Cells of the Immune system
An immune system is a system of biological structures and processes within an organism that protects against disease. The state of protection from infectious disease is called immunity. Immunity has a less specific component, called innate immunity. Innate immunity provides the first line of defense against infection. Most components of innate immunity are present prior to the onset of infection and constitute a set of disease resistant mechanisms that are not specific to a particular pathogen but include cellular and molecular components that recognise classes of molecules peculiar to frequently encountered pathogens. Innate immune defenses are non-specific, meaning these systems respond to pathogens in a generic way. This system does not confer long-lasting immunity against a pathogen. The innate immune system is the dominant system of host defense in most organisms. Important roles in innate immunity is played by skin, variety of antimicrobial compounds synthesized by the host, and phagocytic cells. Immunity has a more specific component, called adaptive immunity which creates immunological memory after an initial response to a specific pathogen, leading to an enhanced response to subsequent encounters with that same pathogen. This process of acquired immunity is the basis of vaccination. The adaptive or "specific" immune system is activated by the “non-specific” and evolutionarily older innate immune system. The major agents of adaptive immunity are lymphocytes and the antibodies and other molecules they produce.
Lymphocytes are the central cells of the immune system, constituting 20%- 40% of the body's white blood cells and 99% of the cells in the lymph. The lymphocytes are responsible for adaptive immunity and the immunologic attributes of diversity, specificity, memory, and self/non-self recognition. The lymphocytes can be broadly subdivided into three populations- B cells, T cells and natural killer cells. B and T lymphocytes make up the adaptive immune system whereas NK cells come under the innate immune system.
B lymphocytes also known as B cells are developed in the bone marrow in mammals and in the bursa of Fabricius in birds. Arising from lymphoid progenitors, immature B cells proliferate and differentiate within the bone marrow, and stromal cells within the bone marrow interact directly with the B cells and secret various cytokines that are required for development.
B lymphocytes are naïve or effector type, depending upon interaction with antigen. Naïve B lymphocytes are small, motile, non-phagocytic cells which have not interacted with antigen. The naïve B lymphocytes are resting cells in the G0 phase of the cell cycle and they have thought to have a short life span. Small lymphocytes interacting with antigen, in the presence of certain cytokines, enter into the cell cycle by progressing from G0 into G1 and subsequently into S, G2, and M. Lymphocytes gradually progress through the cell cycle and enlarge into lymphoblasts which have more organellar complexity than small lymphocytes. Lymphoblasts proliferate and differentiate into effector cells and memory cells. Effector cells, also known as plasma cells having a short life span function in various ways to eliminate antigen (small fragments of pathogen). The presence of memory cells is responsible for life-long immunity to many pathogens. Mature B cells express membrane-bound immunoglobulin molecules, which serve as receptors for antigen. B cells are involved in the humoral immune response.
T cells also known as T lymphocytes, develop in the thymus. T lymphocyte like B lymphocyte can be grouped into naïve cell and effector cell. Naïve T cell which has not previously engaged in an immune response, undergoes clonal expansion and differentiation to become an effector cell, which has the capability of mounting an immune response. T cells recognize a “non-self” target, such as a pathogen, only after antigens (small fragments of the pathogen) have been processed and presented in combination with a “self” receptor called a major histocompatibility complex (MHC) molecule. T cells like B cells express distinctive membrane molecules. All T cell subpopulations express the T-cell receptor which is a complex of polypeptides.
