Microbial adherence occurs through pattern recognition receptors (see Chapters 6 and 7), followed by phagocytosis. Coating microbes with complement components and/or antibodies (opsonization) enhances phagocytosis by monocytes/macrophages and is mediated by specialized complement receptors and antibody receptors expressed by the phagocytic cells (see Chapters 3 and 4).

Q. How do macrophages recognize microbes that have been coated with antibody?

A. Macrophages have Fc receptors that recognize the constant domains of the heavy chains of an antibody molecule (see Figs 1.10 and 1.11).

There are three different types of polymorphonuclear granulocyte

The polymorphonuclear granulocytes (often referred to as polymorphs or granulocytes) consist mainly of neutrophils (PMNs). They:

• are released from the bone marrow at a rate of around 7 million per minute;

• are short-lived (2–3 days) relative to monocytes/macrophages, which may live for months or years.

Like monocytes, PMNs marginate (adhere to endothelial cells lining the blood vessels) and extravasate by squeezing between the endothelial cells to leave the circulation (see Fig. 1.17) to reach the site of infection in tissues. This process is known as diapedesis. Adhesion is mediated by receptors on the granulocytes and ligands on the endothelial cells, and is promoted by chemo-attractants (chemokines) such as interleukin-8 (IL-8) (see Chapter 6).

Like monocytes/macrophages, granulocytes also have pattern recognition receptors, and PMNs play an important role in acute inflammation (usually synergizing with antibodies and complement) in providing protection against microorganisms. Their predominant role is phagocytosis and destruction of pathogens.

The importance of granulocytes is evident from the observation of individuals who have a reduced number of white cells or who have rare genetic defects that prevent polymorph extravasation in response to chemotactic stimuli (see Chapter 16). These individuals have a markedly increased susceptibility to bacterial and fungal infection.

Neutrophils comprise over 95% of the circulating granulocytes

Neutrophils have a characteristic multilobed nucleus and are 10–20 μm in diameter (Fig. 2.5). Chemotactic agents attracting neutrophils to the site of infection include:

• protein fragments released when complement is activated (e.g. C5a);

• factors derived from the fibrinolytic and kinin systems;

• the products of other leukocytes and platelets; and

• the products of certain bacteria (see Chapter 6).

Fig. 2.5 Morphology of the neutrophil

Morphology of the neutrophil. At the ultrastructural level, the azurophilic (primary) granules are larger than the secondary (specific) granules with a strongly electron-dense matrix; the majority of granules are specific granules and contain a variety of toxic materials to kill microbes. A pseudopod (to the right) is devoid of granules. Arrows indicate nuclear pores. Go: Golgi region. Inset: A mature neutrophil in a blood smear showing a multilobed nucleus. × 1500.

(From Zucker-Franklin D, Grossi CE, eds. Atlas of blood cells: function and pathology, 3rd edn. Milan: Edi Ermes; 2003.)

Neutrophils have a large arsenal of enzymes and antimicrobial proteins stored in two main types of granule:

• the primary (azurophilic) granules are lysosomes containing acid hydrolases, myeloperoxidase, and muramidase (lysozyme); they also contain the antimicrobial proteins including defensins, seprocidins, cathelicidins, and bacterial permeability inducing (BPI) protein; and

• the secondary granules (specific to neutrophils) contain lactoferrin and lysozyme (see Fig. 2.5).

During phagocytosis the lysosomes containing the antimicrobial proteins fuse with vacuoles containing ingested microbes (termed phagosomes) to become phagolysosomes where the killing takes place.

Neutrophils can also release granules and cytotoxic substances extracellularly when they are activated by immune complexes (antibodies bound to their specific antigen molecules) through their Fc receptors. This is an important example of collaboration between the innate and adaptive immune systems, and may be an important pathogenetic mechanism in immune complex diseases (type III hypersensitivity, see Chapter 25).

Granulocytes and mononuclear phagocytes develop from a common precursor

Studies in which colonies have been grown in vitro from individual stems cells have shown that the progenitor of the myeloid lineage (CFU-GEMM) can give rise to granulocytes, monocytes and megakaryocytes (Fig. 2.6). Monocytes and neutrophils develop from a common precursor cell, the CFU-granulocyte macrophage cells (CFU-GMs) (see Fig. 2.6). Myelopoiesis (the development of myeloid cells) commences in the liver of the human fetus at about 6 weeks of gestation.

