Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
Blood, lymphoid tissues and haemopoiesis
Blood is an opaque fluid with a viscosity greater than that of water (mean relative viscosity 4.75 at 18 °C), and a specific gravity of 1.06 at 15 °C. It is bright red when oxygenated, as in the systemic arteries, and dark red to purple when deoxygenated, in systemic veins. Blood is a mixture of a clear liquid, plasma, and cellular elements, and consequently the hydrodynamic flow of blood in vessels behaves in a complex manner that is not entirely predictable by simple Newtonian equations.
Plasma is a clear, yellowish fluid that contains many substances in solution or suspension: low-molecular-weight solutes give a mean freezing-point depression of 0.54 °C. Plasma contains high concentrations of sodium and chloride ions, as well as potassium, calcium, magnesium, phosphate, bicarbonate and traces of many other ions; also present are glucose, amino acids and vitamins. It also includes high molecular weight plasma proteins, e.g. clotting factors, particularly prothrombin; immunoglobulins and complement proteins involved in immunological defence; glycoproteins, lipoproteins, polypeptide and steroid hormones, and globulins for the transport of hormones and iron. The plasma is involved in the transport of most molecules that are released or secreted by cells in response to pathological or physiological stimuli and so the routine chemical analysis of plasma is of great diagnostic importance. There is increasing interest in using metabolomics approaches for the high-throughput analysis of small molecules or metabolites in the serum, as a potential aid to diagnosis and understanding of disease ( , ).
The precipitation of the protein fibrin from plasma to form a clot ( Fig. 4.1 ) is initiated by the release of specific materials from damaged cells and blood platelets in the presence of calcium ions. If blood or plasma samples are allowed to stand, they will separate into a clot and a clear yellowish fluid, the serum. Clot formation is prevented by removal of calcium ions, e.g. by addition of citrate, oxalate or various calcium chelators (EDTA, EGTA) to the sample. Heparin is also widely used as an anticlotting agent because it interferes with fibrin clot formation.
Erythrocytes enmeshed in filaments of fibrin in a clot.
Courtesy of Michael Crowder MD.
In postnatal life, blood cells are formed in the bone marrow. Haemopoiesis produces red cells (erythrocytes) and a wide variety of immune defense cells (white blood cells, or leukocytes). The latter include neutrophil, eosinophil and basophil granulocytes, B lymphocytes and monocytes. T lymphocytes develop in the thymus from bone marrow-derived progenitors. These cells all contribute to the immune system (for an overview of the immune system, see ). Platelets are produced in the bone marrow as cellular fragments of megakaryocytes. Only erythrocytes and platelets are generally confined to the blood vascular system, whereas all leukocytes can leave the circulation and enter extravascular tissues. The numbers of cells doing so increases greatly during inflammation caused by local infections or tissue damage.
The lymphoid tissues are the thymus, lymph nodes, spleen and the lymphoid follicles associated mainly with the alimentary and respiratory tracts. Lymphocytes populate lymphoid tissues and are concerned with immune defence. Lymphoid tissue also contains supportive stromal cells, which are non-haemopoietic in origin (e.g. thymic epithelium); non-haemopoietic follicular dendritic cells of lymph nodes and splenic follicles; haemopoietically derived dendritic cells; and macrophages of the mononuclear phagocyte system. Dendritic cells and blood monocyte-derived macrophages are found additionally in most tissues and organs, where they function as antigen-presenting cells (APCs).
Cells of Peripheral Blood
Erythrocytes
Erythrocytes (red blood cells, RBCs) account for the largest proportion of blood cells (99% of the total number), with normal values of 4.1–6.0 ×10 6 /μl in adult males and 3.9–5.5 ×10 6 /μl in adult females. Polycythaemia (increased red cell mass) can occur in individuals living at high altitude, or pathologically in conditions resulting in arterial hypoxia. Reduction in red cell mass (anaemia) has many underlying causes but in rare instances can be due to structural defects in erythrocytes (see below).
