K93 Na1 Full Version Blood Elements Group
Clotilde Wilks
The cells that make up the body's tissues and organs are covered with surface markers, orantigens. Red blood cells are no different. This chapter will describe the types of redblood cell antigen and explain why they are so important in medicine today.
The membrane of each red blood cell contains millions of antigens that are ignored by theimmune system. However, when patients receive blood transfusions, their immune systems willattack any donor red blood cells that contain antigens that differ from their self-antigens.Therefore, ensuring that the antigens of transfused red blood cells match those of thepatient's red blood cells is essential for a safe blood transfusion.
For example, the antigens of the ABO blood group are sugars. They are produced by a seriesof reactions in which enzymes catalyze the transfer of sugar units. A person's DNAdetermines the type of enzymes they have, and, therefore, the type of sugar antigens thatend up on their red blood cells.
In contrast, the antigens of the Rh blood group are proteins. A person's DNA holds theinformation for producing the protein antigens. The RhD gene encodes the D antigen, which isa large protein on the red blood cell membrane. Some people have a version of the gene thatdoes not produce D antigen, and therefore the RhD protein is absent from their red bloodcells.
The figure below shows the red blood cell membrane and some of the blood group antigensattached to it. Aside from the sugar (glycan or carbohydrate) antigens, the red blood cellmembrane contains three types of protein that carry blood group antigens: single-passproteins, multi-pass proteins, and glycosylphosphatidylinositol (GPI)-linked proteins. Clickon the blood groups to find out more about the antigens that define it.
The antigens expressed on the red blood cell determine an individual's blood group. Themain two blood groups are called ABO (with blood types A, B, AB, and O) and Rh (with RhD-positive or Rh D-negative blood types).
The functions of many of the blood group antigens are not known, and if they are missingfrom the red blood cell membrane, there is no ill effect. This suggests that if the bloodgroup antigens used to have a function, e.g., one particular blood group antigen made redblood cells more resistant to invasion from a parasite, it is no longer relevant today.
But the presence or absence of red blood cell antigens becomes extremely important whenblood from different people mixes, e.g., when a patient receives a blood transfusion from ablood bank. This also happens when a mother becomes pregnant because during labor, a smallamount of fetal blood enters her circulation. In these circumstances, exposure to theforeign 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 twopeople mix, but the danger can be minimized. Before a blood transfusion takes place, theblood to be donated must be "typed and cross matched" with the patient's blood to ensureimmune compatibility (see Chapter 3). Inpregnancy, the risk of the mother's immune system attacking the foreign antigens present onher fetus' red blood cells is prevented by giving the mother antibodies to cover fetal redblood cell antigens and removing them from the mother's circulation before her immune cellsfind them (see Chapter 4).
The distribution of the four ABO blood types, A, B, AB, and O, varies in populationsthroughout the world. It is determined by the frequency of the three alleles of the ABO genein 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 bycollecting 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 type O (57%) was found in Hispanic donors (a group that includesdonors of Mexican, Puerto Rican, and Cuban descent). The next highest percentage of type Owas found in North American Indian (55%) and black (50%) donors.
People with blood type O are said to be "universal donors" because their blood iscompatible with all ABO blood types. It is also the most common blood type in populationsaround the world, including the USA (1) andWestern Europe (2, 3). Among indigenouspopulations of Central and South America, the frequency of O blood type is extremely high,approaching 100%. It is also high among Australian aborigines.
Type A is common in Central and Eastern Europe. In countries such as Austria, Denmark,Norway, and Switzerland, about 45-50% of the population have this blood type, whereasabout 40% of Poles and Ukrainians do so.
Traditionally, newly discovered red blood cell antigens were named alphabetically (e.g.ABO, MNS, P) or were named for the first person who produced antibody against them (e.g.Duffy, Diego). In 1980, The International Society of BloodTransfusion (ISBT) Working Party on Terminology for Red Cell Surface antigenswas formed to create a standard for blood group terminology. Under this terminology, eachblood group antigen has a number, and it belongs to a blood group system, a collection, or aseries (4).