Fig. 2.6 Development of granulocytes and monocytes

Pluripotent hematopoietic stem cells generate colony-forming units (CFUs) that can give rise to granulocytes, erythrocytes, monocytes, and megakaryocytes (CFU-GEMMs). CFU-GEMMs therefore have the potential to give rise to all blood cells except lymphocytes. IL-3 and granulocyte–macrophage colony-stimulating factor (GM-CSF) are required to induce the CFU-GEMM stem cell to enter one of five pathways (i.e. to give rise to megakaryocytes, erythrocytes via burst-forming units, basophils, neutrophils, or eosinophils). IL-3 and GM-CSF are also required during further differentiation of the granulocytes and monocytes. Eosinophil (Eo) differentiation from CFU-Eo is promoted by IL-5. Neutrophils and monocytes are derived from the CFU-GM through the effects of G-CSF and M-CSF, respectively. Both GM-CSF and M-CSF, and other cytokines (including IL-1, IL-4, and IL-6), promote the differentiation of monocytes into macrophages. Thrombopoietin (TP) promotes the growth of megakaryocytes. (B, basophil; BFU-E, erythrocytic burst-forming unit; DC, dendritic cell; Epo, erythropoietin; G, granulocyte; M, monocyte)

CFU-GEMMs mature under the influence of colony-stimulating factors (CSFs) and several interleukins (see Fig. 2.6). These factors, which are relevant for the positive regulation of hemopoiesis, are:

• derived mainly from stromal cells (connective tissue cells) in the bone marrow;

• also produced by mature forms of differentiated myeloid and lymphoid cells.

Bone marrow stromal cells, stromal cell matrix, and cytokines form the microenvironment to support stem cell differentiation into individual cell lineages. Stromal cells produce an extracellular matrix which is very important in establishing cell–cell interactions and enhancing stem cell differentiation. The major components of the matrix are proteoglycans, fibronectin, collagen, laminin, haemonectin and thrombospondin.

Other cytokines, such as transforming growth factor-β (TGFβ) may downregulate hemopoiesis. CFU-GMs taking the monocyte pathway give rise initially to proliferating monoblasts. Proliferating monoblasts differentiate into pro-monocytes and finally into mature circulating monocytes which serve as a replacement pool for the tissue-resident macrophages (e.g. lung macrophages).

Monocytes express CD14 and significant levels of MHC class II molecules

The non-differentiated hemopoietic stem cell marker CD34, like other early markers in this lineage, is lost in mature neutrophils and mononuclear phagocytes. Other markers may be lost as differentiation occurs along one pathway, but retained in the other. For example, the common precursor of monocytes and neutrophils, the CFU-GM cell, expresses major histocompatibility complex (MHC) class II molecules, but only monocytes continue to express significant levels of this marker.

Q. What is the functional significance of the expression of MHC molecules on monocytes?

A. Monocytes can present antigens to helper T cells, but neutrophils generally cannot.

Mononuclear phagocytes and granulocytes display different functional molecules. Mononuclear phagocytes express CD14 which is part of the receptor complex for the lipopolysaccharide of Gram-negative bacteria). In addition, they acquire many of the same surface molecules as mature or activated neutrophils (e.g. the adhesion molecules CD11a and b and Fc receptors which recognize the constant regions of antibodies e.g. CD64 and CD32 – FcγRI and FcγRII respectively).

Neutrophils express adhesion molecules and receptors involved in phagocytosis

CFU-GMs go through several differentiation stages to become neutrophils. As the CFU-GM cell differentiates along the neutrophil pathway, several distinct morphological stages are distinguished. Myeloblasts develop into promyelocytes and myelocytes, which mature and are released into the circulation as neutrophils.

The one-way differentiation of the CFU-GM into mature neutrophils is the result of acquiring specific receptors for growth and differentiation factors at progressive stages of development. Surface differentiation markers disappear or are expressed on the cells as they develop into granulocytes. For example, MHC class II molecules are expressed on the CFU-GM, but not on mature neutrophils.

Other surface molecules acquired during the differentiation process include:

• adhesion molecules (e.g. the leukocyte integrins CD11a, b, and c, associated with CD18 β₂ chains); and

• receptors involved in phagocytosis including complement and antibody Fc receptors.

