basic immunology

Basic immunology The immune system is the body’s natural defence in combating organisms. Immunology has developed rapidly over the past decade owing to the refinements made in the molecular tests employed in this area of research. Therefore, the keen reader is encouraged to peruse the ophthalmic and immunological literature in order to keep abreast of the latest developments in this field. The College of Optometrists has awarded this article 2 CET credits. There are 12 MCQs with a pass mark of 60%. Owing to the complex nature of this subject, it is far beyond the scope of this article to cover all aspects of immunology. Rather, the aims of the article are twofold: first to acquaint the busy practitioner with the basic concepts of the immune system; and second, to introduce the reader to the more specific topic of ocular immunology – the study of the ocular immune system. Finally, since it is envisaged that optometrists will one day prescribe therapeutic agents, the discussion is limited to the anterior segment and anterior uvea. Innate & adaptive immune systems The immune system can be thought of as having two “lines of defence”: the first, representing a non-specific (no memory) response to aannttiiggeenn (substance to which the body regards as foreign or potentially harmful) known as the iinnnnaattee iimmmmuunnee ssyysstteemm; and the second, tthhee aaddaa ppt 关于艾滋病ppt课件精益管理ppt下载地图下载ppt可编辑假如ppt教学课件下载triz基础知识ppt tiivvee iimmmmuunnee ssyysstteemm, which displays a high degree of memory and specificity. The innate system represents the first line of defence to an intruding pathogen. The response evolved is therefore rapid, and is unable to “memorise” the same said pathogen should the body be exposed to it in the future. Although the cells and molecules of the adaptive system possess slower temporal dynamics, they possess a high degree of specificity and evoke a more potent response on secondary exposure to the pathogen. The adaptive immune system frequently incorporates cells and molecules of the innate system in its fight against harmful pathogens. For example, ccoommpplleemmeenntt (molecules of the innate system - see later) may be activated by aannttiibbooddiieess (molecules of the adaptive system) thus providing a useful addition to the adaptive system’s armamentaria. A comparison of the two systems can be seen in TTaabbllee 11.. ot www.optometry.co.uk Gregory Heath BSc (Hons), MCOptom, Dip. Clin. Optom ABDO has awarded this article 2 CET credits (GD). February 8, 2002 OT26 Sponsored by a Cells of the innate immune system Phagocytes Although sub-divided into two main types, namely neutrophils and macrophages, they both share the same function - to engulf microbes (phago - I eat, Latin). Neutrophils Microscopically, these cells possess a characteristic, salient feature - a multilobular nucleus ((FFiigguurree 22)). As such, these cells have been referred to as polymorphonuclear leukocytes (PMNs) and have a pivotal role to play in the development of acute inflammation. In addition to being phagocytic, neutrophils contain granules and can also be classed as one of the granulocytes. The granules contain acidic and alkaline phosphatases, defensins and peroxidase - all of which represent the requisite molecules required for successful elimination of the unwanted microbe(s). Macrophages Macrophages (termed monocytes when in the blood stream) have a horseshoe-shaped nucleus and are large cells. Properties of macrophages include phagocytosis and antigen presentation to T cells (see later). Unlike neutrophils (which are short-lived cells), they are seen in chronic inflammation as they are long-lived cells. Mononuclear phagocytic system The cells comprising the monocyte phagocytic system are tissue bound and, as a result, are further sub-divided depending on their location. A list of the cells together with their corresponding location can be found in TTaabbllee 22. Table 1: Cells and molecules of the innate and adaptive immune systems Immunity Cells Molecules Innate Natural killer (NK) cells Cytokines Mast cells Complement Dendritic cells Acute phase proteins Phagocytes Adaptive T and B cells Cytokines Antibodies Components of the immune system can be seen in Figure 1. Figure 1 The principle components of the immune system are listed, indicating which cells produce which soluble mediators. Complement is made primarily by the liver, with some synthesised by mononuclear phagocytes. Note that each cell only produces a particular set of cytokines, mediators etc Figure 2 Morphology of the neutrophil. This shows a neutrophil with its characteristic multilobed nucleus and neutrophilic granules in the cytoplasm. Giemsa stain, x 1500 27www.optometry.co.uk Table 2: Examples of cells of the mononuclear phagocytic system and their respective locations Cells Location Monocytes Blood stream Alveolar macrophages Lungs Sinus macrophages Lymph nodes and spleen Kupffer cells Liver Dendritic