Stages in erythrocyte development

Pluripotential, or uncommitted, stem cells, which have the potential to produce any type of blood cell, divide and develop into erythroid stem cells committed to form erythrocytes (Fig. 23). These divide further and mature, synthesizing haemoglobin and eventually forming normoblasts. Nuclear material is extruded and the endoplasmic reticulum resorbed, producing first a reticulocyte, containing a few remnants of endoplasmic reticulum, and then an erythrocyte. Normally only these last two cell types are found in the circulation, with reticulocytes making up less than 2% of the total. This percentage rises during periods of rapid erythrocyte synthesis, when more immature cells enter the circulation. Mature red cells take the form of biconcave discs which deform easily within the narrow capillaries. The normal red cell count in blood is 4 × 10¹² to 6 × 10¹² L⁻¹ (4–6 × 10⁶ mm⁻³).

Fig. 23 Production of blood cells.

Vitamin B₁₂ and folate

If B₁₂ or folate levels are reduced, cell division and maturation are adversely affected. This is particularly important at sites of rapid cell turnover, such as the bone marrow. There is a reduction in the red cell count so that the overall haemoglobin concentration falls. The erythrocytes which do form are abnormally large (macrocytes), so this is known as a macrocytic anaemia. Abnormal erythrocyte precursors called megaloblasts are found in the marrow, so the term megaloblastic anaemia is also used. It should be appreciated that there is no problem with haemo-globin synthesis within the developing cells; there are just too few red cells produced.

Deficiency of B₁₂ or folate can arise in two ways. The diet itself may include inadequate amounts of the normal source foods for these vitamins, e.g., animal products (for both B₁₂ and folate) and green vegetables (rich in folate). Vitamin B₁₂ deficiency can also occur with a normal diet as a result of reduced B₁₂ absorption. Parietal cells in the stomach normally secrete intrinsic factor, which binds to B₁₂, and it is the resulting complex that is absorbed from the ileum (Section 6.4). The B₁₂ is then transported to the liver, which normally stores 1–2 years’ supply (liver is an excellent dietary source of B₁₂). In pernicious anaemia, there is reduced secretion of intrinsic factor. This leads to malabsorption of B₁₂ and a megalo-blastic, macrocytic anaemia results. These patients often also appear mildly jaundiced because of increased bilirubin production following haemo-lysis of the abnormally fragile red cells. Treatment involves regular intramuscular injections with B₁₂, thereby bypassing the normal intestinal absorption mechanisms.

Iron

If the supply of iron is inadequate, haemoglobin synthesis is restricted. This leads to an anaemia in which erythrocytes contain less haemoglobin than normal (they are hypochromic) and are, as a result, smaller than normal (microcytic). Iron-deficiency anaemia can occur whenever iron demand exceeds supply. That may be because of reduced iron intake or increased iron loss, e.g., because of chronic bleeding. Men normally lose approximately 1 mg of iron daily, chiefly through the shedding of intestinal epithelium. In menstruating women, this can rise to 2 mg or more. Since the average diet provides 10–15 mg of iron each day, of which about 10% is normally absorbed, iron balance is usually maintained in men, but women run an appreciable risk of becoming iron depleted. Pregnancy increases iron demand further and iron supplements may be necessary.

Iron is mainly absorbed in the ferrous form by means of intestinal transferrin and an iron–transferrin receptor (Section 6.4). The rate of absorption increases when iron demand is increased within the body. Plasma transferrin transports iron within the circulation and excess iron is stored as ferritin or haemosiderin, particularly in the liver, spleen and bone marrow. Mean iron stores are 2–3 times larger in men than women, presumably as a consequence of menstrual losses.

ABO system

There are two main antigens in this system, A and B, and these give rise to four different blood groups as shown in Table 2. Of these, blood groups O and A are almost equally common and together account for over 85% of the population in Western Europe. It should be noted that plasma always contains preformed antibodies (IgM class) against A or B antigens which are not already present on our own erythrocytes, whether we have been sensitized by exposure to foreign red cells or not. This breaks the general rule in immune responses, since antibodies against all other foreign antigens are only secreted in appreciable amounts after exposure to that antigen (Section 2.5). It may be that we are all exposed to A and B antigens from another source, e.g., intestinal bacteria, and only become tolerant to the antigens also present on our erythrocytes. Whatever the mechanism, the consequence is that a major immune reaction can be expected on the first exposure to blood of the wrong ABO group.

Table 2 Summary of the ABO blood group system. Inheritance of the A and B antigens is autosomal codominant so there are four possible combinations of red cell antigens. The plasma contains antibodies to any antigen not present on the red cells

Rhesus (Rh) factor

Blood is either Rh positive or Rh negative depending on whether red cells carry one of the Rh antigens or not. There are three main Rh antigens, C, D and E, but D is the most common. Inheritance is dominant so that the genotypes Dd and DD both result in D positive blood. Over 85% of Western Europeans are Rh positive.