Neutrophils constitutively express FcγRIII and FcγRII, and FcγRI is induced on activation.

It is difficult to assess the functional activity of different developmental stages of granulocytes, but it seems likely that the full functional potential is realized only when the cells are mature.

There is some evidence that neutrophil activity, as measured by phagocytosis or chemotaxis, is lower in fetal than in adult life. However, this may be due, in part, to the lower levels of opsonins (e.g. complement components and antibodies) in the fetal serum, rather than to a characteristic of the cells themselves.

To become active in the presence of opsonins, neutrophils must interact directly with microorganisms and/or with cytokines generated by a response to antigen. This limitation could reduce neutrophil activity in early life.

Activation of neutrophils by cytokines and chemokines is also a prerequisite for their migration into tissues (see Chapter 9).

Eosinophils, basophils, mast cells and platelets in inflammation

Eosinophils are thought to play a role in immunity to parasitic worms

Eosinophils comprise 2–5% of blood leukocytes in healthy, non-allergic individuals. Human blood eosinophils usually have a bilobed nucleus and many cytoplasmic granules, which stain with acidic dyes such as eosin (Fig. 2.7). Although not their primary function, eosinophils appear to be capable of phagocytosing and killing ingested microorganisms.

Fig. 2.7 Morphology of the eosinophil

The ultrastructure of a mature eosinophil shows granules (G) with central crystalloids. × 17 500. (ER, endoplasmic reticulum; Nu, nucleus; P, nuclear pores) Inset: A mature eosinophil in a blood smear is shown with a bilobed nucleus and eosinophilic granules. × 1000.

(From Zucker-Franklin D, Grossi CE, eds. Atlas of blood cells: function and pathology, 3rd edn. Milan: Edi Ermes; 2003.)

The granules in mature eosinophils are membrane-bound organelles with crystalloid cores that differ in electron density from the surrounding matrix (see Fig. 2.7). The crystalloid core contains the major basic protein (MBP), which:

• is a potent toxin for helminth worms;

• induces histamine release from mast cells;

• activates neutrophils and platelets; and

• of relevance to allergy, provokes bronchospasm.

Other proteins with similar effects are found in the granule matrix, for example:

• eosinophil cationic protein (ECP); and

• eosinophil-derived neurotoxin (EDN).

Release of the granules on eosinophil activation is the only way in which eosinophils can kill large pathogens (e.g. schistosomula), which cannot be phagocytosed. Eosinophils are therefore thought to play a specialized role in immunity to parasitic worms using this mechanism (see Fig. 15.13).

Basophils and mast cells play a role in immunity against parasites

Basophils are found in very small numbers in the circulation and account for less than 0.2% of leukocytes (Fig. 2.8).

Fig. 2.8 Morphology of the basophil

Morphology of the basophil: ultrastructural analysis shows a segmented nucleus (N) and the large cytoplasmic granules (G). Arrows indicate nuclear pores. × 11 000. Inset: This blood smear shows a typical basophil with its deep violet-blue granules. × 1000.

(Adapted from Zucker-Franklin D, Grossi CE, eds. Atlas of blood cells: function and pathology, 3rd edn. Milan: Edi Ermes; 2003.)

The mast cell (Fig. 2.9), which is present in tissues and not in the circulation, is indistinguishable from the basophil in a number of its characteristics, but displays some distinctive morphological features (Fig. 2.10). Their shared functions may indicate a convergent differentiation pathway.

Fig. 2.9 Histological appearance of human connective tissue in mast cells

This micrograph of a mast cell shows dark blue cytoplasm with purple granules. Alcian blue and safranin stain. × 600.

(Courtesy of Dr TS Orr.)

Fig. 2.10 Some distinctive and common characteristics of basophils and mast cells

Some distinctive and common characteristics of basophils and mast cells.

The stimulus for mast cell or basophil degranulation is often an allergen (i.e. an antigen causing an allergic reaction). To be effective, an allergen must cross-link IgE molecules bound to the surface of the mast cell or basophil via its high-affinity Fc receptors for IgE (FcεRI). Degranulation of a basophil or mast cell results in all contents of the granules being released very rapidly. This occurs by intracytoplasmic fusion of the granules, followed by discharge of their contents (Fig. 2.11).

