Non-specific defences

Many non-specific mechanisms prevent invasion of the body by micro-organisms:

Fig. 9.1 Mechanical and secretory barriers to infection.

Innate immunity

The innate immune system is activated by pattern recognition receptors on dendritic cells recognising conserved polysaccharide molecular patterns on microbes. Key components include:

Specific immunity

The immune system has four essential features:

A specific or adaptive immune response consists of two parts: a specific response to the particular antigen and a non-specific augmentation of the effect of that response. For the specific response there is a quicker and larger response the second time that a particular antigen is encountered; memory of the initial specific immune response provides the efficiency.

The immune system has to recognise all pathogens, past and future, and must have considerable diversity of response. This diversity is partly genetic (germline encoded) and partly generated by somatic mutation during maturation of the immune system.

Immune responses, both innate and adaptive, have two phases: first the recognition phase, involving antigen-presenting cells and T-lymphocytes (see Key molecules), in which the antigen is recognised as foreign; and second the effector phase, in which antibodies and effector T-lymphocytes eliminate the antigen, often by recruiting innate mechanisms such as complement or macrophage activation.

Antigens

Antigens are substances able to provoke an immune response and react with the immune products. They react both with the T-cell recognition receptor and with antibody. An antigenic molecule may have several antigenic determinants (epitopes); each epitope can bind with an individual antibody, and a single antigenic molecule can therefore provoke many antibody molecules with different binding sites. Some low molecular weight molecules, called haptens, are unable to provoke an immune response themselves, although they can react with existing antibodies. Such substances need to be coupled to a carrier molecule in order to have sufficient epitopes to be antigenic. For some chemicals, such as drugs, the carrier may be a host protein—called an auto-antigen. The tertiary structure, as well as the amino acid sequence, is important in determining antigenicity.

Antigens are conventionally divided into thymus-dependent and thymus-independent antigens. Thymus-dependent antigens require T-cell participation to provoke the production of antibodies; most proteins are examples. Thymus-independent antigens require no T-cell co-operation for antibody production; they directly stimulate specific B-lymphocytes by cross-linking antigen receptors on the B-cell surface but provoke poor immunological memory. Such antigens include bacterial cell wall polysaccharides.

Factors other than the intrinsic properties of the antigen also influence the quality of the immune response. These include:

Substances that improve a host’s immune response to a separate antigen are called adjuvants; these are routinely used in immunisation programmes in childhood.

Superantigen is the term given to foreign proteins that simultaneously activate large numbers of T-lymphocytes carrying a particular T-cell receptor V-beta gene (see T-cell receptors). Widespread T-cell activation results in florid cytokine release, as exemplified by toxic shock syndrome induced by certain streptococcal toxins.

Antibody

Humoral immunity is dependent on the production of antibodies and their actions. All antibodies belong to the immunoglobulin class of proteins and are produced by plasma cells, themselves derived from B-lymphocytes. The basic structure of an immunoglobulin molecule is shown in Figure 9.2. It has a four-chain structure: two identical heavy (H) chains (molecular weight 50 kD) and two identical light (L) chains (mol wt 25 kD). There are two alternative types of light chain, known as kappa and lambda; an antibody molecule has either two kappa or two lambda light chains, never one of each. In contrast, there are five types of heavy chain, each with important functional differences (Table 9.1). The heavy chains determine the class (isotype) of the antibody and the physiological function of the antibody molecule. Once the antigen-binding site has reacted with its antigen, the molecule undergoes a change in the conformation of its heavy chains in order to take part in effector functions (Table 9.1).

Fig. 9.2 Basic structure of an immunoglobulin molecule. The two identical light chains and two identical heavy chains are held together by disulphide bonds. (Fab, fragment antigen binding; Fc, fragment crystallisable; V_L, variable domain of a light chain; V_H, variable domain of a heavy chain.)

