Describe the components and functions of blood, explain blood cell production and hemostasis, and use antigens, antibodies and Punnett squares to explain ABO and Rh blood types

Learning goal: Explain the function of blood and of each component of whole blood (plasma and the formed elements: red blood cells, white blood cells and platelets); describe how blood cells and platelets are produced and how hemostasis stops bleeding; explain the relationship between red-cell antigens, plasma antibodies and ABO/Rh blood type, including agglutination and transfusion compatibility; use Punnett squares to explain how blood types are inherited; and analyze homeostatic imbalances of blood such as sickle cell anemia, hemophilia, deep vein thrombosis, leukemia and lymphoma.

Blood is a connective tissue with three jobs. Blood is a fluid connective tissue: cells and cell fragments suspended in a liquid matrix. Its primary functions fall into three groups. Transportation: it carries oxygen from the lungs to the tissues, carbon dioxide back to the lungs, nutrients from the digestive tract, wastes to the kidneys and liver, and hormones from glands to their target cells. Protection: white blood cells and antibodies defend against infectious disease and foreign cells, and platelets and clotting proteins prevent blood loss. Regulation: blood distributes heat, helps hold the pH of body fluids near 7.35 to 7.45 with its buffers, and its dissolved proteins help keep the right amount of water in the vessels.

The components of whole blood. If a tube of blood treated to prevent clotting is spun in a centrifuge, it separates into layers. The top layer, typically a little more than half of the volume, is plasma, the liquid portion and the most abundant component of blood. Plasma is mostly water, and it transports dissolved nutrients such as glucose and amino acids, wastes such as urea, hormones, antibodies, salts, and plasma proteins such as albumin (which holds water in the vessels) and fibrinogen (a clotting protein). The bottom layer, usually a little under half of the volume, is packed red blood cells (erythrocytes). Between them lies a thin pale layer, the buffy coat, of white blood cells (leukocytes) and platelets (thrombocytes). The red cells, white cells and platelets together are called the formed elements. The percentage of the volume that is red cells is the hematocrit; it varies with sex, altitude and health, which is why a lab reports it against a reference range rather than a single number.

Structure fits function in each formed element. A red blood cell is a small, flexible, biconcave disc. A mature human red cell has no nucleus and no mitochondria, which leaves room for about a quarter of a billion molecules of hemoglobin, and it makes its energy without using the oxygen it carries. Each hemoglobin molecule is made of four protein chains, each holding an iron-containing heme group, and each heme binds one oxygen molecule, so one hemoglobin carries up to four oxygen molecules. The biconcave shape gives a large surface for gas diffusion and lets the cell fold to pass through capillaries narrower than itself. Red cells last roughly four months before the spleen and liver remove them. White blood cells have nuclei and come in five kinds: neutrophils and monocytes engulf microbes by phagocytosis, lymphocytes make antibodies or kill infected cells, eosinophils act against parasites and in allergies, and basophils release histamine during inflammation. Platelets are not whole cells; they are fragments pinched off from large bone marrow cells, and they bind together where a blood vessel is damaged.

Production of blood cells. All formed elements come from hematopoietic stem cells in red bone marrow. A stem cell divides, and its daughters differentiate along separate lines: one line becomes red cells, another becomes the granular white cells and monocytes, another becomes lymphocytes, and another becomes the large cells that shed platelets. Red cell production is under negative feedback: when oxygen delivery to the kidneys falls, they release the hormone erythropoietin, which stimulates the marrow to make more red cells, which raises oxygen delivery and reduces the signal.

Hemostasis: stopping bleeding in three overlapping steps. First, vascular spasm: the smooth muscle of a damaged vessel contracts and narrows it. Second, the platelet plug: platelets stick to the exposed collagen of the torn wall, release chemicals that activate more platelets, and pile up into a plug, a positive feedback loop that ends when the leak is sealed. Third, coagulation: a cascade of clotting factors in the plasma ends with the enzyme thrombin converting soluble fibrinogen into insoluble fibrin threads, which weave through the plug and trap red cells to form a firm clot. Later, the clot shrinks and is dissolved as the vessel heals.

Blood type is a statement about antigens. An antigen is a molecule the immune system can recognize; an antibody is a protein that binds a specific antigen. The ABO blood type is determined by the presence or absence of the A and B antigens on the surface of red blood cells. Type A cells carry A antigen, type B carry B, type AB carry both and type O carry neither. Blood type antibodies are found in the plasma, and a person's plasma contains antibodies against whichever ABO antigen their own cells lack: type A plasma has anti-B, type B has anti-A, type O has both anti-A and anti-B, and type AB has neither. A separate antigen, usually called the D antigen, is present (Rh positive) or absent (Rh negative) on red cells and determines the Rh factor. Unlike anti-A and anti-B, anti-Rh antibodies are not normally present; an Rh-negative person makes them only after being exposed to Rh-positive cells.