Fig. 2.11 Electron micrograph study of rat mast cells

Rat peritoneal mast cells show electron-dense granules (1). Vacuolation with exocytosis of the granule contents has occurred after incubation with anti-IgE (2). Transmission electron micrographs. × 2700.

(Courtesy of Dr D Lawson.)

Mediators such as histamine, released by degranulation, cause the adverse symptoms of allergy, but, on the positive side, also play a role in immunity against parasites by enhancing acute inflammation.

Platelets have a role in clotting and inflammation

Blood platelets (Fig. 2.12) are not cells, but cell fragments derived from megakaryocytes in the bone marrow. They contain granules, microtubules, and actin/myosin filaments, which are involved in clot contraction. Platelets also participate in immune responses, especially in inflammation.

Fig. 2.12 Ultrastructure of a platelet

Cross-section of a platelet showing two types of granule (G) and bundles of microtubules (MT) at either end. × 42 000.

(Adapted from Zucker-Franklin D, Grossi CE, eds. Atlas of blood cells: function and pathology, 3rd edn. Milan: Edi Ermes; 2003.)

The adult human produces 10¹¹ platelets each day. About 30% of platelets are stored in the spleen, but may be released if required.

Q. What circumstance might require the release of additional platelets into the circulation?

A. Severe blood loss.

Platelets express class I MHC products and receptors for IgG (CD32; FcγRII), which are important in platelet activation via IgG immune complexes. In addition, megakaryocytes and platelets carry:

• receptors for clotting factors (e.g. factor VIII); and

• other molecules important for their function, such as the GpIIb/IIIa complex (CD41) responsible for binding to fibrinogen, fibronectin, vitronectin (tissue matrix), and von Willebrand factor (another clotting factor).

Both receptors and adhesion molecules are important in the activation of platelets.

Following injury to endothelial cells, platelets adhere to and aggregate at the damaged endothelial surface. Release of platelet granule contents, which include de novo synthesized serotonin and endocytosed fibrinogen, results in:

• increased capillary permeability, a feature of inflammation;

• activation of complement (and hence attraction of leukocytes); and

• clotting.

NK cells

NK cells account for up to 15% of blood lymphocytes and express neither T cell nor B cell antigen receptors. They are derived from the bone marrow and morphologically have the appearance of large granular lymphocytes (see Fig. 2.19).

Fig. 2.19 Morphological heterogeneity of lymphocytes

Lymphocyte morphology. (1) The small lymphocyte has no granules, a round nucleus, and a high N:C ratio. (2) The large granular lymphocyte (LGL) has a lower N:C ratio, indented nucleus, and azurophilic granules in the cytoplasm. Giemsa stain. (3) Ultrastructure of the LGL shows characteristic electron-dense peroxidase-negative granules (primary lysosomes, PL), scattered throughout the cytoplasm, with some close to the Golgi apparatus (GA) and many mitochondria (M). × 10 000.

((1) Adapted from Zucker-Franklin D, Grossi CE, eds. Atlas of blood cells: function and pathology, 3rd edn. Milan: Edi Ermes; 2003. (2) Courtesy of Dr A Stevens and Professor J Lowe.)

Functional NK cells are found in the spleen, and cells found in lymph nodes that express CD56 but not CD16 (see below) might represent immature NK cells.

Nevertheless many surface markers are shared with T cells, monocytes/macrophages or neutrophils.

CD16 and CD56 are important markers of NK cells

The presence of CD16 (FcγRIII) is commonly used to identify NK cells in purified lymphocyte populations. CD16 is involved in one of the activation pathways of NK cells and is also expressed by neutrophils, some macrophages and γδ T cells (see below). However on neutrophils, CD16 is linked to the surface membrane by a glycoinositol phospholipid (GPI) linkage, whereas NK cells and γδ T cells express the transmembrane form of the molecule. The CD56 molecule, a homophilic adhesion molecule of the immunoglobulin superfamily (NCAM), is another important marker of NK cells. Combined with the absence of the T cell receptor (CD3), CD56 and CD16 are currently the most reliable markers for NK cells in humans.

Resting NK cells also express the β chain of the IL-2 receptor, and the signal transducing common γ chain of IL-2 and other cytokine receptors (see Fig. 8.18). Therefore, direct stimulation with IL-2 activates NK cells.