Each erythrocyte is a biconcave disc (see Fig. 4.1 ; Fig. 4.2 ) with a mean diameter in dried smear preparations of 7.1 μm; in fresh preparations the mean diameter is 7.8 μm, decreasing slightly with age. Mature erythrocytes lack nuclei. They are pale red by transmitted light, with paler centres because of their biconcave shape. The properties of their cell coat cause them to adhere to one another by their rims to form loose piles of cells (rouleaux). In normal blood, a few cells assume a shrunken, star-like, crenated form; this shape can be reproduced by placing normal biconcave erythrocytes in a hypertonic solution, which causes osmotic shrinkage. In hypotonic solutions erythrocytes take up water and become spherical; they may eventually lyse to release their haemoglobin (haemolysis), leaving red-cell ghosts.
Erythrocytes have a plasma membrane that encloses mainly a single protein, haemoglobin, as a 33% solution. The plasma membrane of erythrocytes is 60% lipid and glycolipid, and 40% protein.
More than 15 classes of protein are present, including two major types. Glycophorins A and B (each with a molecular mass of approximately 50 kDa) span the membrane, and their negatively charged carbohydrate chains project from the outer surface of the cell. Their sialic acid groups confer most of the fixed charge on the cell surface. A second transmembrane macromolecule, band 3 protein, forms an important anion channel, exchanging bicarbonate for chloride ions across the membrane and allowing the release of CO 2 in the lungs.
The filamentous protein, spectrin, is responsible for maintaining the shape of the erythrocyte. A dimer is formed of α1 and β1 spectrin monomers, and two dimers then come together to form a tetramer ( ). These are joined by junctional complexes that contain (among other proteins) ankyrin, short actin filaments, tropomyosin and protein 4.1, forming a hexagonal lattice that supports the plasma membrane ( ). The junctional complex also interacts with transmembrane proteins. This structure gives the membrane great flexibility; red cells are deformable but regain their biconcave shape and dimensions after passing through the smallest capillaries, which are 4 μm in diameter ( ). Erythrocyte membrane flexibility also contributes to the normally low viscosity of blood. Molecular defects in the cytoskeleton result in abnormalities of red cell shape, membrane fragility, premature destruction of erythrocytes in the spleen and haemolytic anaemia ( ).
Fetal erythrocytes up to the fourth month of gestation differ markedly from those of adults, in that they are larger, are nucleated and contain a different type of haemoglobin (HbF). After this time they are progressively replaced by the adult type of cell.
Haemoglobin
Haemoglobin (Hb) is a globular protein with a molecular mass of 67 kDa. It consists of globulin molecules bound to haem, an iron-containing porphyrin group. The oxygen-binding power of Hb is provided by the iron atoms of the haem groups, and these are maintained in the ferrous (Fe ++ ) state by the presence of glutathione within the erythrocyte. The Hb molecule is a tetramer, made up of four subunits, each a coiled polypeptide chain holding a single haem group.
In normal blood, five types of haemoglobin polypeptide chain can occur: namely, α, β and two β-like polypeptides, γ and δ. A third, β-like η chain is restricted to early fetal development. Each haemoglobin molecule contains two α-chains and two others, so that several combinations, and hence a number of different types of haemoglobin molecule, are possible. For example, haemoglobin A (HbA), which is the major adult class, contains 2 α- and 2 β-chains; a variant, HbA 2 with 2 α- and 2 δ-chains, accounts for only 2% of adult haemoglobin. Haemoglobin F (HbF), found in fetal and early postnatal life, consists of 2 α- and 2 γ-chains. Adult red cells normally contain less than 1% of HbF.
Mutations in the haemoglobin chains can result in a range of pathologies ( ).
