Physiology 2.01 - Physiology of Immunity

2.01

PHYSIOLOGY

JULY 22, 2013

Physiology of Immunity Delfin Arnel D. Razon, MD., DPBA

Immunity Our bodies have special system for combating different infectious and toxic agents. 2 ways in preventing disease 1. destroying the invading bacteria or viruses by phagocytosis 2. formation of antibodies and sensitized lymphocytes which destroy or inactivate the invader Immunology is the study of the physiological defences by which the body (the “host”) recognizes itself from nonself (“foreign matter”). Nonspecific immune defences protect against foreign substances or cells without having to recognize their specific identities. The mechanisms of protection used by these defenses are not unique to the particular foreign substance or cell. Specific immune defenses (also called “acquired immunity”) depend upon specific recognition, by lymphocytes, of the substance or cell to be attacked. This is followed by an attack that is unique for that substance or cell. Cells Mediating the Immune Response Name Site Produced Functions Neutrophils Bone marrow 1. Phagocytosis 2. Release chemicals involved in inflammation (vasodilators, chemotaxins, etc.) Basophils

Bone marrow

Have functions in blood similar to those of mast cells in tissues (see below)

Eosinophils

Bone marrow

1. Destroy multicellular parasites 2. Participate in immediate hypersensitivity reactions

Monocytes

Bone marrow

Lymphocyte s

Mature in bone marrow (B cells and NK cells) and thymus (T cells);

Cytotoxic T cells

Bind to antigens on plasma membrane of target cells (virusinfected cells, cancer cells, and tissue transplants) and directly destroy the cells

Helper T cells

Secrete cytokines that help to activate B cells, cytotoxic T cells, NK cells, and macrophages

NK cells

1. Bind directly and nonspecifically to virus-infected cells and cancer cells and kill them 2. Function as killer cells in Ab-dependent cellular cytotoxicity (ADCC)

Plasma cells

Peripheral lymphoid organs; differentiate from B cells during immune responses

Secrete antibodies

Macrophag es

Bone marrow; reside in almost all tissues and organs; differentiate from monocytes

1. Phagocytosis 2. Extracellular killing via secretion of toxic chemicals 3. Process and present antigens to helper T cells 4. Secrete cytokines involved in inflammation, activation and differentiation of helper T cells, and systemic responses to infection or injury (the acute phase response)

Macrophag e-like cells

Almost all tissues and organs; microglia in the central nervous system

Same as macrophages

1. Have functions in blood similar to those of macrophages in tissues (see below) 2. Enter tissues and are transformed into macrophages Serve as “recognition cells” in specific immune responses and are essential for all aspects of these responses

activated in peripheral lymphoid organs B cells

which are immunoglobulins 2. During activation are transformed into plasma cells, which secrete antibodies 3. Present antigen to helper T cells

1. Initiate antibodymediated immune responses by binding specific antigens to the B cell’s plasma membrane receptors,

TRANSCRIBERS: Maja, Catie, Eunika, Eli, Kenan, Rissa, Trisha, Von

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PHYSIOLOGY Mast cells

Bone marrow; reside in almost all tissues and organs; differentiate from bone marrow cells

Physiology of Immunity Release histamine and other chemicals involved in inflammation

Leukocytes (WBC)  Mobile units of the protective system  Formed in the bone marrow (granulocytes and monocytes) and in lymph tissue (lymphocytes and plasma cells)  Transported to areas of serious infection and inflammation Types of WBCs: Granulocytes – neutrophils, eosinophils, basophils Agranulocytes – monocytes, lymphocytes Occasionally occurring - plasma cells LIFE SPAN & GENERAL CELL INFO Granulocytes (BEN)  released in bone marrow ; 4-8 hrs circulating in blood  4-5 days in tissue  Main action is to protect tissues/body by ingesting Invaders  in seriously infected tissue infection, life span shortens because they proceed rapidly to the infected area and in the process they are destroyed (I gave my life so that you can live-granulocyte) Monocytes  Giant cells in the blood stream that protects by acting to phagocytize invading organisms  Formed only in the blood marrow  10-20 hrs in the blood  once in the tissue, they swell and become tissue macrophages can live for months Lymphocytes  pass out of the blood into the tissue by diapedesis then enter the lymph return to blood again and again  Enter the circulatory system continuously; serve as “recognition cells” (Memory cells?)  Has several sub-types, namely: B and T cells