cells Dendritic cells consist of Langerhans’ and interdigitating cells and form an important bridge between innate and adaptive immunity, as the cells present the antigenic peptide to the T helper cell (adaptive immunity). Such cells are therefore known as professional antigen presenting cells ((AAPPCCss)). TTaabbllee 33 illustrates the various types of dendritic cells together with an example of their location. Eosinophils Eosinophils (so called because their granules stain with eosin – FFiigguurree 44) are granulocytes that possess phagocytic properties. Despite the fact that they represent only 2-5 % of the total leukocyte population, they are instrumental in the fight against parasites that are too big to be phagocytosed. Phagocytosis - the process Phagocytosis is the process by which cells engulf microorganisms and particles ((FFiigguurree 33)). Firstly, the phagocyte must move towards the microbe under the influence of chemotactic signals, e.g. complement (see later). For the process to continue, the phagocyte must attach to the microbe either by recognition of the microbial sugar residues (e.g. mannose) on its surface or complement/antibody, which is bound to the pathogen. Following attachment, the phagocyte’s cell surface invaginates and the microbe becomes internalised into a phagosome. The resultant pphhaaggoossoommee fuses with multiple vesicles containing O2 free radicals and other toxic proteins known as lysosomes to form a phagolysosome. The microbe is subsequently destroyed. Opsonisation (“to make tasty” - Greek) OOppssoonniinnss are molecules, which enhance the efficiency of the phagocytic process by coating the microbe and effectively marking them for their destruction. Important opsonins are the complement component C3b and antibodies. Natural killer (NK) cells NNKK cells are also known as “large granular lymphocytes” (LGLs) and are mainly found in the circulation. They comprise between 5-11% of the total lymphocyte fraction. In addition to possessing receptors for immunoglobulin type G (IgG), they contain two unique cell surface receptors known as killer activation receptor and killer inhibition receptor. Activation of the former initiates cytokine (“communication”) molecules from the cell whilst activation of the latter inhibits the aforesaid action. NK cells serve an important role in attacking virally-infected cells in addition to certain tumour cells. Destruction of infected cells is achieved through the release of perforins and granyzymes from its granules, which induce apoptosis (programmed cell death). NK cells are also able to secrete interferon-γ (IFN-γ). This interferon serves two purposes: first, to prevent healthy host cells from becoming infected by a virus; and second, to augment the T cell response to other virally infected cells (see later). Mast cells and basophils Morphologically, mast cells and basophils are very similar in that both contain electron dense granules in the cytoplasm. Basophils are so-called owing to the fact that their granules stain with a basic dye. Unlike mast cells, which are present in close proximity to blood vessels in connective tissue, basophils reside in the circulation. Both cell types are instrumental in initiating the acute inflammatory response. Degranulation is achieved either by binding to components of the complement system or by cross-linking of the IgE antibody which results in the release of pro-inflammatory mediators including histamine and various cytokines. The former induces vasodilation and augments vascular permeability whilst the latter are important in attracting both neutrophils and eosinophils. Table 3: Dendric cells and location Cells Location Langerhans cell Limbus, skin Interdigitating cell T cell areas in lymph nodes Figure 4 Morphology of the eosinophil. The multilobed nucleus is stained blue and the cytoplasmic granules are stained red. Leishman stain, x 1800 Figure 3 Phagocytes arrive at a site of inflammation by chemotaxis. They may then attach to microorganisms via their non-specific cell surface receptors. Alternatively, if the organism is opsonised with a fragment of the third complement component (C3b), attachment will be through the phagocyte’s receptors for C3b. If the phagocyte membrane now becomes activated by the infectious agent, it is taken into a phagosome by pseudopodia extending around it. Once inside, lysosomes fuse with the phagosome to form a phagolysosome and the infectious agent is killed. Undigested microbial products may be released to the outside ot February 8, 2002 OT www.optometry.co.uk for the successful eradication of an invading virus by the innate immune system. Type II IFN, IFN-γ, is produced by T Helper cells and NK cells and is able to augment both the antigen presenting properties together with the phagocytic properties of the APCs (e.g. macrophages and dentritic cells). Adaptive immunity As mentioned previously, there is a great deal of synergy between the adaptive