Helper T cells
T cells that express the membrane glycoprotein molecule CD4, are restricted to recognizing antigen bound to class II MHC molecules and these specific type of T cells are known as CD4+ T cells or helper T cells (TH cells). TH cells regulate both the innate and adaptive immune responses and help determine which types of immune responses the body will make to a particular pathogen. These cells have no cytotoxic activity and do not kill infected cells or clear pathogens directly. They instead control the immune response by directing other cells to perform these tasks. TH cells are activated by recognition of an antigen-class II MHC complex on an antigen-presenting cell (figure-1). After activation, the TH cell starts to divide and gives rise to a clone of effector cells. These TH cells secret various cytokines which help in activation of B cells, T cells and also help to influence the activity of many cell types.
|Figure 1: Stimulation of B cells and macrophages by T-helper cells|
Cytotoxic T cells
T cells that express the dimeric membrane glycoprotein molecule CD8, are restricted to recognition of antigen bound to class-I MHC molecules. T cells expressing CD8, are known as cytotoxic T cells(TC cells) or killer T cells. Killer T cell are a sub-group of T cells that kill cells that are infected with viruses (and other pathogens), or are otherwise damaged or dysfunctional (figure-2). As with B cells, each type of T cell recognises a different antigen. TC cells are activated when they interact with an antigen-class I MHC complex on the surface of an altered self cell in the presence of appropriate cytokines. This activation leads to proliferation which causes the TC cell to differentiate into an effector cell also known as cytotoxic T lymphocyte (CTL). CTLs secret less cytokines than TH cells. T cell killing of host cells is particularly important in preventing the replication of viruses. T cell activation is tightly controlled and generally requires a very strong MHC/antigen activation signal, or additional activation signals provided by "helper" T cells.
|Figure 2: Interaction between killer T cell and target cell carrying foreign antigens|
T supressor (TS) cells
Supressor TS cells otherwise known as regulatory T cells help to supress the humoral and the cell mediated branches of the immune system. This is an important "self-check" built into the immune system to prevent excessive reactions. Regulatory T cells come in many forms with the most well understood being those that express CD4, CD25, and Foxp3 (CD4+CD25+ regulatory T cells, or "Tregs"). These cells are involved in shutting down immune responses after they have successfully eliminated invading organisms, and also in preventing autoimmunity. The critical role regulatory T cells play within the immune system is evidenced by the severe autoimmune syndrome that results from a genetic deficiency in regulatory T cells. The molecular mechanism by which regulatory T cells exert their suppressor/regulatory activity has not been definitively characterized and is the subject of intense research. Some immunologists believe that the suppression mediated by T cells observed in some systems is simply the consequence of activities of TH or TC subpopulations whose end results are suppressive.
Natural Killer Cells(NK Cells)
Natural killer cells can be defined as large granular lymphocytes, constitute 5%-10% of lymphocytes and in human peripheral blood, do not express the membrane molecules and receptors that distinguish B cell and T cell lineages. They were named “natural killers” because of the initial notion that they do not require activation in order to kill cells that are missing “self” markers of major histocompatibility complex (MHC) class 1. NK cells are known to differentiate and mature in the bone marrow, lymph node, spleen, tonsils and thymus where they then enter into the circulation.
Natural killer cells are leukocytes that attack and destroy tumor cells, or cells that have been infected by viruses. NK cells express two types of receptors i.e., inhibitory receptors and activating receptors. Natural killer cell activation is determined by the balance of inhibitory and activating receptor stimulation i.e. if the inhibitory receptor signaling is more prominent then NK cell activity will be inhibited, similarly if the activating signal is dominant then NK cell activation will result. NK cells express CD16, a membrane receptor (activating receptor) for the carboxy-terminal end of the IgG molecule, called the FC region, through which they can attach to the secreted antitumor or antiviral antibodies in response to the antigens and subsequently destroy the target cells. This type of killing target cells is known as antibody dependent cell mediated cytotoxicity.
NK cells are cytotoxic; small granules in their cytoplasm contain proteins such as perforin and proteases known as granzymes. Upon release in close proximity to a cell slated for killing, perforin forms pores in the cell membrane of the target cell, creating an aqueous channel through which the granzymes and associated molecules can enter, inducing either apoptosis or osmotic cell lysis. Cytokines play a crucial role in NK cell activation. As these are stress molecules released by cells upon viral infection, they serve to signal to the NK cell the presence of viral pathogens. Cytokines involved in NK activation include IL-12, IL-15, IL-18, IL-2, and CCL5. NK cells are activated in response to interferons or macrophage-derived cytokines. They serve to contain viral infections while the adaptive immune response is generating antigen-specific cytotoxic T cells that can clear the infection. NK cells work to control viral infections by secreting IFNγ and TNFα, IFNγ activates macrophages for phagocytosis and lysis and TNFα acts promote direct NK tumor cells killing.
