The Hematopoietic and Lymphoid Systems

Chapter 9 The Hematopoietic and Lymphoid Systems

Marin Nola, MD, PhD, Snježana Dotlić, MD

RED BLOOD CELL DISORDERS

4 List three major groups of anemia according to their etiology and pathogenesis

Anemia due to blood loss

Acute blood loss: massive bleeding from ruptured vessels, wounds, and trauma

Chronic blood loss: bleeding lesions of gastrointestinal tract and gynecologic disturbances

5 How are anemias classified according to the red cell size and shape and their hemoglobin content?

According to red cell size

Microcytic (small)

Normocytic (normal)

Macrocytic (large)

According to degree of hemoglobinization, reflected in the color of red cells

Hypochromic (decreased)

Normochromic (normal)

According to the red cell shape

See Table 9-1 and Fig. 9-1.

TABLE 9-1 Abnormal Red Blood Cell (RBC) Morphology

Abnormality	Features	Significance
Anisocytosis	Variation in RBC size	Nonspecific
Poikilocytosis	Variation in RBC shape	Nonspecific
Target cells	Targetlike appearance	Thalassemia, hemoglobinopathies
Sickle cells	Bipolar (sickle) or hollyleaf	Sickle cell anemia RBCs
Schistocytes	RBC fragments	Microangiopathic hemolytic anemia
Teardrops	Tennis racket RBCs	Myelofibrosis, severe anemias
Spherocytes	Spherical RBCs with dense hemoglobin content	Hereditary spherocytosis, alcoholism
Bite cells	Smooth semicircle taken from one edge	G6PD deficiency

Figure 9-1 Morphology of red blood cells in various forms of anemia.

(Modified from Damjanov I: Pathology for Health Professions, 3rd ed. St. Louis, Mosby, 2006.)

6 Define the main hematologic parameters

The ratio of RBCs to serum expressed in percentages:

In anemia, hematocrit is low, whereas in polycythemia it is high

Normal values: men, 39% to 49%; women, 33% to 43%

MCV of erythrocytes

The average calculated volume of a single RBC (hematocrit/erythrocyte count)

On the basis of MCV, anemias may be defined as microcytic, normocytic, or macrocytic

Normal values: 76 to 100 mm³.

The average content of hemoglobin in each RBC (hemoglobin/erythrocyte count)

According to MCH, anemia can be defined as hypochromic or normochromic

Normal values: 27 to 33 pg

7 Discuss how reticulocyte counts are used in clinical practice

The reticulocyte is a stage of RBC maturation, normally present in both the marrow and the blood. Under normal conditions, the peripheral blood contains less than 1.5% reticulocytes, but in some anemic patients, their number may be increased. Reticulocyte count is used to assess the capacity of the bone marrow to increase RBC production in response to increased demand. On the basis of the reticulocyte count, anemias may be classified as hypoproliferative, normoproliferative, and hyperproliferative.

Hypoproliferative: Patients with anemia caused by defects in erythrocyte proliferation or maturation tend to have low reticulocyte counts. Patients suffering from pernicious anemia have low reticulocyte count, which will, however, increase after vitamin B₁₂ treatment.

Normoproliferative or hyperproliferative: Patients with anemia caused by decreased survival of erythrocytes with a normal bone marrow proliferative response often exhibit increased peripheral blood reticulocytes. If the degree of reticulocytosis is adequate to replace the loss of erythrocytes, the anemia is said to be compensated.

9 Are there any pathologic tissue findings characteristic of anemia in general?

No. Most of the tissue changes caused by anemia are nonspecific. These findings include consequences of prolonged mild ischemia and hypoxia, such as fatty change of liver and heart cells. Prolonged and severe anemia leads to a loss of neurons in the brain, but such a loss cannot be readily recognized in routine histologic sections.

10 Describe the typical hematologic changes following acute massive blood loss

Massive blood loss may result in shock and even death. If the person survives, the blood volume is rapidly restored by entry of water from the interstitial spaces into the circulation. This redistribution of water results in hemodilution, which lowers the hematocrit. The RBC counts performed at this point will usually show anemia, which is typically normocytic and normochromic. Thrombocytosis and leukocytosis may be found in peripheral blood because of mobilization of these cells from the marginal pools. Reactive reticulocytosis occurs a few days later. It is mediated by erythropoietin released from the kidneys in response to low oxygen tension in the blood depleted of RBCs. Reticulocytosis will reach its peak 7 to 10 days after hemorrhage and may be as high as 10% to 15% (i.e., 10 times higher than normal).