The function of NK cells is to recognize and kill virus-infected cells (Fig. 2.13) and certain tumor cells by mechanisms described in chapter 10.

Fig. 2.13 An NK cell attached to a target cell

An NK cell (NK) attached to a target cell (TC). × 4500.

(Courtesy of Dr G Arancia and W Malorni, Rome.)

Classical and non-classical MHC class I molecules (see Fig. 5.15) are ligands for inhibitory receptors on the NK cells which prevent killing and this explains why normal body cells (all of which normally express MHC class I molecules) are not targeted by NK cells.

Downregulation or modification of MHC molecules in virus-infected cells and some tumors makes them susceptible to NK cell-mediated killing.

Q. What advantage is there for a virus in causing the loss of MHC class I molecules in the cell it has infected?

A. The infected cell can no longer be recognized by cytotoxic T cells (see Fig. 1.12).

NK cells are also able to kill targets coated with IgG antibodies via their receptor for IgG (FcγRIII, CD16). This property is referred to as antibody-dependent cellular cytotoxicity (ADCC).

NK cells release interferon-γ (IFNγ) and other cytokines (e.g. IL-1 and GM-CSF) when activated, which might be important in the regulation of hemopoiesis and immune responses.

Antigen presenting cells

APCs are a heterogeneous population of leukocytes that are important in innate immunity (see Fig. 2.2) and play a pivotal role in the induction of functional activity of T helper (TH) cells.

In this regard, APCs are seen as a critical interface between the innate and adaptive immune systems. There are professional APCs (dendritic cells, macrophages and B cells) constitutively expressing MHC class II and co-stimulatory molecules, and non-professional APCs which express MHC class II and co-stimulatory molecules for short periods of time throughout sustained inflammatory responses. This group is comprised of fibroblasts, glial cells, pancreatic β cells, thymic epithelial cells, thyroid epithelial cells and vascular endothelial cells.

Both macrophages and B cells are rich in membrane MHC class II molecules, especially after activation, and are thus able to process and present specific antigens to (activated) T cells (see Chapter 8).

Somatic cells other than immune cells do not normally express class II MHC molecules, but cytokines such as IFNγ and tumor necrosis factor-α (TNFα) can induce the expression of class II molecules on some cell types, and thus allow them to present antigen (non-professional APCs). This induction of ‘inappropriate’ class II expression might contribute to the pathogenesis of autoimmune diseases and to prolonged inflammation (see Chapter 20).

Dendritic cells are derived from several different lineages

Functionally, dendritic cells (DC) are divided into those that both process and present foreign protein antigens to T cells – ‘classical’ dendritic cells (DCs) – and a separate type that passively presents foreign antigen in the form of immune complexes to B cells in lymphoid follicles – follicular dendritic cells (FDCs; Fig. 2.14).

Fig. 2.14 Different kinds of antigen-presenting cells (APCs)

There are two main types of dendritic cells – classical DC and follicular dendritic cells (FDCs). (1) Immature DCs are derived from bone marrow and interact mainly with T cells. They are highly phagocytic, take up microbes, process the foreign microbial antigens into small peptides, and become mature APCs carrying the processed antigen (a peptide) on their surface with specialized MHC molecules. Specific T cells recognize the displayed peptide in a complex with MHC and, in the presence of cytokines produced by the mature DC, proliferate and also produce cytokines. (2) FDCs are not bone marrow derived and interact with B cells. In the B cell follicles of lymphoid organs and tissues they bind small immune complexes (IC, called iccosomes). Antigen contained within the IC is presented to specific B cells in the lymphoid follicles. This protects the B cell from cell death. The B cell then proliferates and with T cell help can leave the follicle and become a plasma cell or a memory cell (see Fig. 2.48).

Most DCs derive from one of two precursors:

• a myeloid progenitor (DC1) that gives rise to myeloid DCs, otherwise called bone-marrow derived or bm-DCs; and

• a lymphoid progenitor (DC2) that develops into plasmacytoid DCs (pDCs).

A summary of the main properties of myeloid and plasmacytoid dendritic cells is shown in Figure 2.15.