Response is antigenindependent

Response is antigendependent

There is immediate maximal response

There is a lag time between exposure and maximal response

Not antigen-specific

Antigen-specific

Exposure results in no immunologic memory

Exposure results in immunologic memory

A. Innate Immunity (Non-specific immunity) Sequence of Events in a Nonspecific Local Inflammatory Response to Bacteria 1. Entry of bacteria into tissue; injury to tissues causes release of chemicals to initiate the following events 2. Vasodilation of the microcirculation in the infected area, leading to increased blood flow 3. Large increase in protein permeability of the capillaries and venules in the infected area, with resulting diffusion of protein and filtration of fluid into the interstitial fluid 4. Chemotaxis: Movement of leukocytes from the venules into the interstitial fluid of the infected area 5. Destruction of bacteria in the tissue either through phagocytosis or by other mechanisms 6. Tissue repair Chemotaxis Process of circulating neutrophils moving out of the blood across the endothelium of capillaries and venules to enter the inflamed area Chemotaxins are messenger molecules released by cells in injured area including endothelial cells . Process of Chemotaxis 1. Margination – exposure of neutrophils to chemotaxins released in the injured area. The chemotaxins act on the neutrophil to induce rapid appearance of molecules that bind tightly to their matching molecules in the endothelial cells. Thus, the neutrophils collect along the site of injury, rather than being washed away with the flowing blood.

2. Diapedesis - huge numbers of neutrophils migrate into the inflamed area. Once in the interstitial fluid, Natural Killer Cells - these cells do not need to recognize neutrophils migrate toward the site of tissue damage the pathogen (therefore non-specific), they detect (chemotaxis). This occurs because damaged cells glycoproteins on the surface of an injured cell, which release chemoattractants. Thus, neutrophils tend to triggers their killing activity, very potent killers, with week move toward the microbes that entered into an injured to month-long lifespan area. o Plasma Cells - Antibody secreting cells from B Lymphocytes Phagocytosis o Macrophages - Monocytes that decide to become 1. Contact between the surfaces of the phagocyte and stationary/attached to the tissues microbe o Mast Cells - for allergic reactions; usually secretes or 2. Interaction of phagocyte receptors with certain releases Histamine carbohydrates or lipids in the microbial cell walls. 3. Opsonin produced by the body can bind the phagocyte Platelets tightly to the microbe and thereby enhance  replaced once in 10 days and formed from phagocytosis. fragments of Monocytes 4. As the phagocyte engulfs the microbe, the internal, microbe-containing sac formed in this step Inflammation is the body’s local response to infection or is called a phagosome. A layer of plasma membrane injury. The functions of inflammation are to destroy or separates the microbe from the cytosol of the phagocyte. inactivate foreign invaders and to set the stage for tissue 5. The phagosome membrane then makes contact with repair. The key mediators are the cells that function as one of the phagocyte’s lysosomes, which is filled with a phagocytes. variety of hydrolytic enzymes. The membranes of the phagosome and lysosome fuse, and the combined Non-specific Immunity Specific Immunity vesicles are now called the phagolysosome. 6. Inside the phagolysosome, the microbe’s macromolecules are broken down by the lysosomal o o

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PHYSIOLOGY

Physiology of Immunity

enzymes. In addition, other enzymes in the phagolysosome membrane produce nitric oxide as well as hydrogen peroxide and other oxygen derivatives, all of which are extremely destructive to the microbe’s macromolecules. Complement- plasma protein that upon activation leads to infection or damage response and will also cause activation of second complement protein which activates a third and so on. FORMATION OF MEMBRANE ATTACK COMPLEX (MAC) 1. The Classical Complement Pathway o The first molecule C1 binds to portion of antibody with antigen, enzymatic portion of C1 is activated. End product is MAC which kills cells to which antibody is bound. C3b function as opsonin. There is only one set of complement molecules, and once activated, they do (According to Guyton): essentially the same thing 1st line of defense – Tissue Macrophage 2nd line of defense – Neutrophil invasion of inflamed area 2. Alternative Pathway 3rd line of defense – Second Macrophage invasion into the o An immediate and automatic response inflamed tissue o Can activate the MAC without the Antigen-Antibody 4th line of defense – Increased production of granulocytes o Complex (unlike Classical pathway) and monocytes o One protein found in this pathway doesn’t have to o be dependent on the complex molecules but rather Intact skin – very effective barrier against pathogens in the o on the carbohydrate substances (glycoproteins) environment o found on the surface of microbes 3. The Lectin Pathway o Mannose binding Lectins refer to proteins that roam, opsonize/cover the glycoprotein/carbohydrate parts of the microbe.