In the genetic condition thalassaemia, only one type of chain is expressed normally, the mutant chain being absent or present at much reduced levels. Thus, a molecule may contain 4 α-chains (β-thalassaemia) or 4 β-chains (α-thalassaemia). In haemoglobin S (HbS) of sickle-cell disease, a point mutation in the β-chain gene (valine substituted for glutamine) causes the haemoglobin to polymerize under conditions of low oxygen concentration, thus deforming the red blood cell.
Lifespan
Erythrocytes last between 100 and 120 days before being destroyed. As erythrocytes age, they become increasingly fragile, and their surface charge decreases as their content of negatively charged membrane glycoproteins diminishes. The lipid content of their membranes also reduces. Aged erythrocytes are taken up by the macrophages of the spleen ( ) and liver sinusoids, usually without prior lysis, and are hydrolysed in phagocytic vacuoles where the haemoglobin is split into its globulin and porphyrin moieties. Globulin is further degraded to amino acids, which pass into the general amino-acid pool. Iron is removed from the porphyrin ring and either transported in the circulation bound to transferrin and used in the synthesis of new haemoglobin in the bone marrow, or stored in the liver as ferritin or haemosiderin. The remainder of the haem group is converted in the liver to bilirubin and excreted in the bile. Haemoglobin that is released by destruction of erythrocytes in the body binds to haptoglobin, and is taken up via CD (antigenic Cluster of Differentiation marker of cell type)163 receptors expressed on the surface of macrophages ( ).
The recognition of effete erythrocytes by macrophages takes place by a number of mechanisms. These include the exposure extracellularly of phospholipids (such as phosphatidyl serine) that are normally found on the inner leaflet of the membrane bilayer, alterations in the carbohydrates expressed by the cells (most notably the loss of sialic acid) and the binding of autoantibodies to antigens exposed on the aged erythrocyte. These lead to the cells being recognised and taken up by macrophages. Red cells are destroyed at the rate of 5 ×10 11 cells a day (or nearly 6 million a second) and are normally replaced from the bone marrow (see Fig. 4.12 ) at the same rate.
Blood groups
Over 300 red cell antigens are recognizable with specific antisera. They can interact with naturally occurring or induced antibodies in the plasma of recipients of an unmatched transfusion, causing agglutination and lysis of the erythrocytes. Erythrocytes of a single individual carry several different types of antigen, each type belonging to an antigenic system in which a number of alternative antigens are possible in different persons. So far, 19 major groups have been identified. They vary in their distribution frequencies between different populations, and include the ABO, Rhesus, MNS, Lutheran, Kell, Lewis, Duffy, Kidd, Diego, Cartwright, Colton, Sid, Scianna, Yt, Auberger, Ii, Xg, Indian and Dombrock systems. Clinically, the ABO and Rhesus groups are of most importance.
In the ABO system, two allelic genes are inherited in simple Mendelian fashion. Thus the genome may be homozygous and carry the AA complement, the blood group being A, or the BB complement, which gives blood group B, or it may carry neither (OO), producing blood group O. In the heterozygous condition the following combinations can occur: AB (blood group AB), AO (blood group A) and BO (blood group B). The ABO blood group antigens are all membrane glycolipids.
Individuals with group AB blood lack antibodies to both A and B antigens, and so can be transfused with blood of any group; they are termed universal recipients. Conversely, those with group O, universal donors, can give blood to any recipient, since anti-A and anti-B antibodies in the donated blood are diluted to insignificant levels. Normally, however, blood is only transfused between persons with corresponding groups because anomalous antibodies of the ABO system are occasionally found in blood and may cause agglutination or lysis. The anti-ABO agglutinins, unlike those of the Rhesus system, belong to the immunoglobulin M (IgM) class and do not cross the placenta during pregnancy.