11 How does chronic blood loss cause anemia?

Chronic blood loss causes a loss of iron, but significant anemia occurs only when the rate of loss exceeds the regenerative capacity of the bone marrow or when iron reserves are markedly depleted. Anemia of chronic blood loss is common in menstruating women, who lose approximately 70 mL of blood during every menstruation. Frequent pregnancies and childbirth are other important causes of anemia. Gastrointestinal diseases are the most common cause of iron-deficiency anemia in men. Nevertheless, in all patients with iron-deficiency anemia, testing of stools and the urine for occult blood loss should be performed.

13 What is the difference between intravascular and extravascular hemolysis?

Extravascular hemolysis: Most frequently, the premature destruction of erythrocytes occurs within the mononuclear phagocyte system of the spleen and liver. In extravascular hemolysis, erythrocytes are destroyed because:

They are rendered “foreign” by autoantibodies that attach to them in autoimmune hemolytic anemia.

They become less deformable as in sickle cell anemia or hereditary spherocytosis.

In both forms of hemolysis, there is anemia and jaundice. Hemoglobinemia and hemoglobinuria occur only in intravascular hemolysis. Hypertrophy of the mononuclear phagocyte system and consequent splenomegaly are seen only in extravascular hemolysis.

14 What are the main features of intravascular hemolysis?

Jaundice: It is related to the excessive formation of unconjugated bilirubin from the heme portion of hemoglobin. Unconjugated bilirubin is bound to albumin and does not appear in urine.

Hemoglobinemia: It results from the release of free hemoglobin released from RBCs. Free hemoglobin binds to haptoglobin, which prevents its excretion into urine. Hemoglobin–haptoglobin complex is rapidly cleared by the mononuclear phagocyte system. Accordingly, the blood concentration of haptoglobin will be reduced. Decreased serum haptoglobin is a reliable sign of intravascular hemolysis.

Methemalbuminemia: When the serum haptoglobin is depleted, the unbound or free hemoglobin is in part rapidly oxidized to methemoglobin, which binds to albumin forming methemalbumin.

Hemoglobinuria: Hemoglobin in urine appears after the haptoglobin binding capacity has been exceeded and the free hemoglobin is filtered through the glomeruli. Both hemoglobin and methemoglobin are excreted through the kidneys, imparting a red-brown color to the urine.

Hemosiderinuria: Hemosiderin in urine is derived from hemoglobin degradation in the renal tubules.

15 What is the mechanism of erythroid hyperplasia of the bone marrow in hemolytic anemia?

All forms of hemolytic anemia are accompanied by premature destruction of erythrocytes. The resulting anemia and lowered oxygen saturation of blood stimulate increased production of erythropoietin in the kidneys, which leads to an increase in the number of normoblasts in the bone marrow and prominent reticulocytosis in the peripheral blood.

17 What is the difference between anemia caused by extrinsic factors (extracorpuscular defects) and anemia caused by intrinsic factors (intracorpuscular defects)?

Because the classification of intravascular and extravascular hemolytic disorders is not entirely satisfactory, a classification based on the underlying cause of red cell destruction is proposed:

Extracorpuscular

Extrinsic underlying cause

Acquired disorders (i.e., antibody mediated, mechanical trauma to red cells, infections, and chemical injury)

Intracorpuscular

Hereditary disorders (i.e., red cell membrane disorders, red cell enzyme deficiencies, and disorders of hemoglobin synthesis)

Acquired disorders (i.e., paroxysmal nocturnal hemoglobinuria)

Key Points: Hematology and Red Blood Cell Disorders

1. Hematology covers diseases of red blood cells (RBCs), platelets, blood coagulation factors, leukocytes, and lymphoid cells.

2. Anemia is a decrease in the red cell mass and hemoglobin content of the blood, reducing the capacity of the blood to transport oxygen.

3. Anemia can be classified etiologically into three groups: anemia due to blood loss, hemolytic anemia, and anemia due to impaired RBC production.

4. Hemolytic anemias develop due to inherent RBC defects or due to antibodies and other extracorpuscular causes.

5. RBC production can be impaired due to deficiency of iron, vitamin B₁₂, and folic acid or as a consequence of bone marrow failure in aplastic anemia.