Phagocytes were first discovered in 1882 by Ilya Ilyich Mechnikov while he was studying starfish larvae. Phagocytes are the white blood cells that protect the body by ingesting (phagocytosing) harmful foreign particles, bacteria, and dead or dying cells. They are essential for fighting infections and for subsequent immunity. Phagocytes are important throughout the animal kingdom and are highly developed within vertebrates. One litre of human blood contains about six billion phagocytes. Phagocytes of human and other animals are of two types that means they are with receptors or without receptors. Phagocytes with receptors on their surface are also known as professional phagocytes which help in detecting harmful organisms and eliminating them. Professional phagocytes are differentiated into neutrophils, monocytes, macrophages, dendritic cells, and mast cells.
Neutrophils are the granulocytes normally found in the bloodstream produced by hematopoiesis in the bone marrow and are the most abundant type of phagocyte, constituting 50% to 60% of the total circulating white blood cells. One litre of human blood contains about five billion neutrophils, which are about 10 micrometers in diameter and live for only about five days. The neutrophil has a multilobed nucleus (figure-3) and a granulated cytoplasm that stains with both acidic and basic dyes. In response to infections, bone marrow releases more neutrophils and as a result of which neutrophils are the first to arrive at a site of inflammation.
|Figure 3: Diagram showing morphology of a Neutrophil|
Movement of circulating neutrophils into tissues, called extravasion in which the cell first adheres to the vascular endothelium, then penetrates the gap between adjacent endothelial cells lining the vessel wall, and finally penetrates the vascular basement membrane, moving out into the tissue spaces. Neutrophils are ferocious eaters and rapidly engulf invaders coated with antibodies and complement, and damaged cells or cellular debris. Neutrophils do not return to the blood; they turn into pus cells and die. Neutrophils also employ both oxygen-dependent and oxygen-independent pathways to generate antimicrobial substances. Neutrophils exhibit a larger respiratory burst than macrophages and consequently are able to generate more reactive oxygen intermediates and reactive nitrogen intermediates.
Monocytes are a type of white blood cell and are part of the innate immune system of vertebrates including all mammals (including humans), birds, reptiles, and fish. During hematopoiesis in the bone marrow, granulocyte-monocyte progenitor cells differentiate into promonocytes, which leave the bone marrow and enter the blood, where they further differentiate into mature monocytes. Monocyte is the largest corpuscle in the blood. Monocytes are usually identified in stained smears by their large kidney shaped (figure-4) or notched nucleus. Monocytes circulate in the bloodstream for about eight hour, during which they enlarge and then typically move into tissues throughout the body. They constitute between three to eight percent of the leukocytes in the blood. Half of them are stored as a reserve in the spleen in clusters in the red pulp's Cords of Billroth. In the tissues monocytes mature into different types of macrophages at different anatomical locations. Monocyte is the largest corpuscle in the blood.
|Figure 4: Diagram showing morphology of a monocyte|
Differentiation of a monocyte into tissue macrophage involves a number of changes during which the cell enlarges five to ten fold, its intracellular organelles increase in both number and complexity; and it acquires increased phagocytic ability produces higher levels of hydrolytic enzymes, and begins to secret soluble factors. Monocytes are attracted to a damaged site by chemical substances through chemotaxis, triggered by a range of stimuli including damaged cells, pathogens and cytokines released by macrophages already at the site. Monocytes can perform phagocytosis, antigen presentation and cytokine production in the immune system. Phagocytosis is the process of uptake of microbes and particles followed by digestion and destruction of this material. Monocytes can perform phagocytosis using intermediary (opsonising) proteins such as antibodies or complement that coat the pathogen, as well as by binding to the microbe directly via pattern-recognition receptors that recognize pathogens. Monocytes are also capable of killing infected host cells via antibody, termed antibody-mediated cellular cytotoxicity. Typical cytokines produced by monocytes are TNF tumor necrosis factor, IL-1 interleukin-1 and IL-12 interleukin-12. The three types of monocytes found in human blood are: the classical monocyte which can be characterized by high level expression of CD14 cell surface receptor, the non-classical monocyte which expresses low level of CD14 with coexpression of CD16 receptor, and the intermediate monocyte which expresses high level of CD14 and low level of CD16 receptor.