Table 9.1Immunoglobulin classes and their functions

The amino (N) terminal regions of the heavy and light chains include the antigen-binding sites. The amino acid sequences of these N- terminal domains vary between different antibody molecules of the same isotype and are known as variable (V) regions. Most of these differences reside in three hypervariable areas of the molecule, each only 6–10 amino acid residues long. In the folded molecules, these hypervariable regions in each chain come together, with their counterparts on the other pair of heavy and light chains, to form the antigen-binding site. The structure of this part of the antibody molecule is unique to that molecule and is known as the idiotypic determinant. In any individual, about 10⁶–10⁷ different antibody molecules could be made up by 10³ different heavy chain variable regions associating with 10³ different light chain variable regions. Somatic mutation during multiple divisions of B-lymphocytes generates further diversity of around 10¹⁴ antibody specificities.

IgM is the oldest class of immunoglobulin in evolutionary terms. It is a large molecule consisting of five basic units held together by a joining (J) chain; it penetrates poorly into tissues on account of its large size (Table 9.1). The major physiological role of IgM is intravascular neutralisation of organisms (especially viruses) aided by its 10 antigen-binding sites. IgM also has multiple complement-binding sites; this results in excellent complement activation and lysis of the organism or removal of the antigen–antibody–complement complexes by complement receptors on phagocytic cells. It is the first class of antibody to be formed in response to an initial encounter with an antigen (primary immune response).

IgG is a smaller immunoglobulin which penetrates tissues easily. It is the most abundant immunoglobulin in the plasma and extracellular fluid. It is the only immunoglobulin that crosses the placenta to provide immune protection to the neonate; this is an active process involving specific placental receptors for the Fc portion of the IgG molecule. Polymorphs and macrophages also have surface receptors for the Fc fragment of IgG; thus binding of IgG to particulate antigen promotes adhesion of these cells and subsequent phagocytosis of the antigen.

There are four subclasses of IgG: IgG₁ and IgG₃ activate complement efficiently and are responsible for clearing most protein antigens; IgG₂ and IgG₄ react predominantly with carbohydrate antigens (in adults).

IgA is sometimes referred to as ‘mucosal antiseptic paint’. It is secreted locally by plasma cells in the intestinal and respiratory mucosa and is an important constituent of breast milk. It consists of two basic units (a dimer) linked by a ‘joining’ or J chain. The addition of a ‘secretory component’ prevents digestion of the immunoglobulin molecule by enzymes present in intestinal or bronchial secretions. Secretory component is a fragment of the polymeric immunoglobulin receptor synthesised by epithelial cells and transports secretory IgA from the mucosa into the lumen of the gut or bronchi.

There is little free IgD or IgE in serum or normal body fluids. These two classes mainly act as cell receptors. IgD is synthesised by antigen-sensitive B-lymphocytes and acts as a cell surface receptor for antigen. IgE is produced by plasma cells but taken up by specific IgE receptors on mast cells and basophils. IgE probably evolved as a way of expelling intestinal parasites via mast cell degranulation.

T-cell receptors

Like B-cells, each T-cell is committed to a given antigen, which it recognises by one of two types of T-cell receptor (TCR). T-cells have either alpha/beta TCR (a heterodimer of alpha and beta chains) or gamma/delta TCR (a heterodimer of gamma and delta chains). Alpha/beta TCRs predominate in adults, although 10% of T-cells in epithelial structures are of the gamma/delta TCR type. Each type of TCR is associated with several transmembrane proteins which make up the cluster differentiation 3 (CD3) molecule (Fig. 9.3) to form the CD3–TCR complex responsible for taking the antigen recognition signal inside the cell (transduction). The CD3 antigen is widely used as a marker of mature T-cells in diagnostic and investigative pathology.

Fig. 9.3 The structure of the T-cell receptor (TCR). The variable regions of the alpha (α) and beta (β) chains make up the T idiotype. The TCR is closely associated on the cell surface with the CD3 molecule.

The TCR complex recognises small processed antigen peptides in the context of major histocompatibility complex (MHC) class I and II antigens (see below), depending on the type of T-cell. Helper T-cells recognise MHC class II molecules in association with foreign antigen and use the CD4 molecule to enhance binding and intracellular signalling. Cytotoxic T-cells recognise antigen associated with MHC class I molecules and use CD8 molecules for increased binding and signalling. However, recognition of processed antigen alone is not enough to activate T-cells. Additional signals through soluble interleukins are needed; some of these are generated during ‘antigen processing’.