Fig. 2.15 Myeloid and plasmacytoid dendritic cells (DCs)

There are two main types of dendritic cells defined by their origin. They have differences in their localization markers and cytokine.

Myeloid DCs can also be divided into at least three types: Langerhans’ cells (LCs), dermal or interstitial DCs (DDC-IDCs) and blood monocyte-derived DCs (moDCs).

Different populations of DCs can be identified by their surface markers. Myeloid DCs, but not pDCs express CD1a and CD208, whilst DDC-IDC and moDC express also CD11b. Langerhans’ cells have so called Birbeck granules containing Langerin. It appears that various populations of myeloid DCs may represent different stages in their maturation and migration in the body (see below).

BM-DCs express various receptors that are involved in antigen uptake:

• C-type lectin receptors – for glycosyl groups, e.g. macrophage mannose receptor (MMR) family;

• Fc receptors for IgG, IgE;

• receptors for heat shock protein–peptide complexes;

• receptors for apoptotic corpses;

• ‘scavenger’ receptors – for sugars, lipid etc;

• toll-like receptors (TLRs).

Before DCs take up antigen (become loaded) they are called immature DCs and express various markers characteristic for this, resting, stage, the most important being chemokine receptors CCR1, CCR5 and CCR6. DCs are attracted to the infection site by chemokines through these receptors (see Chapter 6).

Mature DCs loaded with antigen down-regulate expression of CCR1, 5, 6 and up-regulate CCR7. This encourages their migration from various tissues into peripheral lymphatics, where CCR7 interacts with secondary lymphoid tissue chemokine SLC (CCL21) expressed on vascular endothelium (see Fig. 6.15).

DCs are found primarily in the skin, lymph nodes, and spleen, and within or underneath most mucosal epithelia. They are also present in the thymus, where they present self antigens to developing T cells.

Langerhans’ cells and interdigitating dendritic cells are rich in MHC class II molecules

Langerhans’ cells in the epidermis and in other squamous epithelia migrate via the afferent lymphatics into the paracortex of the draining lymph nodes (Fig. 2.16). Here, they interact with T cells and are termed interdigitating cells (IDCs, Fig. 2.17). These DCs are rich in class II MHC molecules, which are important for presenting antigen to helper T cells.

Fig. 2.16 Migration of antigen-presenting cells (APCs) into lymphoid tissues

Bone marrow-derived dendritic cells (DCs) are found especially in lymphoid tissues, in the skin, and in mucosa. DCs in the form of Langerhans’ cells are found in the epidermis and in mucosa and are characterized by special granules (the tennis racquet-shaped Birbeck granules; not shown here). Langerhans’ cells are rich in MHC class II molecules, and carry processed antigens. They migrate via the afferent lymphatics (where they appear as ‘veiled’ cells) into the paracortex of the draining lymph nodes. Here they make contact with T cells. These ‘interdigitating dendritic cells (IDCs)’, localized in the T cell areas of the lymph node, present antigen to T helper cells. Antigen is exposed to B cells on the follicular dendritic cells (FDCs) in the germinal centers of B cell follicles. Some macrophages located in the outer cortex and marginal sinus may also act as APCs. In the thymus, APCs occur as IDCs in the medulla. (HEV, high endothelial venule)

Fig. 2.17 Ultrastructure of an interdigitating dendritic cell (IDC) in the T cell area of a rat lymph node

Intimate contacts are made by APCs with the membranes of the surrounding T cells. The cytoplasm contains a well-developed endosomal system and does not show the Birbeck granules characteristic of skin Langerhans’ cells. × 2000. (I, IDC nucleus; Mb, IDC membrane; T, T cell nucleus)

(Courtesy of Dr BH Balfour.)

Q. What function could the migration of Langerhans’ cells to the lymph nodes from the mucosa or skin serve?

A. The migration of Langerhans’ cells provides an efficient mechanism for carrying antigen from the skin and mucosa to the TH cells in the lymph nodes, and are rich in class II MHC molecules, which are important for presenting antigen to TH cells. Lymph nodes provide the appropriate environment for lymphocyte proliferation.

BM-DCs are also present within the germinal centers (GCs) of secondary lymphoid follicles (i.e. they are the MHC class II molecule-positive germinal center DCs [GCDCs]). In contrast to FDCs, they are migrating cells, which on arrival in the GC interact with germinal center T cells and are probably involved in antibody class switching (see Chapter 9).

