from the father, and the mother develops anti-Rh agglutinins from exposure to the fetus’s Rh antigen. In turn, the mother’s agglutinins diffuse through the placenta into the fetus and cause red blood cell agglutination.
Incidence of the Disease
An Rh-negative mother having her first Rh-positive child usually does not develop sufficient anti-Rh agglutinins to cause any harm. However, about 3 percent of second Rh-positive babies exhibit some signs of erythroblastosis fetalis; about 10 percent of third babies exhibit the disease; and the incidence rises progressively with subsequent pregnancies.
Effect of the Mother’s Antibodies on the Fetus
After anti-Rh antibodies have formed in the mother, they diffuse slowly through the placental membrane into the fetus’s blood. There they cause agglutination of the fetus’s blood. The agglutinated red blood cells subsequently hemolyze, releasing hemoglobin into the blood. The fetus’s macrophages then convert the hemoglobin into bilirubin, which causes the baby’s skin to become yellow (jaundiced). The antibodies can also attack and damage other cells of the body.
Clinical Picture of Erythroblastosis
The jaundiced, erythroblastotic newborn baby is usually anemic at birth, and the anti-Rh agglutinins from the mother usually circulate in the infant’s blood for another 1 to 2 months after birth, destroying more and more red blood cells.
The hematopoietic tissues of the infant attempt to replace the hemolyzed red blood cells. The liver and spleen become greatly enlarged and produce red blood cells in the same manner that they normally do during the middle of gestation. Because of the rapid production of red cells, many early forms of red blood cells, including many nucleated blastic forms, are passed from the baby’s bone marrow into the circulatory system, and it is because of the presence of these nucleated blastic red blood cells that the disease is called erythroblastosis fetalis.
Although the severe anemia of erythroblastosis fetalis is usually the cause of death, many children who barely survive the anemia exhibit permanent mental impairment or damage to motor areas of the brain because of precipitation of bilirubin in the neuronal cells, causing destruction of many, a condition called kernicterus.
Treatment of the Erythroblastotic Neonate
One treatment for erythroblastosis fetalis is to replace the neonate’s blood with Rh-negative blood. About 400 milliliters of Rh-negative blood is infused over a period of 1.5 or more hours while the neonate’s own Rh-positive blood is being removed. This procedure may be repeated several times during the first few weeks of life, mainly to keep the bilirubin level low and thereby prevent kernicterus. By the time these transfused Rh-negative cells are replaced with the infant’s own Rh-positive cells, a process that requires 6 or more weeks, the anti-Rh agglutinins that had come from the mother will have been destroyed.
Prevention of Erythroblastosis Fetalis
The D antigen of the Rh blood group system is the primary culprit in causing immunization of an Rh-negative mother to an Rh-positive fetus. In the 1970s, a dramatic reduction in the incidence of erythroblastosis fetalis was achieved with the development of Rh immunoglobulin globin, an anti-D antibody that is administered to the expectant mother starting at 28 to 30 weeks of gestation. The anti-D antibody is also administered to Rh-negative women who deliver Rh-positive babies to prevent sensitization of the mothers to the D antigen. This greatly reduces the risk of developing large amounts of D antibodies during the second pregnancy.
The mechanism by which Rh immunoglobulin globin prevents sensitization of the D antigen is not completely understood, but one effect of the anti-D antibody is to inhibit antigen-induced B lymphocyte antibody production in the expectant mother. The administered anti-D antibody also attaches to D-antigen sites on Rh-positive fetal red blood cells that may cross the placenta and enter the circulation of the expectant mother, thereby interfering with the immune response to the D antigen.
Transfusion Reactions Resulting from Mismatched Blood Types
If donor blood of one blood type is transfused into a recipient who has another blood type, a transfusion reaction is likely to occur in which the red blood cells of the donor blood are agglutinated. It is rare that the transfused blood causes agglutination of the recipient’s cells, for the following reason: The plasma portion of the donor blood immediately becomes diluted by all the plasma of the recipient, thereby decreasing the titer of the infused agglutinins to a level usually too low to cause agglutination. Conversely, the small amount of infused blood does not significantly dilute the agglutinins in the recipient’s plasma. Therefore, the recipient’s agglutinins can still agglutinate the mismatched donor cells.
As explained earlier, all transfusion reactions eventually cause either immediate hemolysis resulting from hemolysins or later hemolysis resulting from phagocytosis of agglutinated cells. The hemoglobin released from the red cells is then converted by the phagocytes into bilirubin and later excreted in the bile by the liver, as discussed in Chapter 70. The concentration of bilirubin in the body fluids often rises high enough to cause jaundice—that is, the person’s internal tissues and skin become colored with yellow bile pigment. But if liver function is normal, the bile pigment will be excreted into the intestines by way of the liver bile, so jaundice usually does not appear in an adult person unless more than 400 milliliters of blood is hemolyzed in less than a day.
Acute Kidney Shutdown After Transfusion Reactions
One of the most lethal effects of transfusion reactions is kidney failure, which can begin within a few minutes to few hours and continue until the person dies of renal failure.