Major histocompatibility complex antigens

Histocompatibility antigens were so named because of the vigorous reactions they provoked during mismatched organ transplantation. However, these antigens play a fundamental role in the normal immune response by presenting antigenic peptides to T-cells. Human major histocompatibility complex (MHC) antigens are also known as human leukocyte antigens(HLAs). MHC antigens are cell surface glycoproteins of two basic types: class I and class II (Fig. 9.4). They exhibit extensive genetic polymorphism with multiple alleles at each locus. As a result, genetic variability between individuals is very great and most unrelated individuals possess different HLA molecules. This means that it is very difficult to obtain perfect HLA matches between unrelated persons for transplantation.

Fig. 9.4 The major histocompatibility complex on chromosome 6 and MHC class I and II antigens.

The antigen-specific receptor of an individual T-cell (TCR) will only recognise antigen as part of a complex of antigenic peptide and that individual’s MHC. This process of dual recognition of peptide and MHC molecule is known as MHC restriction because the MHC molecule restricts the ability of the T-cell to recognise antigen. T-cells from one person will not co-operate with antigen-presenting cells from a person of different HLA type.

MHC class I antigens are subdivided into three groups: A, B and C. Each group is controlled by a different gene locus within the major histocompatibility complex on chromosome 6 (Fig. 9.4). The products of the genes at all three loci are chemically similar. MHC class I antigens (Fig. 9.5) are made up of a heavy chain (alpha) controlled by a gene in the relevant MHC locus, associated with a smaller chain called beta-2-microglobulin, controlled by a gene on chromosome 15. The differences between individual MHC class I antigens are due to variations in the alpha chains; the beta-2-microglobulin component is constant. The detailed structure of class I antigens was determined by X-ray crystallography. This shows that small antigenic peptides are tightly bound to a groove in the surface alpha chains.

Fig. 9.5 MHC class I and class II antigens.

MHC class II antigens have a folded structure similar to class I antigens with the peptide-binding groove found between the alpha and beta chains (Fig. 9.5). Whereas class I molecules are expressed by most nucleated cells, expression of class II molecules is restricted to dendritic cells, B-lymphocytes, activated T-cells, macrophages, inflamed vascular endothelium and some epithelial cells. However, other cells (e.g. thyroid, pancreas, gut epithelium) can be induced to express class II molecules under the influence of interferon-gamma released during inflammation. In humans, there are three groups of class II antigen: the loci are known as HLA-DP, HLA-DQ and HLA-DR.

MHC class III antigens (Fig. 9.4) constitute early complement proteins C4 and C2. Other inflammatory proteins, e.g. tumour necrosis factor (TNF), are encoded in adjacent areas.

Accessory and co-stimulatory molecules

T-cell activation needs more than just binding between the T-cell receptor and the MHC class II molecule and processed antigen complex on the antigen-presenting cell.

Accessory and co-stimulatory molecules are needed for efficient binding and signalling (Fig. 9.6). Each accessory molecule has a corresponding protein, or ligand, to which it binds. The interaction between antigen-presenting cells and T-cells is strongly influenced by accessory molecules which function as co-stimulators; for example, CD80 and CD86 on the activated dendritic cell engage with their counter-receptors CD28 and CTLA-4 (CD152) on the T-cell surface (Fig. 9.6). A functional co-stimulatory pathway is essential for T-cell activation. In the absence of a co-stimulatory signal, interaction between the dendritic cell and T-cell leads to T-cell unresponsiveness (a state called anergy).

Fig. 9.6 Accessory and co-stimulatory molecules on T-lymphocytes and their ligands on antigen-presenting cells (APCs).

Adhesion molecules mediate cell-to-cell adhesion as well as adhesion between leukocytes and endothelial cells, and are grouped into two main families: integrins and selectins.

The migration of leukocytes to sites of inflammation depends on three key sequential steps mediated by adhesion molecules:

Integrins are subdivided into five families (beta-1 to beta-5 integrins) which mediate binding of lymphocytes and monocytes to the endothelial adhesion receptor called vascular cell adhesion molecule (VCAM-1). Defective expression of certain integrins is associated with a severe immunodeficiency characterised by marked neutrophil leukocytosis because neutrophils are unable to migrate from blood vessels into sites of infection.

