BLOOD AND HEMATOPOIESIS

Plasma ( 6-1 )

Plasma is the fluid component of blood. Plasma contains salts and organic compounds (including amino acids, lipids, vitamins, proteins and hormones). In the absence of anticoagulants, the cellular elements of blood, together with plasma proteins (mostly fibrinogen ), form a clot in the test tube. The fluid portion is called serum , which is essentially fibrinogen-free plasma.

Red blood cells (RBC; erythrocytes) ( 6-2 )

RBCs, also called erythrocytes (Greek erythros , red; kytos , cell), are non-nucleated, biconcave-shaped cells measuring about 7.8 μm in diameter (unfixed). RBCs lack organelles and consist only of a plasma membrane, its underlying cytoskeleton, hemoglobin and glycolytic enzymes.

RBCs (average number: 4 to 6 × 10 ⁶ per mm ³ ) circulate for 120 days . Senescent RBCs are removed by phagocytosis or destroyed by hemolysis in the spleen. RBCs are replaced in the circulation by reticulocytes , which complete their hemoglobin synthesis and maturation 1 to 2 days after entering the circulation. Reticulocytes account for 1% to 2% of circulating RBCs. RBCs transport oxygen and carbon dioxide and are confined to the circulatory system.

Cytoskeletal and hemoglobin abnormalities of red blood cells ( 6-2 )

The main determinant of anemia in hemolytic anemias is RBC destruction . Normal RBC destruction takes place in the spleen but acute and chronic RBC hemolysis occurs within blood vessels as the result of membrane cytoskeleton, metabolic or hemoglobin abnormalities.

• 1.

  Defects of the membrane cytoskeleton: Elliptocytosis and spherocytosis are alterations in the shape of RBCs caused by defects in the membrane cytoskeleton.

  Elliptocytosis , an autosomal dominant disorder characterized by the presence of oval-shaped RBCs, is caused by defective self-association of spectrin subunits, defective binding of spectrin to ankyrin, protein 4.1 defects and abnormal glycophorin.

  Spherocytosis is also an autosomal dominant condition involving a deficiency in spectrin . RBCs are spherical, of different diameter and many of them lack the typical central pale area seen in normal RBCs.

  The common clinical features of elliptocytosis and spherocytosis are anemia, jaundice (resulting from increased bilirubin production) and splenomegaly (enlargement of the spleen). Splenectomy is usually curative, because the spleen is the primary site responsible for the destruction of elliptocytes and spherocytes.

• 2.

  Metabolic defects: Normal RBCs produce energy to maintain cell shape and electrolyte and water content by metabolizing glucose through the glycolytic (Embden-Meyerhof glycolytic pathway) and pentose phosphate (hexose monophosphate shunt) pathways.

  The most abundant phosphate in RBC is 2,3-diphosphoglycerate (2,3-DPG), involved in the release of oxygen from hemoglobin. The enzyme glucose 6-phosphate dehydrogenase (G6PD) protects the membrane and hemoglobin from oxidant damage, a frequent metabolic cause of intravascular hemolysis caused by severe infection, hepatitis or diabetic ketoacidosis observed in the presence of G6PD deficiency. Pyruvate kinase deficiency is another metabolic defect found in hemolytic anemia.

• 3.

  Hemoglobin defects: Hemoglobin genetic defects (α ₂ βS ₂ ) cause sickle cell anemia and thalassemia (Greek thalassa , sea; observed in populations along the Greek and Italian coasts).

Sickle cell anemia results from a point mutation in which glutamic acid is replaced by valine at the sixth position in the β-globin chain.

Defective hemoglobin (Hb S) tetramers aggregate and polymerize in deoxygenated RBCs, changing the biconcave disk shape into a rigid and less deformable sickle-shaped cell. Hb S leads to severe chronic hemolytic anemia and obstruction of postcapillary venules (see Spleen in Chapter 10 , Immune-Lymphatic System).

Thalassemia syndromes are heritable anemias characterized by defective synthesis of either the αor βchains of the normal hemoglobin tetramer (α ₂ β ₂ ). The specific thalassemia syndromes are designated by the affected globin chain: α -thalassemia and β -thalassemia .

Thalassemia syndromes are defined by anemia caused by defective synthesis of the hemoglobin molecule and hemolysis.