The Rhesus antigen system is determined by three sets of alleles: namely, Cc, Dd and Ee. The most important clinically is Dd. Inheritance of the Rh factor also obeys simple Mendelian laws; it is therefore possible for a Rhesus-negative mother to bear a Rhesus-positive child. Under these circumstances, fetal Rh antigens can stimulate the production of anti-Rh antibodies by the mother; as these belong to the IgG class of antibody they are able to cross the placenta. For most of the pregnancy the stroma stops the blood group antibodies from crossing into the fetal circulation. However, immediately prior to birth, the antibodies can cross this barrier and cause destruction of fetal erythrocytes. In the first such pregnancy little damage usually occurs because anti-Rh antibodies have not been induced, but in subsequent Rh-positive pregnancies massive destruction of fetal red cells may result, causing fetal or neonatal death (haemolytic disease of the newborn). Sensitization of the maternal immune system can also result from abortion or miscarriage, or occasionally even from amniocentesis, which may introduce fetal antigens into the maternal circulation. Treatment is by exchange transfusion of the neonate or, prophylactically, by giving Rh-immune (anti-D) serum to the mother after the first Rh-positive pregnancy, which destroys the fetal Rh antigen in her circulation before sensitization can occur.
Leukocytes also bear highly polymorphic antigens encoded by allelic gene variants. These belong to the group of major histocompatibility complex (MHC) antigens, also termed human leukocyte antigens (HLA) in humans. HLA class I antigens are expressed by all nucleated cells. Class II antigens are expressed on antigen-presenting cells (APCs) of the immune system, but can also be induced on many parenchymal cell types, e.g. after exposure to interferon. HLA class I and II antigens play important roles in cell–cell interactions in the immune system, particularly in the presentation of antigens to T lymphocytes by APCs.
Leukocytes
Leukocytes (white blood cells) belong to at least five different categories (see Fig. 4.12 ) and are distinguishable by their size, nuclear shape and cytoplasmic inclusions. In practice, leukocytes are often divided into two main groups: namely, those with prominent stainable cytoplasmic granules, the granulocytes, and those without.
Granulocytes
This group consists of eosinophil granulocytes, with granules that bind acidic dyes such as eosin; basophil granulocytes, with granules that bind basic dyes strongly; and neutrophil granulocytes, with granules that stain only weakly with either type of dye. Granulocytes ( Fig. 4.3 ) all possess irregular or multilobed nuclei and belong to the myeloid series of blood cells (see Fig. 4.12 ).
Neutrophil granulocytes
Neutrophil granulocytes (neutrophils) are also referred to as polymorphonuclear leukocytes (polymorphs) because of their irregularly segmented (multilobed) nuclei. They form the largest proportion of the white blood cells (40–75% in adults, with a normal count of 2500–7500/μl) and have a diameter of 12–14 μm. The cells may be spherical in the circulation, but they can flatten and become actively motile within the extracellular matrix of connective tissues.
The numerous cytoplasmic granules are heterogeneous in size, shape and content, but all are membrane-bound and contain hydrolytic and other enzymes. Two major types can be distinguished according to their developmental origin and contents. Non-specific or primary (azurophilic) granules are formed early in neutrophil maturation. They are relatively large (0.5 μm) spheroidal lysosomes containing myeloperoxidase, acid phosphatase, elastase and several other enzymes. Specific or secondary granules are formed later, and occur in a wide range of shapes including spheres, ellipsoids and rods. These contain strong bacteriocidal components including alkaline phosphatase, lactoferrin and collagenase, none of which is found in primary granules. Conversely, secondary granules lack peroxidase and acid phosphatase. Some enzymes, e.g. lysozyme, are present in both types of granule.
In mature neutrophils the nucleus is characteristically multilobed with up to six (usually three or four) segments joined by narrow nuclear strands; this is known as the segmented stage. Less mature cells have fewer lobes. The earliest to be released under normal conditions are juveniles (band or stab cells), in which the nucleus is an unsegmented crescent or band. In certain clinical conditions, even earlier stages in neutrophil formation, when cells display indented or rounded nuclei (metamyelocytes or myelocytes) may be released from the bone marrow. In mature cells the edges of the nuclear lobes are often irregular. In females 3% of the nuclei of neutrophils show a conspicuous ‘drumstick’ formation, which represents the sex chromatin of the inactive X chromosome (Barr body). Neutrophil cytoplasm contains few mitochondria but abundant cytoskeletal elements, including actin filaments, microtubules and their associated proteins, all characteristic of highly motile cells.