6. Polycythemia vera can occur in two forms: as polycythemia vera, which is a neoplastic disorder, and as a secondary polycythemia, which is related to excessive stimulation of RBC precursors by erythropoietin.

18 What is hereditary spherocytosis?

Hereditary spherocytosis (HS) is caused by several inherited defects in the red cell membrane skeleton that render the RBCs spheroid (round) and less deformable during their passage through the splenic sinusoids. Spherical RBCs are thus more prone to splenic sequestration and destruction, which lead to intracorpuscular hemolysis.

19 What are the exact molecular defects in HS?

The normal membrane cytoskeleton of RBCs is composed of several proteins, the most important of which are α and β spectrin, ankyrin, actin, and proteins known as band 4.1 and band 3 (Fig. 9-2). Together these proteins maintain the normal biconcave shape of RBCs. The mutation of ankyrin gene is the most common defect in autosomal dominant HS, and the mutations of gene encoding protein band 3 account for 20% of cases. Genes encoding other cytoskeletal proteins are less often mutated.

Figure 9-2 Schematic drawing of the erythrocyte cytoskeleton.

(From Palek J, Lux SE: The red cell membrane skeletal defects in hereditary hemolytic anemias. Semin Hematol 20:189, 1983.)

20 How common is HS, and how is it inherited?

The prevalence of HS is as high as 1 in 5000 in Caucasians of northern European origin. The autosomal dominant pattern of inheritance accounts for 75% of these cases. The autosomal recessive form of the disease is less common, but it is much more severe than the autosomal dominant form.

21 Describe the pathologic findings in HS

The most prominent changes are found in the peripheral blood, the bone marrow, and the spleen:

Blood smears: Spheroid red cells appear uniformly red in routine smears (spherocytosis). Their central pallor, corresponding to the central concavity, is lacking. Although this morphology of RBCs is distinctive, it is not pathognomonic of HS and may also be seen in autoimmune hemolytic anemias.

Bone marrow: Erythroid hyperplasia with an increased number of normoblasts is seen in bone marrow biopsy.

Splenomegaly of moderate extent (500–1000 g) results from marked congestion of the red pulp cords. The sinuses appear virtually empty.

Erythrophagocytosis can be seen within the congested cords.

Hemosiderosis of the splenic red pulp is prominent and typically associated with hemosiderosis of the liver.

Biliary system: Pigment gallstones are found in 40% to 50% of affected adults.

24 Explain why glucose-6-phosphate dehydrogenase (G6PD) causes hemolytic anemia

Exposure of RBCs to drugs or toxins that generate oxygen radicals is counteracted by reduced glutathione. To form this antioxidant, the cells must increase the breakdown of glucose through the hexose-monophosphate shunt. G6PD is an enzyme that plays a crucial role in this metabolic pathway and is essential for maintaining the glutathione in its active form. Erythrocytes deficient in G6PD are less resistant to oxidant injury, and consequently any exposure to oxidants will cause hemolysis.

25 What should one know about G6PD deficiency?

It is an X-linked disorder, comprising more than 350 variants, and exclusively affects males.

There is geographic variability. It is most prevalent in Mediterranean countries and western Africa and in Americans whose ancestors came from those regions.

Overall prevalence is 1 in 1000, but it is more common in some populations (e.g. African Americans, 15%).

Periodic hemolysis can be induced by a variety of oxidants:

Oxygen free radicals generated by leukocytes in course of infections

Drugs (antimalarials, such as primaquine, chloroquine, sulfonamides, and nitrofurantoin)

Fava beans (hence the popular term for this disease, favism)

Laboratory findings include anemia, hemoglobinemia, and occasional hemoglobinuria. Peripheral blood smear shows Heinz bodies and bite cells. Heinz bodies can be seen as dark inclusions when the smears are stained with crystal violet. When the spleen pits the Heinz bodies from RBCs, it also removes a portion of the RBC cytoplasm, leading to formation of bite cells.

26 What is sickle cell anemia?

Sickle cell anemia is a hereditary hemoglobinopathy caused by mutation of the b-globin gene. This mutation affects hemoglobin A (HbA), transforming it into an abnormal hemoglobin S (HbS). Aggregation and polymerization of the HbS molecules, initiated by deoxygenation, leads to the formation of HbS fibers and resultant distortion of the red cells by sickling. Initially, sickling is a reversible phenomenon, but with repeated episodes of sickling and unsickling, membrane damage ensues, and the cells become irreversibly deformed.