“How would the non-specific phagocytes know that the invading substance is foreign or harmful to the body? Because the presence of PAMPs (Pathogen associated molecular pattern), which are displayed on all pathogenic organisms, viruses, possible antigens and is considered evolutionary. Our phagocytes have PRR (Pattern recognition receptors) which help in determining foreign and pathogenic substance, and thus makes phagocytosis possible” *IgM and IgG – activates classical pathway Functions of Complement Proteins *IgA – activates alternative pathway (slower and less efficient pathway) B. Acquired Immunity (Adaptive immunity) – Basic types of acquired immunity 1. Humoral or B cell immunity - globulin molecules in blood plasma that attacks invading agent 2. Cell-mediated or T cell immunity - formation of large numbers of T lymphocytes that are crafted in lymph nodes to destroy foreign agents

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PHYSIOLOGY

Physiology of Immunity

C3B Receptor Start of alternative pathway ↓ Non specific attachment of protein to bacteria ↓ Complement proteins creates MAC ↓ MAC embeds itself in the cell membrane and creates a channel from the outside to the inside ↓ Inrushing of fluid into the cytoplasm of the microbes ↓ metabolic arrangement in the invading microbe ↓ neutralized microbe Interferon – virally infected and NK cells -directed to the nucleus thus producing virus Virus infected cell try to direct the nucleus to produce viral proteins ↓ Nucleated cells infected by the virus then produce interferon and secrete them ↓ Interferon seeks uninfected cells (interferon receptor) ↓ Direct the uninfected cell to produce antiviral proteins ↓ Another virus would infect other cells ↓ It will not cause viral protein replication because of the antiviral proteins produced by the interferon receptors in it

B Cells  Beta cell receptors (BCR) bind intact antigens (like diphtheria toxoid, the protein introduced into your body in the DTP vaccine). These may be Lymphoid organs o soluble molecules present in the extracellular fluid; Primary lymphoid organs o intact molecules that the B cell plucks from the  Bone marrow and Thymus surface of antigen-presenting  Supplies the secondary lymphoid organs with mature cells like macrophages and dendritic cells. lymphocytes  The bound antigen molecules are engulfed into the B  Mature lymphocytes – already programmed to perform cell by receptor-mediated endocytosis. their function when activated by antigens  The antigen is digested into fragments which are then Secondary lymphoid organs displayed at the cell surface nestled inside a class II histocompatibility molecule.  Lymph nodes, Spleen, Tonsils and Lymphocyte accumulation in the intestinal, respiratory, genital and  Helper T cells specific for this structure (i.e., with urinary tracts complementary TCRs) bind the B cell and  Site where the lymphocytes are activated during immune  secrete lymphokines that: responses o stimulate the B cell to enter the cell cycle and develop, by repeated mitosis, into a clone of cells Preprocessing of T and B lymphocytes (Lymphocyte with identical BCRs; origins) o switch from synthesizing their BCRs as integral membrane proteins to a soluble version; o differentiate into plasma cells that secrete these Lymphocyte-committed stem cells – origin of all Lymphocytes soluble BCRs, which we now call antibodies. • Thymus Gland Pre-process - the T Lymphocytes or Negative selection o T lymphocyte reacts with “self-antigen” o Destroyed and phagocytized o Positive selection o Cells released are non-reactive against the body’s own antigen o Removal of the thymus gland before birth o prevent development of cell mediated immunity (responsible for rejection of transplanted organs) • Liver and Bone marrow Pre-process - the B lymphocyte or B lymphocytes actively secretes antibodies

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PHYSIOLOGY

Physiology of Immunity

T Cells The surface of each T cell also displays thousands of identical T cell receptors (TCRs). There are two types of T cells that differ in their TCR:  alpha/beta (αβ) T cells. Their TCR is a heterodimer of an alpha chain with a beta chain. Each chain has a variable (V) region and a constant (C) region. The V regions each contain 3 hypervariable regions that make up the antigenbinding site  gamma/delta (γδ) T cells. Their TCR is also a heterodimer of a gamma chain paired with a delta chain.