Neutrophils are important in the defence of the body against microorganisms. They can phagocytose microbes and small particles in the circulation and, after extravasation, they carry out similar activities in other tissues. They function effectively in relatively anaerobic conditions, relying largely on glycolytic metabolism, and they fulfil an important role in the acute inflammatory phase of tissue injury, responding to chemotaxins released by damaged tissue. Phagocytosis of cellular debris or invading microorganisms is followed by fusion of the phagocytic vacuole with granules, which results in bacterial killing and digestion. Actively phagocytic neutrophils are able to reduce oxygen enzymatically to form reactive oxygen species including superoxide radicals and hydrogen peroxide, which enhance bacterial destruction probably by activation of some of the granule contents ( , ). Neutrophils can also produce neutrophil extracellular traps (NETs), which are web-like structures composed of DNA and proteolytic enzymes that can trap bacteria and kill them ( ).
Phagocytosis is greatly facilitated by circulating antibodies to molecules such as bacterial antigens, which the body has previously encountered. Antibodies (acting as opsonins) coat the antigenic target and bind the plasma complement protein, C1, to their non-variable Fc regions. This activates the complement cascade via the classical pathway, which involves some 20 plasma proteins synthesized mainly in the liver, and completes the process of opsonization. The complement cascade involves the sequential cleavage of the complement proteins into a large fragment, which generally binds to the antigenic surface, and a small bioactive fragment, which is released. The final step is the recognition of complement by receptors on the surfaces of neutrophils and macrophages, which promotes phagocytosis of the microorganism.
Complement activation can be triggered via three pathways: classical, lectin and alternative. The classical pathway is activated by C1q binding to antigen-bound antibodies. The lectin pathway is triggered by mannan-binding lectin (MBL) interacting with carbohydrate patterns on the microbial surface. The alternative pathway relies on autoactivation of complement component C3 on the microbial surface. The products of complement activation have functions of opsonization, cytotoxicity and recruitment of infiltrating cells to the site of infection or injury.
Neutrophils are short-lived; they spend some 6–7 hours circulating in the blood and a few days in connective tissues. The number of circulating neutrophils varies, and often rises during episodes of acute bacterial infection. They die after carrying out their phagocytic role; dead neutrophils, bacteria, tissue debris (including tissue damaged by neutrophil enzymes and toxins) and interstitial fluid form the characteristic, greenish-yellow pus of infected tissue. The colour is derived from the natural colour of neutrophil myeloperoxidase.
Granules may also be released inappropriately from neutrophils. Their enzymes are implicated in various pathological conditions, e.g. rheumatoid arthritis, where tissue destruction and chronic inflammation occur.
Eosinophil granulocytes
Eosinophil granulocytes (eosinophils; for a review, see ) are similar in size (12–15 μm), shape and motile capacity to neutrophils, but are present only in small numbers in normal blood (100–400/μl). The nucleus has two prominent lobes connected by a thin strand of chromatin. Their cytoplasmic specific granules are uniformly large (0.5 μm) and give the living cell a slightly yellowish colour. The cytoplasm is packed with granules, which are spherical or ellipsoid and membrane-bound. The core of each granule is composed of a lattice of major basic protein, which is responsible for its strong eosinophilic staining properties. The surrounding matrix contains several lysosomal enzymes including acid phosphatase, ribonuclease, phospholipase and a myeloperoxidase unique to eosinophils.