27 How common is sickle cell anemia?

Sickle cell anemia is most common in Africa, but it is also encountered in Mediterranean countries and India. In some areas of Africa, as many as 40% of inhabitants are heterozygotes for HbS. The prevalence of heterozygosity among African Americans is 10%. One in 650 of these people is monozygous and has clinical signs of sickle cell anemia. Approximately 50,000 people have sickle cell anemia in the United States.

28 Describe the difference between the sickle cell trait and sickle cell anemia

The mutation that causes either sickle cell disease or sickle cell trait is a point mutation in the gene that leads to substitution of a valine for glutamic acid at the sixth position of the β-globin chain of hemoglobin:

When an individual is homozygous for the mutant gene, all the hemoglobin is of the abnormal HbS type. Sickling and hemolysis occur in regular living circumstances, causing the spectrum of clinical signs and symptoms known as sickle cell anemia.

In heterozygotes, only a portion (approximately 40%) of the hemoglobin is HbS, and the remainder is normal HbA. RBC sickling and possibly hemolysis occur in hypoxia and other abnormal conditions. This heterozygous state is referred to as sickle cell trait.

29 Why do dehydration and anoxia (e.g., high altitude) potentiate sickling of RBCs in sickle cell anemia?

The precipitation of HbS fibers has deleterious effects on the red cell membrane in irreversibly sickled cells but also in normal-appearing cells. With membrane injury, the RBCs have difficulty maintaining normal intracellular volume, and consequently intracellular hemoglobin concentration increases (MCHC is higher), with a higher concentration of the HbS within the cell. Dehydration removes the water from RBCs, bringing HbS molecules even closer to each other, thus greatly facilitating sickling.

Hypoxia, as occurs at high altitudes, increases the amount of deoxygenated HbS. Because deoxygenated HbS molecules aggregate and polymerize many times more readily than oxygenated HbS, it is obvious that hypoxia will promote sickling. Similarly, a decrease in pH lowers the affinity of hemoglobin for oxygen and therefore increases the amount of deoxygenated HbS, which will increase sickling.

30 Name two main causes of ischemia in sickle cell anemia

Chronic hemolytic anemia: Anemia results from shortened life span of RBC, which survive on average only 20 days in circulation. In response to chronic hemolysis the bone marrow undergoes massive hyperplasia (Fig. 9-3).

Occlusion of small blood vessels: The altered RBC membranes favor adhesion of these cells to the endothelium. The slow down of the circulation due to increased RBC adhesion promotes hypoxia, which in turn contributes to the sickling of abnormal RBCs and finally vasoocclusion. Hypoxia affects numerous organs and may even cause death.

Figure 9-3 Sickle cell anemia. Clinical symptoms and pathologic findings are a consequence of hemolytic anemia and sickling of abnormal red blood cells (RBCs). Sickling causes occlusion of blood vessels in many organs and consequent ischemia. CNS, Central nervous system; GI, gastrointestinal.

31 How does sickling of RBCs affect the spleen?

Irreversibly sickled cells have rigid and nondeformable membranes and have difficulty negotiating the splenic sinusoids. They become sequestered in the spleen, where they are destroyed by the mononuclear phagocytes. The spleen may become enlarged early in the disease (up to 500 g in children). There is a marked congestion of the red pulp due to the trapped sickled red cells. This erythrostasis leads to thrombosis and infarction in the spleen or at least to marked tissue hypoxia. Continued scarring causes progressive shrinkage of the spleen, resulting in autosplenectomy by adolescence or early adulthood, when only a small nubbin of fibrous tissue may be left.

32 Why does the calvaria of sickle cell patients have a “crew-cut” appearance on x-rays?

The bone marrow in these patients is hyperplastic because of the compensatory expansion of normoblasts. The resorption of bone with secondary new bone formation sometimes develops as a consequence of the expansion of bone marrow. These changes on the calvaria produce the x-ray appearance of a crew cut.

33 Discuss the most common bacteria causing infection, death, or both in children with sickle cell anemia

Septicemia and meningitis caused by Streptococcus pneumoniae and Haemophilus influenzae are the most common causes of severe infection or death in these children. Salmonella species are a common cause of osteomyelitis.

34 What are three types of crisis in sickle cell anemia?

Vasoocclusive (or painful) crises: Episodes of hypoxia and infarction are associated with severe pain in the affected area. Most commonly involved sites are bones, lungs, liver, brain, spleen, and penis.