The selectin family comprises three glycoproteins designated by the prefixes E- (endothelial), L- (leukocyte) or P- (platelet) to denote the cells on which they were first described. Selectins bind strongly to carbohydrate molecules on leukocytes and endothelial cells, and regulate the homing of these cells to sites of inflammation.

Cytokines

Cytokines are soluble mediators secreted by lymphocytes (lymphokines) or by macrophages/monocytes (monokines). They act as stimulatory or inhibitory signals between cells.

Cytokines that act between cells of the immune system are called interleukins; those that induce chemotaxis of leukocytes are called chemokines. All cytokines share common features:

Among the array of cytokines produced by macrophages and T-cells, interleukin-1 (IL-1) and interleukin-2 (IL-2) have a pivotal role in amplifying immune responses. IL-1 acts on a wide range of targets, including T- and B-cells (Table 9.2). In contrast, the effects of IL-2 are restricted largely to lymphocytes: it has a trophic effect on T-cells, IL-2 receptor-bearing B-cells and natural killer (NK) cells. The considerable overlap between individual cytokines and interleukins is summarised in Table 9.3.

Table 9.2 Actions of interleukin-1

Table 9.3 Cytokines and their actions

Cytokine	Source	Action
IL-2	T-cells	Proliferation and maturation of T-cells, induction of IL-2 receptors and activation of NK cells
IL-4 IL-5		Induction of MHC class II, FcR and IL-2R on B- and T-cells; induction of isotype switching in B-cells; facilitate IgE production (mainly IL-4)
IL-6	Macrophages, fibroblasts, T-cells, mast cells	Growth and differentiation of T-, B- and haemopoietic cells; acute phase response
IL-8	Monocytes/macrophages, T-cells, fibroblasts, keratinocytes, endothelial cells, NK cells	Chemotaxis of neutrophils
IL-10	T-cells, monocytes, keratinocytes	Inhibitory cytokine production
IL-12	B-cells, monocytes/macrophages	Synergism with IL-2; regulates IFN-gamma production
IL-13	T-cells	Actions overlap with IL-4
GM-CSF	T-cells, macrophages, endothelial cells	Stimulates growth of polymorphs and mononuclear progenitors
IFN-gamma	T-cells, NK cells	Activation of macrophages, endothelial cells and NK cells; increases expression of MHC class I and II; inhibits IgE production
TNF	Macrophages, T-cells, B-cells	Promotion of inflammation; interferes with catabolism in muscle and fat

T- and B-lymphocyte development

All lymphoid cells originate in the bone marrow although the nature of the uncommitted lymphoid stem cell remains unclear (Fig. 9.7). An understanding of the developmental pathway is important, not only to clarify the physiology of the normal immune response but also because some leukaemias and immunodeficiency states reflect maturation arrest of cells in their early stages of development. Lymphoid progenitors destined to become T-lymphocytes migrate from the bone marrow into the cortex of the thymus where further differentiation into mature T-cells occurs. Passage of T-cells from the thymic cortex to the medulla is associated with the acquisition of characteristic surface glycoprotein molecules so that medullary thymocytes resemble mature peripheral blood T-cells. T-cell development in the thymus is characterised by a process of positive selection whereby T-cells that recognise and bind with low affinity to fragments of self-antigen in association with self-MHC molecules proceed to full maturation. In contrast, T-cells that do not recognise self-MHC or that recognise and bind with high affinity to self-antigen are selected out—negative selection—and do not develop further. Negatively selected T-cells kill themselves by apoptosis, i.e. programmed cell death. This process is an important mechanism in preventing autoimmune disease. In summary, the thymus selects out the useful, neglects the useless and destroys the harmful, i.e. autoreactive T-cells.

Fig. 9.7 Development of lymphocytes from a pluripotential stem cell in the bone marrow.

In contrast, B-cell development occurs in the bone marrow and depends on the secretion of cytokines by stromal cells.