The thymus is of crucial importance in the development and maturation of T cells. In thymus there are cortical DCs and IDCs which are especially abundant in the medulla (see Fig. 2.16). They participate in two important stages in T cell maturation/differentiation in thymus positive and negative selection respectively (see below).

FDCs lack class II MHC molecules and are found in B cell areas

Unlike the APCs that actively process and present protein antigens to T cells, FDCs have a passive role in presenting antigen in the form of immune complexes to B cells. They are therefore found in the primary and secondary follicles of the B cell areas of the lymph nodes, spleen, and MALT (see Fig. 2.16). They are a non-migratory population of cells and form a stable network (a kind of web) by establishing strong intercellular connections via desmosomes.

FDCs lack class II MHC molecules, but bind antigen via complement receptors (CD21 and CD35), which attach to complement associated with immune complexes (iccosomes; Fig. 2.18). They also express Fc receptors. The FDCs produce chemokines that are important in homing of B cells to the follicular areas in lymphoid tissues. They are not bone marrow derived, but are of mesenchymal origin.

Fig. 2.18 Follicular dendritic cell

An isolated follicular dendritic cell (FDC) from the lymph node of an immunized mouse 24 hours after injection of antigen. The FDC is of intermediate maturity with smooth filiform dendrites typical of young FDCs, and beaded dendrites, which participate in the formation of iccosomes (immune complexes) in mature FDCs. The adjacent small white cells are lymphocytes.

(Electron micrograph kindly provided by Dr Andras Szakal; reproduced by permission of the Journal of Immunology.)

Lymphocytes

Lymphocytes are phenotypically and functionally heterogeneous

Large numbers of lymphocytes are produced daily in the primary or central lymphoid organs (i.e. thymus and postnatal bone marrow). Some migrate via the circulation into the secondary lymphoid tissues (i.e. spleen, lymph nodes, and MALT).

The average human adult has about 2 × 10¹² lymphoid cells and lymphoid tissue as a whole represents about 2% of total body weight. Lymphoid cells account for about 20% of the leukocytes in the adult circulation.

Many mature lymphoid cells are long-lived, and persist as memory cells for many years.

Q. Given that there are roughly 10⁹ lymphocytes/liter of blood and an average of 5 liters of blood in an individual, and that roughly 2 × 10⁹ new cells are produced each day, what can you infer about the location and life span of lymphocytes within an individual?

A. This implies that less than 1% of an individual’s lymphocytes are in the circulation. This highly selective population of lymphocytes is mostly en route between tissues. The data also imply that many lymphocytes must die each day to maintain the overall balance of the lymphoid system, and that the average life span of a lymphocyte will be months to years. The actual values are, however, enormously variable, depending on the type of lymphocyte.

Lymphocytes are morphologically heterogeneous

In a conventional blood smear, lymphocytes vary in both size (from 6–10 μm in diameter) and morphology.

Differences are seen in:

• nuclear to cytoplasmic (N:C) ratio;

• nuclear shape; and

• the presence or absence of azurophilic granules.

Two distinct morphological types of lymphocyte are seen in the circulation as determined by light microscopy and a hematological stain such as Giemsa (Fig. 2.19):

• the first type is relatively small, is typically agranular and has a high nuclear to cytoplasmic (N:C) ratio (Fig. 2.19[1]);

• the second type is larger, has a lower N:C ratio, contains cytoplasmic azurophilic granules, and is known as the large granular lymphocyte (LGL).

LGLs should not be confused with granulocytes, monocytes, or their precursors, which also contain azurophilic granules.

Most T cells express the αβ T cell receptor (see below) and, when resting, can show either of the above morphological patterns.

Most T helper (TH) cells (approximately 95%) and a proportion (approximately 50%) of cytotoxic T cells (TC or CTL) have the morphology shown in Figure 2.19(1).

The LGL morphological pattern displayed in Figure 2.19(2) is shown by less than 5% of TH cells and by about 30–50% of TC cells. These cells display LGL morphology with primary lysosomes dispersed in the cytoplasm and a well-developed Golgi apparatus, as shown in Figure 2.19(3).

Most B cells, when resting, have a morphology similar to that seen in Figure 2.19(1) under light microscopy.