The kidney shutdown seems to result from three causes: First, the antigen-antibody reaction of the transfusion reaction releases toxic substances from the hemolyzing blood that cause powerful renal vasoconstriction. Second, loss of circulating red cells in the recipient, along with production of toxic substances from the hemolyzed cells and from the immune reaction, often causes circulatory shock. The arterial blood pressure falls very low, and renal blood flow and urine output decrease. Third, if the total amount of free hemoglobin released into the circulating blood is greater than the quantity that can bind with “haptoglobin” (a plasma protein that binds small amounts of hemoglobin), much of the excess leaks through the glomerular membranes into the kidney tubules. If this amount is still slight, it can be reabsorbed through the tubular epithelium into the blood and will cause no harm; if it is great, then only a small percentage is reabsorbed. Yet water continues to be reabsorbed, causing the tubular hemoglobin concentration to rise so high that the hemoglobin precipitates and blocks many of the kidney tubules. Thus, renal vasoconstriction, circulatory shock, and renal tubular blockage together cause acute renal shutdown. If the shutdown is complete and fails to resolve, the patient dies within a week to 12 days, as explained in Chapter 31, unless treated with an artificial kidney.
Transplantation of Tissues and Organs
Most of the different antigens of red blood cells that cause transfusion reactions are also widely present in other cells of the body, and each bodily tissue has its own additional complement of antigens. Consequently, foreign cells transplanted anywhere into the body of a recipient can produce immune reactions. In other words, most recipients are just as able to resist invasion by foreign tissue cells as to resist invasion by foreign bacteria or red cells.
Autografts, Isografts, Allografts, and Xenografts
A transplant of a tissue or whole organ from one part of the same animal to another part is called an autograft; from one identical twin to another, an isograft; from one human being to another or from any animal to another animal of the same species, an allograft; and from a lower animal to a human being or from an animal of one species to one of another species, a xenograft.
Transplantation of Cellular Tissues
In the case of autografts and isografts, cells in the transplant contain virtually the same types of antigens as in the tissues of the recipient and will almost always continue to live normally and indefinitely if an adequate blood supply is provided.
At the other extreme, in the case of xenografts, immune reactions almost always occur, causing death of the cells in the graft within 1 day to 5 weeks after transplantation unless some specific therapy is used to prevent the immune reactions.
Some of the different cellular tissues and organs that have been transplanted as allografts, either experimentally or for therapeutic purposes, from one person to another are skin, kidney, heart, liver, glandular tissue, bone marrow, and lung. With proper “matching” of tissues between persons, many kidney allografts have been successful for at least 5 to 15 years, and allograft liver and heart transplants for 1 to 15 years.
Attempts to Overcome Immune Reactions in Transplanted Tissue
Because of the extreme potential importance of transplanting certain tissues and organs, serious attempts have been made to prevent antigen-antibody reactions associated with transplantation. The following specific procedures have met with some degrees of clinical or experimental success.
Tissue Typing—the Human Leukocyte Antigen (HLA) Complex of Antigens
The most important antigens for causing graft rejection are a complex called the HLA antigens. Six of these antigens are present on the tissue cell membranes of each person, but there are about 150 different HLA antigens to choose from. Therefore, this represents more than a trillion possible combinations. Consequently, it is virtually impossible for two persons, except in the case of identical twins, to have the same six HLA antigens. Development of significant immunity against any one of these antigens can cause graft rejection.
The HLA antigens occur on the white blood cells, as well as on the tissue cells. Therefore, tissue typing for these antigens is done on the membranes of lymphocytes that have been separated from the person’s blood. The lymphocytes are mixed with appropriate antisera and complement; after incubation, the cells are tested for membrane damage, usually by testing the rate of trans-membrane uptake by the lymphocytic cells of a special dye.
Some of the HLA antigens are not severely antigenic, for which reason a precise match of some antigens between donor and recipient is not always essential to allow allograft acceptance. Therefore, by obtaining the best possible match between donor and recipient, the grafting procedure has become far less hazardous. The best success has been with tissue-type matches between siblings and between parent and child. The match in identical twins is exact, so transplants between identical twins are almost never rejected because of immune reactions.
Prevention of Graft Rejection by Suppressing the Immune System
If the immune system were completely suppressed, graft rejection would not occur. In fact, in a person who has serious depression of the immune system, grafts can be successful without the use of significant therapy to prevent rejection. But in the normal person, even with the best possible tissue typing, allografts seldom resist rejection for more than a few days or weeks without use of specific therapy to suppress the immune system. Furthermore, because the T cells are mainly the portion of the immune system important for killing grafted cells, their suppression is much more important than suppression of plasma antibodies. Some of the therapeutic agents that have been used for this purpose include the following:
1. Glucocorticoid hormones isolated from adrenal cortex glands (or drugs with glucocorticoid-like activity), which suppress the growth of all lymphoid tissue and, therefore, decrease formation of antibodies and T cells.
2. Various drugs that have a toxic effect on the lymphoid system and, therefore, block formation of antibodies and T cells, especially the drug azathioprine.
3. Cyclosporine, which has a specific inhibitory effect on the formation of helper T cells and, therefore, is especially efficacious in blocking the T-cell rejection reaction. This has proved to be one of the most valuable of all the drugs because it does not depress some other portions of the immune system.
Use of these agents often leaves the person unprotected from infectious disease; therefore, sometimes bacterial and viral infections become rampant. In addition, the incidence of cancer is several times as great in an immunosuppressed person, presumably because the immune system is important in destroying many early cancer cells before they can begin to proliferate.
Transplantation of living tissues in human beings has had important success mainly because of the development of drugs that suppress the responses of the immune system. With the introduction of improved immunosuppressive agents, successful organ transplantation has become much more common. The current approach to immunosuppressive therapy attempts to balance acceptable rates of rejection with moderation in the adverse effects of immunosuppressive drugs.
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