A blood group system contains antigens controlled by a single gene (or by multipleclosely linked loci), and the system is genetically distinct. At the time of writing,there are 22 blood group systems, including the ABO, Rh, and Kell blood groups whichcontain antigens that can provoke the most severe transfusion reactions.
Each blood group antigen is assigned a six-digit number by the ISBT. The first threedigits represent the blood group (e.g., ABO is 001, Rh is 004), and the last threeidentify the antigen in the order it was discovered. For example, for ABO, the A antigenwas the first to be discovered and has the number 001.001 whereas the B antigen was nextand is designated 001.002.
A collection contains antigens that are related in some way, e.g., by genetics orbiochemistry, but they do not meet the criteria to form a blood group. Once a collectionof antigens can be proven to be genetically distinct, they are given the status of a bloodgroup. At the time of writing, there are six collections of antigens.
Red cell antigens that do not fit into a blood group or a collection are sorted into twoseries: if they are rare (frequency of less than 1%), they are placed in the 700 series,if they are common (frequency greater than 90%), they are placed in the 901 series. At thetime of writing, there are 22 antigens in the 700 series and 11 antigens in the 901series.
The average human adult has more than 5 liters (6 quarts) of blood in his or herbody. Blood carries oxygen and nutrients to living cells and takes away their wasteproducts. It also delivers immune cells to fight infections and contains plateletsthat can form a plug in a damaged blood vessel to prevent blood loss.
Through the circulatory system, blood adapts to the body's needs. When you areexercising, your heart pumps harder and faster to provide more blood and henceoxygen to your muscles. During an infection, the blood delivers more immune cells tothe site of infection, where they accumulate to ward off harmful invaders.
All of these functions make blood a precious fluid. Each year in the USA, 30 millionunits of blood components are transfused to patients who need them. Blood is deemedso precious that is also called "red gold" because the cells and proteins itcontains can be sold for more than the cost of the same weight in gold.
The straw-colored fluid that forms the top layer is called plasma and forms about 60%of blood. The middle white layer is composed of white blood cells (WBCs) andplatelets, and the bottom red layer is the red blood cells (RBCs). These bottom twolayers of cells form about 40% of the blood.
A sample of blood can be further separated into its individual components by spinningthe sample in a centrifuge. The force of the spinning causes denser elements tosink, and further processing enables the isolation of a particular protein or theisolation of a particular type of blood cell. With the use of this method,antibodies and clotting factors can be harvested from the plasma to treat immunedeficiencies and bleeding disorders, respectively. Likewise, RBCs can be harvestedfor blood transfusion.
Every second, 2-3 million RBCs areproduced in the bone marrow and released into the circulation. Also known aserythrocytes, RBCs are the most common type of cell found in the blood, with eachcubic millimeter of blood containing 4-6 million cells. With a diameter of only 6µm, RBCs are small enough to squeeze through the smallest blood vessels.They circulate around the body for up to 120 days, at which point the old or damagedRBCs are removed from the circulation by specialized cells (macrophages) in thespleen and liver.
If a patient has a low level of hemoglobin, a condition called anemia, they mayappear pale because hemoglobin gives RBCs, and hence blood, their red color. Theymay also tire easily and feel short of breath because of the essential role ofhemoglobin in transporting oxygen from the lungs to wherever it is needed around thebody.
Despite their differences in appearance, all of the various types of WBCs have a rolein the immune response. They circulate in the blood until they receive a signal thata part of the body is damaged. Signals include interleukin 1 (IL-1), a moleculesecreted by macrophages that contributes to the fever of infections, and histamine,which is released by circulating basophils and tissue mast cells, and contributes toallergic reactions. In response to these signals, the WBCs leave the blood vessel bysqueezing through holes in the blood vessel wall. They migrate to the source of thesignal and help begin the healing process.
Neutrophils are also known aspolymorphonuclear cells because they contain a nucleus whose shape (morph) isirregular and contains many (poly) lobes. They also belong to a group of WBCsknown as granulocytes because their cytoplasm is dotted with granules thatcontain enzymes that helps them digest pathogens.
aa06259810