Primary and secondary lymphoid organs

The thymus and the bone marrow are primary lymphoid organs. They contain cells undergoing a process of maturation from stem cells to antigen-sensitive but antigen-restricted cells. This process of maturation is independent of antigenic stimulation. In contrast, secondary lymphoid organs are those that contain antigen-reactive cells in the process of recirculating through the body. They include the lymph nodes, spleen and mucosa-associated lymphoid tissues. Antigenic stimulation changes the relative proportions of mature cell types in secondary tissues.

Peripheral T- and B-cells circulate in a characteristic pattern through the secondary lymphoid organs. Most of the recirculating cells are T-cells and the complete cycle takes about 24 hours; some B-cells, including long-lived memory B-cells, also recirculate. Lymphocyte circulation is strongly influenced by chemokine receptors on lymphocyte surfaces which act as homing agents. Adhesion molecules direct cells to their respective ligands on high endothelial venules of lymph nodes.

Lymph node architecture is well adapted to its function (Fig. 9.8). Lymphatic vessels draining the tissues penetrate the lymph node capsule and drain into the marginal sinus from which a branching network of sinuses passes through the cortex to the medulla and into the efferent lymphatic. This network provides a filtration system for antigens entering the node from peripheral tissue.

Fig. 9.8 Structure of a normal lymph node. The locations of T- and B-lymphocytes are shown in Figs 9.10–9.12.

The cortex contains primary follicles of B-lymphocytes, surrounded by T-cells in the ‘paracortex’ (Figs 9.9–9.11). There is a meshwork of dendritic cells that express MHC class II antigen throughout the lymph node, and these cells filter and present antigen to lymphoid cells. On antigen challenge, the ‘primary’ follicles of the lymph node develop into ‘secondary’ follicles which contain germinal centres. These comprise mainly B-cells with a few helper T-cells and a mantle zone of the original primary follicle B-cells. B-cells in a secondary follicle are antigen-activated and more mature; most have IgG on their surfaces, whereas those in the primary follicle and mantle zone bear both IgD and IgM. Activated B-cells migrate from the follicle to the medulla, where they develop into plasma cells (Fig. 9.12) in the medullary cords before releasing antibody into the efferent lymph.

Fig. 9.9 Lymphoid follicle in a lymph node, surrounded by paracortex.

Fig. 9.10 A lymphoid follicle stained with a monoclonal antibody (anti-CD20) that reacts with B-cells. (Positive cells stain brown.)

Fig. 9.11 Immunohistochemical identification of T-cells. This section, adjacent to that in Fig. 9.10, is stained with a monoclonal antibody that reacts with T-cells (anti-CD3). Cells of the paracortex are stained (arrow) and there are few T-cells in the follicle.

Fig. 9.12 Plasma cells. Note the eccentrically placed nuclei.

The majority of naive T-cells entering the lymph node will leave again immediately via efferent lymphatics. Naive T-cells that recognise specific antigen differentiate into effector T-cells before re-entering the circulation.

Antigen presentation

The first stage of an immune response to any antigen is the processing and presentation of that antigen to lymphocytes by specialised antigen-presenting cells (APCs). T-cells cannot recognise antigen without it. The interaction between APCs and T-cells is strongly influenced by a group of cell-surface molecules that act as co-stimulators. Thus, CD80 and CD86 on the APC engage with their counter-receptors CD28 and CTLA-4 (cytotoxic T-lymphocyte antigen 4; CD152) on the T-cell surface (Fig. 9.6). Normal functioning of theco-stimulatory pathway is vital for T-cell activation. In the absence of a co-stimulatory signal, interaction between the APC and T-cell leads to T-cell unresponsiveness, or anergy. Antagonists to co-stimulatory molecules disrupt immune responses, an observation of potential therapeutic importance; for instance, antagonists to CTLA-4 are being used experimentally to treat severe autoimmune diseases and to prevent graft rejection.

Processed antigen is presented to T-cells alongside MHC class II antigens on the APC surface because T-cells do not recognise processed antigen alone. The most efficient APCs are the interdigitating dendritic cells found in the T-cell regions of a lymph node. Such cells have high concentrations of MHC class I and II molecules, co-stimulatory molecules (CD80, CD86) and adhesion molecules on their surfaces but limited enzymatic powers, so enabling effective processing and presentation of antigen without complete digestion.