Macrophages are cells produced by the differentiation of monocytes in tissues. Macrophages are dispersed throughout the body. Some are fixed macrophages by staying in particular tissues whereas others remain motile and are called free macrophages. Human macrophages are about 21 micrometres (0.00083 in) in diameter. Macrophages function in both non-specific defense (innate immunity) as well as help initiate specific defense mechanisms (adaptive immunity) of vertebrate animals. Macrophages serve different functions in different tissues and named according to their tissue location as alveolar macrophages in the lung, histiocytes in connective tissues, kupffer cells in the liver, mesangial cells in the kidney, micriglial cells in the brain and osteoclasts in the bone. Their role is to phagocytose (engulf and then digest) cellular debris and pathogens, either as stationary or as mobile cells. They also stimulate lymphocytes and other immune cells to respond to pathogens. They are specialized phagocytic cells that attack foreign substances, infectious microbes and cancer cells through destruction and ingestion. They move by action of amoeboid movement.
Though macrophages are activated by a variety of stimuli in the course of an immune response, phagocytosis of particulate antigens serve as an initial activating stimulus. Macrophages can be activated to perform functions that a resting monocyte cannot. T helper cells are responsible for the activation of macrophages. TH1 cells activate macrophages by signaling with IFN-gamma and displaying the protein CD40 ligand. Activated macrophages are more effective than the resting ones in eliminating potential pathogens, because they exhibit greater phagocytic activity. Activated macrophages secret various cytotoxic proteins that help them eliminate a broad range of pathogens including virus infected cells, tumor cells, and intracellular bacteria.
During the process of phagocytosis, macrophages are attracted by and move toward a variety of substances generated in an immune response and the process is called chemotaxis. Then antigens adhere to the macrophage cell membrane, which is induced by mebrane protrusions on the cell membrane of macrophage, also known as pseudopodia (figure-5). Pseudopodia help to extend around the attached material and the fusion of pseudopodia encloses the material within a membrane bounded structure called a phagosome, which enters the endocytic processing pathway , where it fuses with a lysome to form a phagolysosome. lysosomes contain lysozyme and a variety of other hydrolytic enzymes that digests the ingested material. During phagocytosis, a metabolic process known as respiratory burst occurs in activated macrophages, which results in the activation of a membrane-bound oxidase that catalyzes the reduction of oxygen to superoxide anion, a reacive oxygen intermediate that is extremely toxic to ingested microorganisms. The superoxide anion also generates powerful oxidizing agents including hydroxyl radicals and hydrogen peroxide. When macrophages are activated with bacterial cell wall components such as lipopolysaccharide or muramyl dipeptide, together with a T cell derived cytokine (IFN-γ), they begin to express high levels of nitric oxide synthetase, an enzyme that oxidises L-arginine to L-citrulline and nitric oxide. Nitric oxide has potent antimicrobial activity and when it combines with the superoxide anion, it produces even more potent antimicrobial substances. Activated macrophages also synthesize lysozymes and various hydrolytic enzymes whose degradative activities do not require oxygen. Activated macrophages also produce a group of antimicrobial and cytotoxic peptides known as defensins which form ion-permeable channels in bacterial cell membranes.
|Figure 5: Diagram showing the morphology of a macrophage|
Macrophages are versatile cells that play many roles. As scavengers, they rid the body of worn-out cells and other debris. Along with dendritic cells, they are foremost among the cells that "present" antigen, a crucial role in initiating an immune response. As secretory cells, monocytes and macrophages are vital to the regulation of immune responses and the development of inflammation; they produce a wide array of powerful chemical substances (monokines) including enzymes, complement proteins, and regulatory factors such as interleukin-1. At the same time, they carry receptors for lymphokines that allow them to be "activated" into single-minded pursuit of microbes and tumour cells.