Hemoglobin A1c (glycated hemoglobin) and diabetes mellitus

A valuable clinical indicator of average plasma glucose concentration is the measurement of hemoglobin A1c (glycohemoglobin or glycated [coated] hemoglobin). Glucose links to hemoglobin A1 in a non-enzymatic irreversible reaction.

The normal range for the hemoglobin A1c is between 4% and 5.6%. Hemoglobin A1c levels between 5.7% and 6.4% indicate increased risk of diabetes mellitus and levels of 6.5% or higher indicate diabetes mellitus. Determination of glycated hemoglobin is an efficient way to assess pre-diabetes or diabetes mellitus conditions as well as the treatment to achieve long-term regulation of serum glucose levels to prevent cardiovascular, renal and retinal complications.

Leukocytes

Leukocytes (6 to 10 × 10 ³ per mm ³ ; see Box 6-B ) are categorized as either granulocytes or agranulocytes . Granulocytes contain primary and specific, or secondary, cytoplasmic granules (see Box 6-C ). Agranulocytes contain only primary granules .

Granulocytes

Granulocytes are phagocytic cells with a multilobed nucleus and measuring 12 to 15 μm in diameter. Their average life span varies with cell type. Three types of granulocytes can be distinguished by their cytoplasmic granules:

Neutrophils ( 6-4 )

Neutrophils have a multilobed nucleus. Their cytoplasm contains secondary (specific) and primary granules (see Box 6-C ). In stained smears, neutrophils appear very pale pink. Neutrophils, which constitute 50% to 70% of circulating leukocytes, have a life span of 6 to 7 hours and may live for up to 4 days in the connective tissue.

After leaving the circulation through postcapillary venules, neutrophils act to eliminate opsonized bacteria or limit the extent of an inflammatory reaction in the connective tissue. The mechanism of bacterial opsonization and the relevant role of neutrophils in acute inflammation are discussed in Chapter 10 , Immune-Lymphatic System.

Elastase, defensins and myeloperoxidase are enzymes contained in the primary granules. Lactoferrin, gelatinase, lysozyme and other proteases are seen in secondary granules. Neutrophils have specific receptors for C5a produced by the complement system pathway (see Chapter 10 , Immune-Lymphatic System). L-selectin and integrins in neutrophils bind to endothelial cell ligands intercellular-adhesion molecules 1 and 2 (ICAM-1 and ICAM-2) . These ligands enable the antibacterial and antiinflammatory function of neutrophils in the extravascular space.

Eosinophils ( 6-5 )

Eosinophils have a characteristic bilobed nucleus. Their cytoplasm is filled with large, refractile granules that stain red in blood smears and tissue sections.

The various components of the eosinophil granules and other secretory molecules are listed in Figure 6-5 . Eosinophil degranulation occurs when the cytokines interferon-γ and chemokine ligand 11(CCL11) bind to eosinophil surface receptors. Cytokine interleukin-5 (IL-5) is a major regulator of eosinophil function.

Eosinophils constitute 1% to 5% of circulating leukocytes and have a half-life of about 18 hours. Eosinophils leave the circulation, recruited to the connective tissue by IL-5.

These cells are the first line of defense against parasites and also participate in triggering bronchial asthma (see Chapter 13 , Respiratory System). Eosinophilic esophagitis , associated with eosinophilia, is clinically defined by dysphagia and abdominal pain. This condition is triggered by fungal and insect allergens (see Box 6-D ).

Basophils ( 6-6 )

Basophils contain large, metachromatic cytoplasmic granules that often obscure their bilobed nucleus.

Basophils represent only 1% of circulating leukocytes. Basophils complete their maturation in bone marrow. In contrast, mast cells enter the connective tissue or mucosae as immature cells lacking cytoplasmic granules.

In addition, basophils and mast cells differ in the presence of c-kit receptor and CD49b but share FcεR1. Basophils are c-kit ^- FcεR1 ⁺ CD49b ⁺ ; mast cells are c-kit ⁺ FcεR1 ⁺ CD49b ^- .

Basophils have a short life span (about 60 hours), whereas mast cells survive for weeks and months. The relationship between basophil and mast cell lineages is further discussed in the Hematopoiesis section of this chapter.

Basophils play a role in bronchial asthma and type 2 immunity in response to allergens (allergic skin reaction) and parasitic worms (helminths) .