Agglutination and transfusion, worked fully. When an antibody meets cells carrying its antigen, each antibody can bind two cells at once, so the cells clump: agglutination. In a transfusion, the key question is whether the recipient's plasma antibodies will attack the donor's red cells. Suppose a patient is type A negative. Their plasma contains anti-B, and, because they are Rh negative, they may make anti-Rh if exposed. Donor type O negative cells carry no A, B or Rh antigen, so nothing in the patient's plasma binds them: compatible. Donor type B cells carry B antigen, which the patient's anti-B would clump: incompatible, and clumped cells can block small vessels and burst, releasing hemoglobin that damages the kidneys. Donor A positive cells carry A antigen, which this patient's plasma has no antibody against, but their Rh antigen could sensitize the patient, so A negative or O negative is the choice whenever such units are available. That matters most for two groups: anyone who already has anti-D from an earlier exposure, whose plasma would attack Rh-positive cells, and girls and women who may later become pregnant, because anti-D made now could cross the placenta and destroy the red cells of a future Rh-positive fetus. In a life-threatening emergency with no Rh-negative blood available, hospitals may give Rh-positive red cells to some Rh-negative patients, accepting the risk of sensitization and planning follow-up. Reasoning this way explains the shorthand terms: for red cells, O negative is the universal donor (no A, B or Rh antigens to be attacked) and AB positive is the universal recipient (no anti-A, anti-B or anti-Rh in the plasma). Real transfusion services also cross-match each unit, because many other red cell antigens exist.

Inheritance of blood types. The ABO gene has three common alleles: I^A and I^B are codominant, and i is recessive to both. Genotypes I^A I^A and I^A i give type A; I^B I^B and I^B i give type B; I^A I^B gives type AB; and ii gives type O. For Rh, a simplified model uses one gene with a dominant allele D (Rh positive) and a recessive allele d (Rh negative). A Punnett square shows that two parents of genotypes I^A i and I^B i can have children of all four ABO types, each with probability 1/4.

Homeostatic imbalances. Sickle cell anemia results from a single change in a hemoglobin gene; the altered hemoglobin makes red cells stiff and sickle-shaped when oxygen is low, so they block capillaries and are destroyed early. Carriers with one copy are partly protected against severe malaria, which is why the allele is common among descendants of people from regions where malaria is or was common; it is linked to that history of malaria, not to any one race. Hemophilia is an inherited lack of a clotting factor, so coagulation is slow. Deep vein thrombosis is an unwanted clot in a deep vein, usually of the leg, which can break off and lodge in the lungs. Leukemia is a cancer of the blood-forming cells of the marrow that produces large numbers of abnormal white cells, and lymphoma is a cancer of lymphocytes that usually grows in lymph nodes and other lymphatic tissue.

Investigation: simulated blood typing. Use a commercial simulated blood-typing kit (no real blood). Add a drop of simulated anti-A serum and a drop of anti-B serum to two wells of each unknown sample, and anti-Rh to a third. Record clumping (agglutination) or no clumping. Clumping with anti-A only means type A; with anti-B only, B; with both, AB; with neither, O; clumping with anti-Rh means Rh positive. Then write transfusion guidelines for each sample. The model shows the antigen-antibody logic but cannot show the other red cell antigens, the time course of a reaction, or its effects on the kidneys.

Misconceptions. "Type O blood has no antibodies" is backwards: type O cells have no A or B antigens, but type O plasma has both anti-A and anti-B. "Rh-negative people always have anti-Rh antibodies" is wrong; they make them only after exposure.

Worked example 1 (transfusion compatibility): A patient is type B positive. Which donor red cell types are safe? The patient's plasma contains anti-A, and because the patient is Rh positive, Rh antigens are not foreign. Donor cells must lack A antigen: B positive, B negative, O positive and O negative all qualify. A and AB cells carry A antigen and would agglutinate. Answer: B+, B-, O+, O-.

Worked example 2 (Punnett square): Parents are type A (genotype I^A i) and type B (genotype I^B i). Gametes: I^A or i from one parent, I^B or i from the other. The four boxes: I^A I^B (AB), I^A i (A), I^B i (B), ii (O). Each type has probability 1/4, or 25 percent. So two parents with types A and B can have a type O child, but only if both carry the i allele.

Worked example 3 (Rh inheritance): Two Rh-positive parents have an Rh-negative child. Under the simplified one-gene model, Rh negative is dd, so the child received d from each parent. Each parent must therefore be Dd. For a Dd by Dd cross, the probability of another Rh-negative child is 1/4.

Worked example 4 (lab data analysis): A hypothetical patient reports fatigue and shortness of breath. The report shows a hematocrit and hemoglobin below their reference ranges, with red cells of normal shape, and normal white cell and platelet counts. Low red cells or hemoglobin means reduced oxygen-carrying capacity: this pattern is anemia. If the red cells had been sickle-shaped and the family history showed the trait on both sides, sickle cell anemia would be the better explanation; very high counts of abnormal white cells would instead point toward leukemia. The reasoning follows the variable that is out of range; an actual diagnosis is made by a clinician with further tests.

Representation task: Draw one labeled red blood cell for each of the four ABO types and both Rh factors, showing the antigens on the cell and the antibodies in the surrounding plasma. Then draw a centrifuged tube of whole blood, label plasma, buffy coat and red cells, and write one function for each component and each formed element.

Independent performance: Carry out the simulated blood-typing investigation and write transfusion guidelines a medical assistant could use. Success criteria: (a) the agglutination result for anti-A, anti-B and anti-Rh is recorded for every sample and each blood type is justified from those results; (b) a compatibility table lists which donor red cell types each recipient can receive, with a reason in terms of antigens and antibodies; (c) universal donor and universal recipient are explained, including why cross-matching is still done; (d) a Punnett square shows the possible blood types of the children of two stated genotypes; (e) one paragraph explains the three steps of hemostasis and names which one fails in hemophilia; (f) the limitations of the simulated kit are stated.