A Rh-negative recipient will mount an immune response against Rh-positive blood but, unlike ABO agglutinins, there are normally no anti-Rh antibodies in plasma from a Rh-negative individual. Therefore, there is unlikely to be any Rh-dependent agglutination following an initial transfusion with Rh-positive blood. This exposure sensitizes the immune system to the Rh antigen, however, so that subsequent mismatched transfusions can lead to prompt agglutination and haemolysis of the donor cells. Rhesus sensitization can also occur when a Rh-negative mother gives birth to a Rh-positive baby. Fetal red cells, normally separated from the maternal circulation by the placenta, may enter the mother’s blood during delivery as the placenta is sheared off the uterine wall. This stimulates production of anti-Rh antibodies by the mother and, if there is a subsequent Rh-positive pregnancy, these antibodies (IgG class) cross into the fetal circulation, leading to haemolysis. The resulting jaundice, anaemia and heart failure may threaten the baby’s life. These problems of the ‘Rhesus baby’ have largely been eradicated by injecting all D-negative mothers with anti-D antibodies shortly after the birth of each baby. Any circulating D-positive cells become coated with exogenous antibody and are destroyed before they can stimulate the maternal immune system. This is an example of temporary passive immunity (conferred by the injected immunoglobulins) preventing the development of permanent active immunity. It should be noted that fetal/maternal ABO incompatibility is common but has no damaging consequences because the IgM class anti-A and anti-B antibodies are too large to cross the placenta.

Transfusions and cross matching

The terms universal donor (blood group O, Rh negative) and universal recipient (blood group AB, Rh positive) are sometimes used to indicate conditions in which transfusions may be attempted without knowing the blood group of both donor and recipient. The reasoning is that the red cells of the universal donor carry no antigens and so cannot be agglutinated, while the plasma of the universal recipient contains no antibodies and could not agglutinate donor cells, regardless of their group. Possible agglutination of recipient cells is regarded as unlikely, since agglutinins in the donor plasma will be diluted following transfusion. Use of O, Rh negative blood for a patient of unknown blood group is reserved for life-threatening emergencies, however, and donor and recipient blood groups should normally match. Indeed, the existence of a wide range of rarer erythrocyte antigens means that blood of the appropriate group should actually be tested against samples of the patient’s cells and plasma before being transfused. Donor cells are mixed with recipient plasma, while recipient cells are mixed with donor plasma; there should be no agglutination in either case. This is referred to as cross matching.

Reticuloendothelial system

This phagocytic system is almost synonymous with the tissue macrophage system. Mobile macrophages rove freely through connective tissues, but many more remain relatively fixed in a given region. When activated, however, these will also become mobile, being chemotactically attracted to local sites of infection or damage. There are dense aggregations of macrophage-type cells within the reticular tissue of the lymph nodes, spleen and bone marrow. Alveolar macrophages in the lung and the phagocytic Küpffer cells in the liver, along with microglial cells in nervous tissue, are also regarded as part of the reticuloendothelial system, as are the blood monocytes.

Mechanical and chemical barriers against infection

The epithelia covering skin and lining the gastrointestinal, genitourinary and respiratory tracts provide mechanical barriers which help exclude damaging organisms. This is aided by mucus secretion in the respiratory tract, which traps dirt and pathogens and allows them to be swept out by the action of epithelial cilia. Chemical secretions, such as acid in the stomach and vagina, also play a role and may defend against colonization of the epithelial surface, e.g., preventing vaginal yeast infections.

The inflammatory response and phagocytosis

Inflammation is a set of local cellular and vascular responses to tissue damage or infection which accelerates the destruction and phagocytic removal of invading organisms and debris.

Phagocytic responses in inflammation Tissue macrophages adjacent to a site of bacterial invasion, for example, become mobile and phagocytically active. Chemicals released from injured and infected cells (chemotaxins) act as attractants for these cells, directing their movements towards the damaged area in a process called chemotaxis. Local macrophages are soon reinforced by the migration of blood neutrophils and monocytes into the region (Fig. 25). These stick to the endothelium of capillaries in the affected area (leucocyte margination) and then invaginate themselves through the clefts between the endothelial cells using active amoeboid movements (diapedesis). The blood-borne phagocytes escape into the tissues where they are chemotactically attracted to assist in removal of infectious or toxic agents and tissue debris. The resulting congregation of large numbers of neutrophils and macrophages within a tissue is the histological hallmark of acute inflammation.

Fig. 25 Diapedesis of blood granulocytes and monocytes and chemotaxis of local macrophages increase tissue phagocyte density during inflammation.

Vascular responses in inflammation Leucocyte aggregation in inflamed tissues is accelerated by increased blood flow caused by dilatation of local blood vessels. This transports more white cells into the region, while an increase in capillary permeability promotes diapedesis. These vascular changes are stimulated by the generation of a variety of vasodilator substances within damaged tissues.

Kinins. Activation of a cascade of proteolytic enzymes known as the kinin–kallikrein system produces vasoactive kinins, particularly bradykinin.

Vasodilators. Basophils and mast cells release the vasodilators bradykinin, 5-hydroxytryptamine (serotonin) and histamine, as well as the anticoagulant heparin.

Prostaglandins. Activated phagocytic cells also stimulate prostaglandin synthesis and this may amplify the mechanisms of inflammation. Drugs which inhibit prostaglandin production, such as aspirin and glucocorticoid steroid hormones, can be used as antiinflammatory agents.

Local and systemic effects of inflammation The inflammatory response leads to a number of characteristic effects at the site of injury or infection:

Acute inflammation (particularly if caused by infection) may also be associated with generalized (systemic) responses.

Body temperature.This may rise as a result of resetting of the hypothalamic set point (Section 1.1). Fever (pyrexia) seems to increase the activity of phagocytic cells and is promoted both by exogenous pyrogens, including bacterial products known as endotoxins, and by endogenous pyrogens released by phagocytes. Endogenous pyrogens stimulate prostaglandin synthesis and this is blocked by drugs such as aspirin. This probably explains their ability to reduce fever (their antipyretic action).

Blood white cell count. There is an increase in the blood white cell count (a leucocytosis), most of the increase being the result of extra neutrophils (a neutrophilia). This reflects both the rapid mobilization of neutrophils already present in the bone marrow and an increased rate of production in the marrow.