Antibody production

Antibody production involves at least three types of cell: antigen-presenting cells, B-lymphocytes and helper T-cells.

Antibodies are synthesised by B-cells and their mature progeny, called plasma cells (Fig. 9.12). B-cells are readily recognised because they express immunoglobulin on their surfaces. During development, B-cells first show intra-cellular mu chains and then surface IgM. These cells are able to switch from production of IgM to IgG, IgA or IgE as they mature, a process known as isotype switching (Fig. 9.13). This maturation sequence fits with the kinetics of an antibody response: the primary response is mainly IgM and the secondary response predominantly IgG (Fig. 9.14).

Fig. 9.13 Interaction between CD40L on T-cells and CD40 on B-cells leads to isotype switching under the influence of IL-4. (TcR, T-cell receptor.)

Fig. 9.14 Primary and secondary antibody responses.

Isotype switching is mediated by the interaction of two important proteins: CD40 expressed on the B-cell surface engages with its ligand, CD40L (CD154), on activated T-cells (under the influence of IL-4) to induce B-cells to switch immunoglobulin production from IgM to IgG, IgA or IgE (Fig. 9.13). Deficiency of either CD40 or CD40L in humans leads to a severe immunodeficiency characterised by inability to switch from IgM to IgG antibody production.

Each B-cell is committed to the production of an antibody that has a unique Vh–Vl combination, the idiotype, and the surface immunoglobulin and secreted immunoglobulin are identical. Contact with antigen and factors released by helper T-cells (IL-4, -5, -6) stimulate the B-cell to divide and differentiate, generating more antibody-producing cells, all of which make the same antibody with the same idiotype. Simultaneously, a population of memory cells is produced which express the same surface immunoglobulin receptor. The result of these cell divisions is that a greater number of antigen-specific B-cells becomes available when the animal is exposed to the same antigen at a later date. This process, known as clonal expansion, helps to account for the amplified secondary response. As well as being quicker and more vigorous (Fig. 9.14), secondary responses are more efficient because the antibodies bind more effectively to the antigen, i.e. with higher affinity.

A minority of B-cells will respond directly to antigens called T-independent antigens, which have repeating, identical, antigenic determinants and provoke predominantly IgM antibody responses. B-cells, however, will not usually respond directly to antigen, even when presented by appropriate accessory cells. A second signal is needed to trigger the B-cell; this signal is normally provided by CD4+ helper T-cells.

T-cell help is antigen-specific. Only helper T-cells that have responded to antigen presented by macrophages can subsequently help B-cells already committed to that antigen. Helper T-cells recognise both antigen and MHC class II antigens as a complex on the presenting cells. They then recognise the same combination of antigen and class II molecule on the corresponding B-cell but co-stimulation is also required (Fig. 9.13). When helper T-cells meet an antigen for the first time, the limited number reacting with that antigen are activated to provide help for B-cells. They undergo blast transformation and proliferation, i.e. clonal expansion, so the immune response on second and subsequent exposures is quicker and more vigorous.

Other mechanisms help to improve this efficiency. Memory cells (which bear the surface marker CD45RO) have increased numbers of adhesion molecules (LFA-1, CD2, LFA-3, ICAM-1) plus a higher proportion of high-affinity receptors for the relevant antigen. Memory cells are therefore easily activated and produce high concentrations of IL-2 to recruit more helper T-cells. Thus T-cell memory is a combination of a quantitative increase of T-cells and a qualitative change in the efficiency of those T-cells.

Helper T-cells are further grouped into two distinct subsets depending on their cytokine profile. Th1 cells secrete TNF and interferon-gamma and mediate cellular immunity. In contrast, Th2 cells predominantly secrete IL-4, IL-5, IL-10 and IL-13 (Fig. 9.15) and are responsible for stimulating vigorous antibody production by B-cells. T-cells expressing cytokine profiles common to both Th1 and Th2 cells are designated Th0.

Fig. 9.15 Th1 and Th2 cells secrete different cytokines. Some cytokines provide inhibitory feedback (- – – -) on subsets of Th cells.