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antibody-mediated immunity These CD4+ cells, called helper T cells, bind to antigen presented by B cells. The result is the development of clones of plasma cells secreting antibodies against the antigenic material

The TCR (of alpha/beta T cells) binds a bimolecular complex displayed at the surface of some other cell called an antigen-presenting cell (APC). This complex consists of:  a fragment of an antigen lying within the groove of a  histocompatibility molecule The complex has been compared to a "hot dog in a bun". Most of the T cells in the body belong to one of two subsets. These are distinguished by the presence on their surface of one or the other of two glycoproteins designated:  CD4 B cell Receptors  CD8 - Immunoglobulin o Receptors that are from proteins Which of these molecules is present determines what types o Has four interlinked polypeptide chains of cells the T cell can bind to. o Two Long chains are Heavy chains while the  CD8+ T cells bind epitopes that are part of class I o other two short ones are light chains; the area histocompatibility molecules. Almost all the cells o known as “variable portion” varies depending of the body express class I molecules. o on the specificity of the antibody  CD4+ T cells bind epitopes that are part of class II o This variable portion attaches to a particular histocompatibility molecules. Only specialized o type of antigen antigen-presenting cells express class II molecules. These include: o dendritic cells o phagocytic cells like macrophages and o B cells!  Mhc class 1-can be seen in all nucleated cells in the body antigen presentor to cytotoxic t cells  Mhc class 2- macrophages, b cells, antigen presentor to helper t cells CD8+ T cells The best understood CD8+ T cells are cytotoxic T lymphocytes (CTLs). They secrete molecules that destroy the cell to which they have bound. This is a very useful function if the target cell is infected with a virus because the cell is usually destroyed before it can release a fresh crop of viruses able to infect other cells. +

In general, the role of the CD8 T cells is to monitor all the cells of the body, ready to destroy any that express foreign antigen fragments in their class I molecules. +

CD4 T cells + CD4 T cells bind an epitope consisting of an antigen fragment lying in the groove of a class II histocompatibility + molecule. CD4 T cells are essential for both the cellmediated and antibody-mediated branches of the immune system:

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Cell-mediated immunity These CD4+ cells bind to antigen presented by antigen-presenting cells (APCs) like phagocytic macrophages and dendritic cells. The T cells then release lymphokines that attract other cells to the area. The result isinflammation: the accumulation of cells and molecules that attempt to wall off and destroy the antigenic material (an abscess is one example; the rash following exposure to poison ivy is another).

o B cells produce plasma cells> secrete immunoglobulins o B cells produce memory cells > display same receptor> secondary encounter to the same antigen>immune response is stronger and faster. There are 5 major classes of immunoglobulin determined by heavy chain amino acid sequence Antibody Function IgA

• Found in Gut, respiratory tract, urogenital tract (mucosal surfaces) • Precent bacterial adherence to mucosal surface

IgD

• Unknown function • Doesn’t appear to serve a protective function, doesn’t bind to complement, neutrophils or macrophages

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PHYSIOLOGY

Physiology of Immunity

IgE

• Immediate hypersensitivity or allergic reactions • Resistance to parasites

IgG

PREDOMINANT IMMUNOGLOBULIN • Immunity for the new born (can cross placenta • Fixation of Complement • Opsonization for coating antigen for enhanced phagocytosis • Neutralization of toxins and viruses • Participation in agglutination and precipitation reaction

IgM

MACROGLOBULIN • Found mainly in intravascular pool • Primary response

1. Clonal deletion- happens in fetal life, if the body has a lymphocyte that has a receptor to a body protein the lymphocyte is destroyed immediately 2. Clonal anergy - secretion of interleukin 2 is inhibited; instead interleukin 10 is secreted, so it inhibits helper t cell activation 3. Clonal ignorance- physically preventing the meeting of the receptor and the host antigen.