In children, painful bone crises are extremely common, causing the so-called hand–foot syndrome, which can be distinguished from acute osteomyelitis with difficulty.

Involvement of the lungs presents with fever, cough, chest pain, and pulmonary infiltrates. Acute chest syndrome is a major cause of death in patients older than 5 years.

Central nervous system hypoxia may present clinically as seizures or stroke. Retarded mental development is a serious complication of these hypoxia attacks.

The affected spleen is initially enlarged in childhood. Following multiple infarcts, it undergoes progressive shrinkage during adolescence or early adulthood and may completely vanish (autosplenectomy).

Leg ulcers in adult patients develop due to ischemia of skin and the subcutaneous tissue.

Aplastic crises: Temporary cessation of bone marrow activity is usually triggered by intercurrent infections. The most common cause is a parvovirus infection that affects the erythroid progenitor cells. Such crises are marked by sudden and rapid worsening of anemia accompanied by a disappearance of reticulocytes from the peripheral blood.

Sequestration crisis: Sequestration of deformed RBCs in the spleen is accompanied by sudden splenomegaly, hypovolemia, and sometimes shock. Splenic ischemia ultimately leads to autosplenectomy.

36 What is thalassemia, and how is it classified?

Thalassemia is a group of disorders caused by a lack of or decreased synthesis of structurally normal hemoglobin chains. This disease is classified according to the type of chains that are missing. If the α chains are missing, it is called α-thalassemia; if the β chains are missing, it is called β-thalassemia. Low intracellular hemoglobin (hypochromia), as well as the relative excess of the other chain, accounts for the morphologic and functional disturbances seen in these diseases.

37 What are the common features of all forms of β-thalassemia?

A quantitatively defective synthesis of β-globin chains is found in all β-thalassemias. The synthesis of α chains is unimpaired, leading to an excess of α chains. Free α chains tend to aggregate into insoluble inclusions within erythrocytes and their precursors, leading to:

Premature destruction of maturing erythroblasts within the marrow (ineffective erythropoiesis)

Lysis of abnormal mature red cells in the spleen (hemolysis)

38 What are the pathologic features of β-thalassemia?

Most erythroblasts die in the bone marrow (apoptosis), resulting in ineffective erythropoiesis. This, in turn, stimulates erythropoietin secretion, which leads to severe erythroid hyperplasia in the bone marrow and often at extramedullary sites.

Massive erythropoiesis within the bones invades the cortex, impairs bone growth, and produces skeletal abnormalities (crew-cut appearance on x-rays).

If untreated, the disease often results in growth retardation.

Sequestration and destruction of inclusion-bearing red cells is derived from precursors that escape intramedullary death.

Extramedullary hematopoiesis is a common compensatory change involving splenic red pulp.

Splenomegaly results from RBC destruction and extramedullary hematopoiesis.

Iron overload presents as hemosiderosis of Kupffer cells and hepatocytes (secondary hemochromatosis).

Extramedullary hematopoiesis compensates for inefficient bone marrow RBC production that may cause hepatomegaly.

39 What are the causes of iron excess in β-thalassemia?

Excessive absorption of dietary iron due to ineffective hematopoiesis

Iron accumulation due to repeated blood transfusions required by these patients

40 What is the difference between the three clinical types of β-thalassemia (thalassemia major, intermedia, and minor)?

The clinical classification of β-thalassemias is based on:

Severity of the anemia

Type of genetic defect (βo and β⨥)

Gene dosage (homozygous or heterozygous)

β-Thalassemia major

Severe transfusion-dependent anemia that first becomes manifest 6 to 9 months after birth as hemoglobin synthesis switches from HbF to HbA

Either type βo or β⨥(βo/βo or β⨥/β⨥)

β-Thalassemia minor or β-thalassemia trait

Usually asymptomatic with a mild anemia

One normal gene (βo/β or β⨥/β⨥)

41 What is α-thalassemia?

α-Thalassemias are disorders caused by reduced synthesis (α⨥-thalassemias) or absent synthesis (α0-thalassemias) of α-globin chains. Because there are four α-globin genes (two on each member of the chromosome pair), there are many variations. The anemia stems both from lack of adequate hemoglobin and from the effects of excess unpaired non-α chains.

42 List the four main clinical types of α-thalassemia

These are classified on the basis of the number and position of the α-globin genes deleted:

Silent carrier state

There is deletion of a single α-globin gene.