immune system and its innate counterpart. The adaptive immune system comprises two main types of leukocyte known as B and T lymphocytes. Before describing these important cell types, it is necessary to acquaint the reader with both the primary and secondary lymphoid organs and tissues in the body. These are summarised in TTaabbllee 44. The bone marrow represents the dominant site for haemopoiesis (production of blood cells and platelets). Although most of the haemopoietic cells maturate in this region, T lymphocytes do so in the thymus. In the thymus, premature T cells undergo a process of positive and negative selection whereby the former are allowed to progress to maturity whilst the latter are marked for termination via apoptosis (see central tolerance). Lymphocytes Morphologically, there are three types of lymphocytes: T, B and NK cells. However, only T and B lymphocytes exhibit memory and specificity and, as such, are responsible for the unique quality of the adaptive immune system. Resting B lymphocytes are able to react with free antigen directly when it binds to their cell surface immunoglobins which act as receptors. T lymphocytes do not react with free antigen and instead make use of APCs to phagocytose the antigen and then to express its component 28 Molecules of the innate immune system There are many molecules, which work in concert with the cells of the innate immune system and which also foster close functional links with their adaptive counterpart. The three major molecules are: • Complement • Acute phase proteins (APP) • Interferons (IFNs) Complement The complement system represents a large group of independent proteins (denoted by the letter C and followed by a number), secreted by both hepatocytes (liver cells) and monocytes. Although these proteins maybe activated by both the adaptive immune system (ccllaassssiiccaall ppaatthhwwaayy) or innate immune system (aalltteerrnnaattiivvee ppaatthhwwaayy), the nomenclature is derived from the fact that the proteins help (“complement”) the antibody response. Activation of complement via the microbe itself is known as the alternative pathway. The classical pathway requires the interaction of antibody with specific antigen. The C3 component is the pivotal serum protein of the complement system. Binding of the antigen to C3 results in two possible sequelae. In either case, C3 component becomes enzymatically converted to C3b. The bacterial cell wall can either remain bound to C3b and become opsonised (since phagocytes have receptors for C3b) or act as a focus for other complement proteins (namely C5, 6, 7, 8 and 9). The latter form the membrane attack complex (MAC), which induces cellular lysis. The functions of the complement system may be summarised as follows: • Opsonisation • Lysis (destruction of cells through damage/ rupture of plasma membrane) • Chemotaxis (directed migration of immune cells) • Initiation of active inflammation via direct activation of mast cells It is important that complement is regulated to protect host cells from damage and/or their total destruction. This is achieved by a series of regulatory proteins, which are expressed on the host cells themselves. Acute phase proteins These serum proteins are synthesised by hepatocytes and are produced in high numbers in response to cytokines released from macrophages. Interferons (IFNs) IFNs are a group of molecules, which limit the spread of viral infections ((FFiigguurree 55)). There are two categories of IFNs, namely type I and type II. Type I IFNs maybe sub-divided further into IFN-α and β. IFN-γ is the sole type II interferon. Type I IFNs are induced by viruses, pro-inflammatory cytokines and endotoxins from gram negative bacterial cell walls. Their presence remains vital proteins on the cell surface adjacent to special host proteins called major histocompatibility complex (MHC) class II molecules. As discussed, antigen presenting cells which express MHC class II molecules include dendritic cells and macrophages. This “afferent” phase must occur in order for the T cell to recognise the antigen. The “efferent” phase occurs when activated lymphocytes enter the tissue and meet antigen again. This results in multiplication and secretion of cytokines or immunoglobins in order to destroy the antigen. T cells T cells can be broadly divided into both T helper (TH) and cytotoxic T cells (Tc). Furthermore, TH cells may be sub-divided into TH1 and TH2. The former are pro-inflammatory T cells and stimulate macrophages whilst the latter orchestrate B cell differentiation and maturation and hence are involved in the production of humoral immunity (antibody mediated). T cells express cell surface proteins, described by cluster determination (CD) numbers. TH cells express CD4 molecules on their cell surface, which enable the lymphocyte to bind to a MHC class II molecule. The T cell receptor is unique in that it is only able to identify antigen when it is associated with