A Th1 cytokine profile provides protection against intracellular pathogens, while a Th2 profile is found in those diseases associated with overproduction of antibodies, especially IgE.

The effects of helper T-cells are balanced by those of functional suppressor T-cells that express the characteristic surface glycoprotein CD8.

Cell-mediated responses

Antigen-specific cell-mediated responses are carried out by T-lymphocytes. T-cells can lyse cells expressing specific antigens (cytotoxicity), release cytokines that trigger inflammation (delayed hypersensitivity), or regulate immune responses. These T-cell responses are mediated by distinct T-cell subpopulations: cytotoxicity is the role of cytotoxic T-cells and delayed hypersensitivity that of Th1 cells. These cells are responsible for fighting intracellular pathogens (all viruses, parasites and certain bacteria) which are inaccessible to antibodies.

Cytotoxic T-cells kill cells infected with virus (and possibly those tumour cells expressing recognisable tumour antigens). Such cytotoxicity is virus specific—only cells expressing the relevant viral proteins on their surfaces are killed. Since infected cells express surface viral proteins prior to the assembly of new virus particles and viral budding, cytotoxic T-cells are important in the recovery phase of an infection, destroying the infected cells before new virus particles are generated.

Cytotoxic T-cells recognise viral antigens together with MHC class I molecules. They show exquisite specificity for self-MHC antigens, in that they can only lyse cells expressing the same MHC class I molecules, i.e. express MHC restriction.

Regulatory T-cells are a subset of CD4+ T-cells with a distinct phenotype (CD4+, CD25+) under the control of a gene called FoxP3. These cells dampen down activation and expansion of self-reactive T-cells. Mutations in FoxP3 result in severe autoimmune disease and allergy.

Complement

Complement is a complex series of interacting plasma proteins which form a major effector system for antibody-mediated immune reactions. Many complement components exist as inactive precursors; once activated, the component may behave as an enzyme which cleaves several molecules of the next component in the sequence. Each precursor is cleaved into two or more fragments. The major fragment (usually designated ‘b’) has two biologically active sites: one for binding to cell membranes or the triggering complex, and the other for enzymatic cleavage of the next complement component. Minor cleavage fragments (designated ‘a’) have important biological properties in the fluid phase. Control of complement activation involves spontaneous decay of any exposed attachment sites and inactivation by specific inhibitors. The major purpose of the complement pathway is to remove or destroy antigen, either by direct lysis or by opsonisation.

Complement activation

Complement activation occurs in two sequential phases:

• activation of the C3 component

• activation of the ‘attack’ or lytic pathway.

The critical step (Fig. 9.16) is cleavage of C3 by complement-derived enzymes called C3 convertases. The major fragment of activated C3—called C3b—mediates a number of vital biological activities, particularly opsonisation.

Fig. 9.16 The lectin, classical and alternative pathways of complement activation.

The cleavage of C3 is achieved via three main routes, the classical, alternative and lectin pathways, all of which generate C3 convertases but in response to different stimuli (Fig. 9.16).

Classical pathway activation

The classical pathway is activated when binding of IgM or IgG to antigen causes a conformational change in the Fc region of the antibody to reveal a binding site for the first component in the classical pathway, C1.

C1 is a macromolecular complex of three subcomponents—C1q, C1r and C1s. C1q is a collagen-like protein composed of six subunits. C1q reacts with Fc regions via its globular heads but attachment by two critically spaced binding sites is needed for activation. IgM is more efficient than IgG in activating C1q. IgA, IgD and IgE do not activate the classical pathway.

Once C1q is activated, C1r and C1s are sequentially bound to generate enzyme activity (C1 esterase) for C4 and C2, splitting both molecules into a and b fragments. The complex C4b2b is the classical pathway C3 convertase. C4b2b cleaves C3 into two fragments, one (C3a) possessing anaphylatoxic and chemotactic activity (see below) and one that binds to the initiating complex and promotes many of the biological properties of complement. The C4b2b3b complex so generated is an enzyme, C5 convertase, which initiates the final lytic pathway (the ‘attack’ sequence).

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