Eosinophils or eosinophiles (or, less commonly, acidophils), are white blood cells (figure-6) that are one of the immune system components responsible for combating multicellular parasites and certain infections in vertebrates. They are granulocytes that develop during hematopoiesis in the bone marrow before migrating into blood. Eosinophils, like neutrophils, are motile phagocytic cells that can migrate from the blood into the tissue spaces. In normal individuals, eosinophils make up about 1-6% of white blood cells, and are about 12-17 micrometers in size. Eosinophils appear brick-red after staining with eosin, a red dye, using the Romanowsky method. The staining is concentrated in small granules within the cellular cytoplasm, which contain many chemical mediators, such as histamines and proteins such as eosinophil peroxidase, ribonuclease (RNase), deoxyribonucleases, lipase, plasminogen, and major basic protein. These mediators are released by a process called degranulation following activation of the eosinophil, and are toxic to both parasite and host tissues. They are found in the medulla and the junction between the cortex and medulla of the thymus, and, in the lower gastrointestinal tract, ovary, uterus, spleen, and lymph nodes. Eosinophils persist in the circulation for 8–12 hours, and can survive in tissue for an additional 8–12 days in the absence of stimulation.
|Figure 6: Diagram showing the morphology of an eosinophil|
Eosinophils play a role in fighting viral infections, which is evident from the abundance of RNases they contain within their granules, and in fibrin removal during inflammation. Eosinophils along with basophils and mast cells, are important mediators of allergic responses and asthma pathogenesis and are associated with disease severity. They also fight helminth (worm) colonization and may be slightly elevated in the presence of certain parasites.
A mast cell, also known as mastocyte, is a resident cell of several types of tissues and contains many granules rich in histamine and heparin. Mast-cell precursors, which are formed in the bone marrow by hematopoiesis, are released into the blood as undifferentiated cells. They only differentiate only when they leave the blood and enter the tissues. Mast cells are found in a wide variety of tissues including the skin, connective tissues of various organs and mucosal epithelial tissue of the respiratory, genitourinary and digestive tracts.
Mast cells play a key role in the inflammatory process. When activated, a mast cell rapidly releases its characteristic granules and various hormonal mediators into the interstitium. Mast cells can be stimulated to degranulate by direct injury (e.g. physical or chemical [such as opioids, alcohols, and certain antibiotics such as polymyxins]), cross-linking of Immunoglobulin E (IgE) receptors, or by activated complement proteins.
Mast cells express a high affinity receptor (FceRI) for the Fc region of IgE antibody. IgE is produced by plasma cells (the antibody-producing cells of the immune system). IgE molecules, like all antibodies, are specific to one particular antigen. In allergic reactions, mast cells remain inactive until an allergen binds to IgE already in association with the cell. Allergen refers specifically to nonparasitic antigens capable of stimulating type I hypersensitivity responses in allergic individuals. The majority of humans mount significant IgE responses only as a defense against parasitic infections. After an individual has been exposed to a parasite, serum IgE levels increase and remain high until the parasite is completely cleared from the body. But some persons may have an abnormality called atopy, a hereditary predisposition to the development of immediate hypersensitivity reactions against common environmental antigens. Atopic individuals have abnormally high levels of circulating IgE and more susceptible to allergies. A type I hypersensitivity reaction (figure-7) is induced by allergens, where an allergen induce a humoral antibody response, resulting in the generation of antibody secreting plasma cells and memory cells. Plasma cells secrete IgE. This class of antibody binds with high affinity to Fc receptors on the surface of tissue mast cells and blood basophils. After binding of IgE antibodies, the mast cells and basophils are remain sensitized (figure-7). A later exposure to the same allergen cross-links the membrane bound IgE on sensitized mast cells and basophils, causing degranulation of these cells. The pharmacologically active mediators released from the granules act on the surrounding tissues. These chemical mediators cause the characteristic symptoms of allergy.