antigenic stimulation and during maturing infant • Complement fixation • Agglutination • Opsonization • Neutralization of toxins • Most efficient in triggering the classical complement pathway

o B lymphocytes remain dorman before exposure to specific antigen o Activated helper T cell activates B lymphocytes and enlarges then lymphoblast, differentiates into plasma cells forming plasmablasts then expands in the RER and proliferates to produce immunoglobulin then secretion of antibodies into lymph which carries antibodies to the circulating blood Formation of memory B cells (Primary vs. Secondary response) o A few of the lymphoblast formed do not further differentiate into plasma cells, instead forms B lymphocyte similar to original clone o Activated B-cell population become enhanced and new B lymphocytes are added remain dormant until activated again they are called MEMORY CELLS o Exposure to the same antigen will cause a more rapid and more potent antibody response 2nd time around this is because of more memory B cells o This explains why during immunization accomplished by injecting antigen multiple doses with periods of several weeks to several months

Hypersensitivity reaction 1. Typical allergic reaction (IgE) Allergen > triggers allergic reaction>encounters b lymphocyte > present allergen to helper t cells secretes interleukin 4-directs b lymphocyte to produce IgE - attach to mast cells and basophils > Histamines, cytokines in the mast cells and basophils are released- smooth muscle contract, BV dilates, increase secretion Allergenic foods – peanuts, proteins, fats and carbohydrates 2. Type 2 ( IgG) Ex. Blood transfusion reaction 3. Type 3 o Antigen-antibody complex o Invasion of neutrophils in the circulation o Causes clogging in the circulation-causes inflammation and injury in various tissues 4. Type 4-delayed hypersensitivity o cell mediated Helper T cell is involved(helper t cell 1) o has sensitization phase and effector phase o Produces giant granulomas

T Cell Receptors T-cell receptor cannot combine with antigen unless the antigen is first complexed with the body’s own plasma proteins

Mechanisms for Unresponsiveness

1. Sequestration of self-antigens, rendering them inaccessible to the immune system

2. Specific unresponsiveness (tolerance or anergy) of relevant T or B cells

3. Limitation of potential reactivity by regulatory mechanisms Mechanisms that make sure that body will not launch an attack to own cell

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PHYSIOLOGY

Physiology of Immunity

Guide Questions: 1. With innate immunity, a. Each time the body is exposed to a substance, the response is the same b. Specificity and memory are involved c. Body may become immune to a disease d. Antibodies may be produced e. All of these 2.

3.

4.

5.

A(n)__________ is a group of 20 proteins activated by a series of reactions, in which one activated chemical then activates another; and promotes inflammation, phagocytosis, and cell lysis. a. Complement b. Interferon c. Lysozyme d. Mucus e. Sebum Substances that stimulate adaptive immunity are called a. Antibodies b. Antigens c. Pyrogens d. Chemical mediators e. Phagocytes B cells a. b. c. d. e.

Are stimulated by helper T cells Are produced in the thymus Release cytokines Cause cell lysis All of these

The activation of T cells can result in a. Lysis of virus infected cells b. Production of memory T cells c. Production of lymphokines d. Delayed hypersensitivity reactions e. All of these

6.

Given these events that are part of helper T cell activation: 1. interleukin-1 released by macrophage (costimulation) 2. antigen processed by macrophage 3. helper T cell binds to macrophage 4. helper T cell releases interleukin-2 and divides 5. antigen displayed on MHC molecule

Arrange the events in the order that they occur as an antigen is recognized and a helper T cell is activated. a. 1 4 2 5 3 b. 2 5 3 1 4 c. 3 1 4 2 5 d. 4 2 5 3 1 e. 5 3 1 4 2 7.

Complement is involved in a. Only adaptive immunity b. Only innate immunity c. Both

8.

The antigen-presenting cells that are required for the activation of B cells are a. B cells b. Macrophages c. Dendritic cells d. Monocytes e. Mast cells

9.

Interleukins and CD4, B7 and CD28 proteins are a. Innate immunity b. Co-stimulation of helper T cells c. Suppressing B cell or T cell activity d. Inflammation e. Production of T cells in spleen

10. MHC class 2 molecules a. Are found on all cells b. Are found on APCs c. Usually display self-antigens d. Cannot combine with other antigens e. Both a and c

Answers: A, A, B, A, E, B, C, A, B, B

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