Like other leukocytes, eosinophils are motile. When suitably stimulated, they are able to pass into the extravascular tissues from the circulation. They are typical minor constituents of the dermis, and of the connective tissue components of the bronchial tree, alimentary tract, uterus and vagina. The total lifespan of these cells is a few days, of which some 10 hours is spent in the circulation, and the remainder in the extravascular tissues.
Eosinophil numbers rise (eosinophilia) in worm infestations and also in certain allergic disorders, and it is thought that they evolved as a primary defence against parasitic attack. They have surface receptors for IgE that bind to IgE-antigen complexes, triggering phagocytosis and release of granule contents. However, they are only weakly phagocytic and their most important function is the destruction of parasites too large to phagocytose. This antiparasitic effect is mediated via toxic molecules released from their granules (e.g. eosinophil cationic protein and major basic protein). They also release histaminase, which limits the inflammatory consequences of mast cell degranulation. High local concentrations of eosinophils, e.g. in bronchial asthma and in cutaneous contact hyper sensitivity and allergic eczema, can cause tissue destruction as a consequence of the release of molecules such as collagenase from their granules.
Basophil granulocytes
Slightly smaller than other granulocytes, basophil granulocytes are 10–14 μm in diameter, and form only 0.5–1% of the total leukocyte population of normal blood, with a count of 25–200/μl. Their distinguishing feature is the presence of large, conspicuous basophilic granules. The nucleus is somewhat irregular or bilobed, and is usually obscured in stained blood smears by the similar colour of the basophilic granules. The granules are membrane-bound vesicles that display a variety of crystalline, lamellar and granular inclusions: they contain heparin, histamine and several other inflammatory agents, and closely resemble those of tissue mast cells. Both basophils and mast cells have high-affinity membrane receptors for IgE and are therefore coated with IgE antibody. If this binds to its antigen it triggers degranulation of the cells, causing vasodilation, increased vascular permeability, chemotactic stimuli for other granulocytes, and the symptoms of immediate hypersensitivity, e.g. in allergic rhinitis (hay fever). Despite these similarities, basophils and mast cells develop as separate lineages in the myeloid series, from haemopoietic stem cells in the bone marrow. Evidence from experimental animal models suggests that they are closely related (see Fig. 4.12 ) but studies on mast cell disorders in humans indicate that their lineages diverge from a more distant ancestral progenitor ( ). The role of mast cells in the regulation of responses to pain is of interest clinically as a therapeutic target ( ).
Mononuclear leukocytes
Monocytes
Monocytes are the largest of the leukocytes (15–20 μm in diameter) but they form only a small proportion of the total population (2–8% with a count of 100–700/μl of blood). The nucleus, which is euchromatic, is relatively large and irregular, often with a characteristic indentation on one side. The cytoplasm is pale-staining, particulate and typically vacuolated. Near the nuclear indentation it contains a prominent Golgi complex and vesicles. Monocytes are actively phagocytic cells and contain numerous lysosomes. Phagocytosis is triggered by recognition of opsonized material, as described for neutrophils. Monocytes are highly motile and possess a well-developed cytoskeleton.
Monocytes express class II MHC antigens and share other similarities to tissue macrophages and dendritic cells. Most monocytes are thought to be in transit via the blood stream from the bone marrow to the peripheral tissues, where they give rise to macrophages and dendritic cells; different monocyte subsets may target inflamed tissues. Like other leukocytes, they pass into extravascular sites through the walls of capillaries and venules.
Lymphocytes
Lymphocytes ( Fig. 4.4 ; see Figs 4.6 , 4.12 ) are the second most numerous type of leukocyte in adulthood, forming 20–30% of the total population (1500–2700/μl of blood). In young children they are the most numerous blood leukocyte. Most circulating lymphocytes are small, 6–8 μm in diameter; a few are medium-sized and have an increased cytoplasmic volume, often in response to antigenic stimulation. Occasionally, cells up to 16 μm are seen in peripheral blood. Lymphocytes, like other leukocytes, are found in extravascular tissues (including lymphoid tissue); however, they are the only white blood cells that return to the circulation. The lifespan of lymphocytes ranges from a few days (short-lived) to many years (long-lived). Long-lived lymphocytes are necessary for the maintenance of immunological memory.