|Figure 7: General mechanism showing a type I hypersensitivity reaction|
Mast cells can consume and kill gram-negative bacteria (e.g., salmonella), and process their antigens. They specialize in processing the fimbrial proteins on the surface of bacteria, which are involved in adhesion to tissues. In addition to these functions, mast cells produce cytokines that induce an inflammatory response. This is a vital part of the destruction of microbes because the cytokines attract more phagocytes to the site of infection.
Dendritic cells are covered with long membrane extensions that resemble the dendrites of nerve cells. Dendritic cells mainly act as antigen presenting cells as their main function is to process antigen material and present it on the surface to other cells of the immune system. They act as messengers between the innate and adaptive immunity. Dendritic cells are present in tissues in contact with the external environment, such as the skin (where there is a specialized dendritic cell type called Langerhans cells) and the inner lining of the nose, lungs, stomach and intestines. They can also be found in an immature state in the blood. Dendritic cells are derived from hematopoietic bone marrow progenitor cells. These progenitor cells initially transform into immature dendritic cells. These cells are characterized by high endocytic activity and low T-cell activation potential. Immature dendritic cells constantly sample the surrounding environment for pathogens such as viruses and bacteria. This is done through pattern recognition receptors (PRRs) such as the toll-like receptors (TLRs). TLRs recognize specific chemical signatures found on subsets of pathogens. Immature dendritic cells may also phagocytize small quantities of membrane from live own cells, in a process called nibbling. Once they have come into contact with a presentable antigen, they become activated into mature dendritic cells and begin to migrate to the lymph node. Immature dendritic cells phagocytose pathogens and degrade their proteins into small pieces and upon maturation present those fragments at their cell surface using MHC molecules. Mature dendritic cells activate T helper cells and cytotoxic T cells. The activated helper T cells interact with macrophages and B cells to activate them in turn. Four types of dendritic cells are known: langerhans cells, interstitial cells, myeloid cells and lymphoid dendritic cells. Each arises from haematopoitic stem cells via different pathways and in different locations. All types of dendritic cells express high levels of both class II MHC molecules and members of the co-stimulatory B7 family.
Another type of dendritic cell, the follicular dendritic cell, doesn't arise in bone marrow and has a different function from the antigen-presenting dendritic cells. Follicular dendritic cells do not express class-II MHC molecules and therefore do not function as antigen-presenting cells for TH cell activation. Follicular dendritic cells reside in lymph follicles which are rich in B cells. Follicular dendritic cells express high levels of membrane receptors for antibody, which allow the binding of antigen-antibody complexes.
The lifespan of activated dendritic cells, while somewhat varying according to type and origin, is of a similar order of magnitude, but immature dendritic cells seem to be able to exist in an inactivated state for much longer.
Basophils- the non-phagocytic granulocytes
Basophils, are the least common of the granulocytes, representing about 0.01% to 0.3% of circulating white blood cells. Basophils (figure-8) are non-phagocytic in nature.they are susceptible to staining by basic dyes. They function by releasing pharmacologically active substances from their cytoplasm. Basophils along with mast cells play an important role in type-I hypersensitivity reactions. Basophils appear in many specific kinds of inflammatory reactions, particularly those that cause allergic symptoms.
|Figure 8: Diagram showing the morphology of a basophil|
Basophils contain anticoagulant heparin, which prevents blood from clotting too quickly. They also contain the vasodilator histamine, which promotes blood flow to tissues. When activated, basophils degranulate to release histamine, proteoglycans (e.g. heparin and chondroitin), and proteolytic enzymes (e.g. elastase and lysophospholipase). They also secrete lipid mediators like leukotrienes, and several cytokines. Histamine and proteoglycans are pre-stored in the cell's granules while the other secreted substances are newly generated. Each of these substances contributes to inflammation.
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