It is asymptomatic, without anemia.

α-Thalassemia trait

There is deletion of two α-globin genes.

It may affect both genes of one chromosome (Asians) or one gene of each chromosome (Africans).

There is minimal or no anemia and no physical signs; clinical findings are identical to those of β-thalassemia minor (microcytosis and hypochromia).

Hemoglobin H (HbH) disease

There is deletion of three α-globin genes.

Tetramers of β-globin, called HbH, are formed. These tetramers are nonfunctional in oxygen transport, so anemia is disproportionate to hemoglobin level.

There is moderately severe anemia, resembling β-thalassemia intermedia (microcytic, hypochromic anemia with target cells and Heinz bodies in the blood smear).

Hydrops fetalis

There is deletion of all four α-globin genes

In the fetus, excess of γ-globin chains form tetramers (hemoglobin Bart) that are unable to deliver the oxygen to tissues. Without intrauterine transfusions, the fetus invariably dies.

The fetus shows pallor, generalized edema, and massive hepatosplenomegaly.

43 Discuss the cause of paroxysmal nocturnal hemoglobinuria (PNH)

This is the only example of hemolytic anemia in which there is an acquired defect in the cell membrane. In PNH, the gene that is essential for the synthesis of the glycosyl-phosphatidyl-inositol (GPI) anchor is mutated. The absence of several GPI-linked membrane proteins renders blood cells unusually sensitive to lysis by endogenous complement. The somatic mutation affects pluripotent stem cells, and, accordingly, all their descendants (including platelets and granulocytes) are more sensitive to lysis by complement.

44 How does PNH present clinically?

PNH affects young people, and the average age of onset is 20 to 25 years. Patients with PNH present with intravascular hemolysis, which (despite its name) is paroxysmal and nocturnal in only 25% of cases. Hemosiderinuria with loss of iron leads to iron deficiency. Patients may also suffer from multiple episodes of venous thrombosis involving the hepatic, portal, or cerebral veins. These episodes are fatal in 50% of cases. The disease can evolve into aplastic anemia or acute leukemia. Steroid treatment may stabilize the disease in more than 50% of cases, but many patients still die within 10 years of onset.

46 What is the difference between warm antibody and cold antibody hemolytic anemia?

48 What is megaloblastic anemia, and how is it classified?

Megaloblastic anemias are characterized by impaired DNA synthesis and distinctive morphologic changes of affected RBC precursors and their descendants in the blood and bone marrow. It is a disease of older age, most commonly diagnosed in the fifth to eighth decade of life. Two main types are:

Pernicious anemia, the major form of vitamin B₁₂ deficiency anemia

Folate deficiency anemia

A deficiency of vitamin B₁₂ and folic acid or impairment in their use results in defective nuclear maturation due to deranged or inadequate synthesis of DNA.

51 How is vitamin B₁₂ absorbed?

Humans are totally dependent on dietary animal products for their vitamin B₁₂ requirement. Absorption of vitamin B₁₂ requires intrinsic factor (IF), which is secreted by the parietal cells of the stomach along with hydrochloric acid. The vitamin is released from its protein-bound form by action of pepsin and then binds to salivary vitamin B₁₂–binding proteins called cobalophilins, or R-binders. R–vitamin B₁₂ complexes are broken down by pancreatic proteases in the duodenum. The released vitamin B₁₂ then attaches to IF and goes to the ileum, where it adheres to IF-specific receptors on the ileal cells. Vitamin B₁₂ enters the mucosal cells, which pass it on to transcobalamin II, a plasma transport protein.

53 What is the pathogenesis of pernicious anemia?

Pernicious anemia is believed to result from immunologically mediated, possibly autoimmune, destruction of gastric mucosa. The resultant chronic atrophic gastritis is marked by a loss of parietal cells, a prominent infiltrate of lymphocytes and plasma cells, and nuclear changes in the mucosal cells similar to those found in the erythroid precursors. It is suspected that an autoreactive T cell response initiates gastric mucosal injury, which then triggers the formation of autoantibodies. These antibodies cause further injury to the epithelium, and after the mass of IF-secreting cells is significantly depleted (together with reserves of stored vitamin B₁₂), anemia develops. Two processes aggravate anemia:

Ineffective erythropoiesis (megaloblasts are especially prone to intramedullary destruction; premature destruction of granulocytic and platelet precursors also occurs)

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