Blood lymphocytes are a heterogeneous collection, mainly of B and T cells. About 85% of all circulating lymphocytes in normal blood are T cells. Primary immunodeficiency diseases can result from molecular defects in T and B lymphocytes that provide targets for gene therapy for these conditions ( , ).
Small lymphocytes (both B and T cells) contain a rounded, densely staining nucleus that is surrounded by a very narrow rim of cytoplasm, barely visible in the light microscope. By transmission electron microscopy (see Fig. 4.4 ), few cytoplasmic organelles can be seen apart from a small number of mitochondria, single ribosomes, sparse profiles of endoplasmic reticulum and occasional lysosomes; these features indicate a low metabolic rate and a quiescent phenotype. However, these cells become motile when they contact solid surfaces, and can pass between endothelial cells to exit from, or re-enter, the vascular system. They migrate extensively within various tissues, including epithelia ( Fig. 4.5 ).
Larger lymphocytes include T and B cells that are functionally activated or proliferating after stimulation by antigen, and NK cells. They contain a nucleus, which is, at least in part, euchromatic; a basophilic cytoplasm, which may appear granular; and numerous polyribosome clusters, consistent with active protein synthesis. The ultrastructural appearance of these cells varies according to their class and is described below.
B cells
B cells and the plasma cells that develop from them synthesize and secrete antibodies that can specifically recognize and neutralize foreign (non-self) macromolecules (antigens), and can direct various non-lymphocytic cells (e.g. neutrophils, macrophages and dendritic cells) to phagocytose pathogens. B cells differentiate from haemopoietic stem cells in the bone marrow. After deletion of autoreactive cells, the selected B lymphocytes then leave the bone marrow and migrate to peripheral lymphoid sites (e.g. lymph nodes). Here, following stimulation by antigen, they undergo further proliferation and selection, forming germinal centres in the lymphoid tissues. Following this, some B cells differentiate into large basophilic (RNA-rich) plasma cells, either within or outside the lymphoid tissues. Plasma cells produce antibodies in their extensive rough endoplasmic reticulum ( Fig. 4.6 ) and secrete them into the adjacent tissues. They have a prominent pale-staining Golgi complex adjacent to an eccentrically placed nucleus, typically with peripheral blocks of condensed heterochromatin resembling the numerals of a clock (clock-faced nucleus) (see Fig. 4.12 ). Other germinal centre B cells develop into long-lived memory cells capable of responding to their specific antigens not only with a more rapid and higher antibody output, but also with an increased antibody affinity compared with the primary response.
Antibodies are immunoglobulins, grouped into five classes according to their heavy polypeptide chain. Immunoglobulin G (IgG) forms the bulk of circulating antibodies. Immunoglobulin M (IgM) is normally synthesized early in immune responses. Immunoglobulin A (IgA) is present in breast milk, tears, saliva and other secretions of the alimentary tract, coupled to a secretory piece (a 70 kDa protein) that is synthesized by the epithelial cells. The secretory piece (or component) is a cleavage product of the epithelial plasma membrane Ig receptor, following internalization of the receptor–IgA complex and processing during transcytosis. It protects the immunoglobulin from proteolytic degradation after secretion: secretory IgA (sIgA) contributes to mucosal immunity ( ). IgA deficiency is relatively common, particularly in some ethnic groups (reviewed in ). Immunoglobulin E (IgE) is an antibody that binds to receptors on the surfaces of mast cells, eosinophils and blood basophils, and is found only at low concentrations in the circulation. IgE is mostly associated with type I hypersensitivity (allergic) reactions. Immunoglobulin D (IgD) is found together with IgM as a major membrane-bound immunoglobulin on mature, immunocompetent but naïve (prior to antigen exposure) B cells, acting as the cellular receptor for antigen. IgG and IgM, when bound to self or microbial antigens, can activate the classical complement pathway, leading to a cascade of cytolytic and proinflammatory consequences.