a MHC molecule on the surface of the cell. Cytotoxic T cells are primarily involved in the destruction of infected cells, notably viruses. Unlike TH cells, cytotoxic cells possess CD8 cell surface markers, which bind to antigenic peptides expressed on MHC class I molecules. B cells and antibodies (immunoglobulins - Ig) B cells are lymphocytes that produce antibodies (immunoglobulins) and can recognise free antigen directly. They are produced in the bone marrow and migrate to secondary lymphoid organs. B cells are responsible for the Figure 5 When host cells become infected by virus, they may produce interferon. Different cell types produce interferon-α (IFN-α ) or interferon-β (IFN-β); interferon-γ(IFN-γ) is produced by some types of lymphocyte (TH) after activation by antigen. Interferons act on other host cells to induce a state of resistance to viral infection. IFN-γ has many other effects as well Table 4: Primary and secondary lymphoid organs Primary lymphoid organs Secondary lymphoid organs Bone marrow Lymph nodes MALT - mucosa associated lymphoid tissue (includes bronchus, gut, nasal and conjunctival associated mucosal tissues) Spleen www.optometry.co.uk Module 4 Part 2 29 Sponsored by a development of antibody mediated immunity known as hhuummoorraall mmeeddiiaatteedd iimmmmuunniittyy. When activated by foreign antigen, B cells undergo proliferation and mature into antibody secreting ppllaassmmaa cceellllss. The latter are rich in organelles such as rough endoplasmic reticulum and mitochondria, which confer their ability to secrete soluble proteins (antibodies). Not all proliferating B cells develop into plasma cells. Indeed, a significant proportion remain as memory B cells through a process known as clonal selection. This process is vital in eliminating the antigen should the body become re-exposed to it in the future. T cells are also clonally selected and this confers to the production of T memory cells. Although T and B cells behave differently, both are able to recirculate around the body migrating from blood to tissue and vice versa. The ability to recirculate obviously increases the efficiency with which cells of the immune system can home onto the invading antigen. Antibodies Antibodies have two roles to play - the first is to bind antigen and the second is to interact with host tissues and effector systems in order to ensure removal of the antigen ((FFiigguurree 66)). There are five different types (known as iissoottyyppeess) of antibody in the human immune system - namely IgM, IgG, IgA, IgE and IgD. In addition, there are four sub classes of IgG (IgG1-4). The basic antibody unit consists of a glycosylated protein consisting of two heavy and two light, polypeptide chains. The region which binds to the antigen is known as the FFaabb region, while the constant region, FFcc, not only determines the isotype but is the region responsible for evoking effector systems, e.g. mast cell activation. The term immune complex refers to the combination of antigen and antibody and will be discussed later in the article (see type III hypersensitivity). The antibody isotypes together with their corresponding function are illustrated in TTaabbllee 55.. MHC Major histocompatability complex (MHC) are cell surface proteins classified as class I (also termed human leucocytic antigen [HLA] A, B and C), found on all nucleated cells and class II (termed HLA, DP, DQ and DR), found on all antigen presenting cells (APCs). MHC molecules are the sine qua non of T cell induced immunity. Clinically, there is a strong association between HLA and certain systemic and ocular diseases (see later). Cytokines Cytokines (also termed interleukins [IL] meaning “between white blood cells”) are small molecules that act as a signal between cells and have a variety of roles including chemotaxis, cellular growth and cytotoxicity. Owing to their ability to control immune activity, they have been described as the “hormones” of the immune system. Characteristics Crosses placenta thus providing newborn with useful humoral immunity High affinity Predominant antibody in blood and tissue fluid Large pentameric structure in circulation Present in monometric form on B cell surface Secreted form is predominant antibody in early immune response against antigen Reaches 75% of adult levels at 12 months of age Exists in both a monometric and dimeric form Secretory IgA (dimeric form) represents 1st line of defence against microbes invading the mucosal surface, e.g. tears Low levels in circulation Increased levels in worm infections Fc region has high affinity for mast cell thus involved in allergy Antigen receptor on B cells Absent from memory cells Table 5: Antibody isotypes and corresponding functions Antibody IgG IgM IgA IgE IgD Table 6: Various cytokines, their sources and functions Cytokine Source Function IL-1 Macrophages 1. T, B cell activation 2. Mobilisation of PMNs 3. Induction of acute phase proteins IL-2 T cells Prol