Microbes and the human body – Antibody–antigen complex
The surface of the red blood cell (RBC) is coated with antigens (sugars This topic will review clinically relevant RBC antigens and respective antibodies, . WITH SPECIAL REFERENCE TO ITS RELATIONSHIP WITH THE P.  Regular occurrence of ABO antibodies with reciprocal relation of antigen on red cells and the antibody in serum not only helped in confirmation of the ABO. Antibodies, also called immunoglobulins, are proteins manufactured by the body that help How is the relationship between an antibody and an antigen complementary? . What exactly do antibody and antigens present on RBC signifies?.
The reaction between red cells and corresponding antibodies usually results in clumping— agglutination —of the red cells; therefore, antigens on the surfaces of these red cells are often referred to as agglutinogens.
Antibodies are part of the circulating plasma proteins known as immunoglobulins, which are classified by molecular size and weight and by several other biochemical properties. Most blood group antibodies are found either on immunoglobulin G IgG or immunoglobulin M IgM molecules, but occasionally the immunoglobulin A IgA class may exhibit blood group specificity.
Naturally occurring antibodies are the result of immunization by substances in nature that have structures similar to human blood groups. These antibodies are present in an individual despite the fact that there has been no previous exposure to the corresponding red cell antigens—for example, anti-A in the plasma of people of blood group B and anti-B in the plasma of people of blood group A.
Immune antibodies are evoked by exposure to the corresponding red cell antigen.
The combination of pregnancy and transfusion is a particularly potent stimulus. Individual blood group antigens vary in their antigenic potential; for example, some of the antigens belonging to the Rh and ABO systems are strongly immunogenic i. The blood group antigens are not restricted solely to red cells or even to hematopoietic tissues.
The antigens of the ABO system are widely distributed throughout the tissues and have been unequivocally identified on platelets and white cells both lymphocytes and polymorphonuclear leukocytes and in skin, the epithelial lining cells of the gastrointestinal tractthe kidney, the urinary tract, and the lining of the blood vessels.
Evidence for the presence of the antigens of other blood group systems on cells other than red cells is less well substantiated. Among the red cell antigens, only those of the ABO system are regarded as tissue antigens and therefore need to be considered in organ transplantation. Chemistry of the blood group substances The exact chemical structure of some blood groups has been identified, as have the gene products i.
Blood group antigens are present on glycolipid and glycoprotein molecules of the red cell membrane. The carbohydrate chains of the membrane glycolipids are oriented toward the external surface of the red cell membrane and carry antigens of the ABOHh, Ii, and P systems.
Red cell antigens: Structure and function
Glycoproteinswhich traverse the red cell membrane, have a polypeptide backbone to which carbohydrates are attached. Another integral membrane glycoprotein, glycophorin Acontains large numbers of sialic acid molecules and MN blood group structures; another, glycophorin Bcontains Ss and U antigens. The genes responsible for inheritance of ABH and Lewis antigens are glycosyltransferases a group of enzymes that catalyze the addition of specific sugar residues to the core precursor substance.
For example, the H gene codes for the production of a specific glycosyltransferase that adds l-fucose to a core precursor substance, resulting in the H antigen; the Le gene codes for the production of a specific glycosyltransferase that adds l-fucose to the same core precursor substance, but in a different place, forming the Lewis antigen; the A gene adds N-acetyl-d-galactosamine H must be presentforming the A antigen; and the B gene adds d-galactose H must be presentforming the B antigen.
The P system is analogous to the ABH and Lewis blood groups in the sense that the P antigens are built by the addition of sugars to precursor globoside and paragloboside glycolipids, and the genes responsible for these antigens must produce glycosyltransferase enzymes. The genes that code for MNSs glycoproteins change two amino acids in the sequence of the glycoprotein to account for different antigen specificities.
They are produced by a series of reactions in which enzymes catalyze the transfer of sugar units. A person's DNA determines the type of enzymes they have, and, therefore, the type of sugar antigens that end up on their red blood cells.
In contrast, the antigens of the Rh blood group are proteins. A person's DNA holds the information for producing the protein antigens. The RhD gene encodes the D antigen, which is a large protein on the red blood cell membrane.
Some people have a version of the gene that does not produce D antigen, and therefore the RhD protein is absent from their red blood cells. The figure below shows the red blood cell membrane and some of the blood group antigens attached to it. Aside from the sugar glycan or carbohydrate antigens, the red blood cell membrane contains three types of protein that carry blood group antigens: Click on the blood groups to find out more about the antigens that define it.
Red blood cell antigens determine your blood group The antigens expressed on the red blood cell determine an individual's blood group. The functions of many of the blood group antigens are not known, and if they are missing from the red blood cell membrane, there is no ill effect. This suggests that if the blood group antigens used to have a function, e. But the presence or absence of red blood cell antigens becomes extremely important when blood from different people mixes, e.
This also happens when a mother becomes pregnant because during labor, a small amount of fetal blood enters her circulation. In these circumstances, exposure to the foreign antigens on the red blood cells can trigger immune reactions.
It is not possible to completely remove the danger of adverse reactions when blood from two people mix, but the danger can be minimized. Before a blood transfusion takes place, the blood to be donated must be "typed and cross matched" with the patient's blood to ensure immune compatibility see Chapter 3. In pregnancy, the risk of the mother's immune system attacking the foreign antigens present on her fetus' red blood cells is prevented by giving the mother antibodies to cover fetal red blood cell antigens and removing them from the mother's circulation before her immune cells find them see Chapter 4.
It is determined by the frequency of the three alleles of the ABO gene in different populations. Blood type O is the most common worldwide, followed by group A. Group B is less common, and group AB is the least common. The frequencies of ABO and Rh type in the United States were recently examined by collecting data from blood donors over a 10 year period 1.
The charts below summarize the findings for blood type and race: The highest percentage of RhD- was found in white donors It is also the most common blood type in populations around the world, including the USA 1 and Western Europe 2, 3. It is also high among Australian aborigines. The highest frequencies are found in small, unrelated populations.