When circulating antibodies bind to antigens they form immune complexes. If present in abnormal quantities, these may cause pathological damage to the vascular system and other tissues, either by interfering mechanically with the permeability of the basal lamina (e.g. some types of glomerulonephritis), or by causing local activation of the complement system that generates inflammatory mediators (e.g. C5a), attacks cell membranes and causes vascular disease. In pregnancy, maternal IgG crosses the placenta and confers passive immunity on the fetus. Maternal milk contains secretory immunoglobulins (IgA) that help to combat bacterial and viral organisms in the alimentary tract of the baby during the first few weeks of postnatal life.
T cells
There are a number of subsets of T (thymus-derived) lymphocytes, all progeny of haemopoietic stem cells in the bone marrow. They develop and mature in the thymus, and subsequently populate peripheral secondary lymphoid organs, which they constantly leave and re-enter via the circulation. As recirculating cells, their major function is immune surveillance. Their activation and subsequent proliferation and functional maturation are under the control of APCs. T cells undertake a wide variety of cell-mediated defensive functions that are not directly dependent on antibody activity, and which constitute the basis of cellular immunity. T-cell responses focus on the destruction of cellular targets such as virus-infected cells, certain bacterial infections, fungi, some protozoal infections, neoplastic cells and the cells of grafts from other individuals (allografts, when the tissue antigens of the donor and recipient are not sufficiently similar). Targets may be killed directly by cytotoxic T cells, or indirectly by accessory cells (e.g. macrophages) that have been recruited and activated by cytokine-secreting helper T cells. A third group, regulatory T cells, acts to regulate or limit immune responses.
Functional groups of T cells are classified according to the molecules they express on their surfaces. The majority of cytokine-secreting helper T cells express CD4, while cytotoxic T cells are characterized by CD8. Regulatory T cells co-express CD4 and CD25. The CD (cluster of differentiation) prefix provides a standard nomenclature for all cell-surface molecules. At present, more than 330 different CD antigens have been designated; each one represents a cell surface molecule that can be identified by specific antibodies. Further details of the classification are beyond the scope of this publication and are given in .
Structurally, T lymphocytes present different appearances depending on their type and state of activity. When resting, they are typically small lymphocytes and are morphologically indistinguishable from B lymphocytes. When stimulated, they become large (up to 15 μm), moderately basophilic cells, with a partially euchromatic nucleus and numerous free ribosomes, rough and smooth endoplasmic reticulum, a Golgi complex and a few mitochondria, in their cytoplasm. Cytotoxic T cells contain dense lysosome-like vacuoles that function in cytotoxic killing.
Cytotoxic T cells
Cytotoxic T lymphocytes (CTLs) express CD8. They are responsible for the direct cytotoxic killing of target cells (e.g. virus-infected cells); the requirement for direct cell–cell contact ensures the specificity of the response. Recognition of antigen, presented as a peptide fragment on MHC class I molecules, triggers the calcium-dependent release of lytic granules by the CTLs. These lysosome-like granules contain perforin (cytolysin), which forms a pore in the target cell membrane, very similar to the membrane attack complex generated at the end of complement activation. They also contain several different serine protease enzymes (granzymes), which enter the target cell via the perforin pore and induce programmed cell death (apoptosis; p. 27 ) of the target. CD8 + CTLs protect their own membranes from lysis by perforin and granzymes using cathepsin B, which is bound to the granules, and which can degrade their suicidal contents. CTLs and natural killer (NK) cells of the innate immune system have similar effector functions in IFN-γ production and cytotoxicity (see below).
You're Reading a Preview
Become a Clinical Tree membership for Full access and enjoy Unlimited articles
If you are a member. Log in here