Dr.gold Immunology Notes

Immunology -‐ Gold Immunology – study of physiological defense by which the body recognizes itself from non-‐self (foreign matter)

o In the process of doing so, the body: o Protects against infection by microbes, bacteria, fungi, and parasites o Isolates or removes non-‐microbial foreign substances o Destroys cancer cells

• Nonspecific immune defense – protect against foreign cells without having to recognize specific identities

• Specific immune defense – (Acquired immunity) specific recognition by lymphocytes of the substance or cell to be attacked

Bacteria – unicellular organisms that have outer coating (cell wall), no intracellular membrane bound organelles. Damage tissue cells at sites of bacterial replication or release toxins into body Viruses – Nucleic acids surrounded by protein coat; unlike bacteria, they lack enzyme machinery for metabolism and ribosomes essential for protein synthesis; must exist inside other cells to survive; different types of viruses Cells Mediating Immune Defense Leukocytes

• Most numerous of immune cells • Neutrophils, basophils, eosinophils, monocytes, and lymphocytes

o Lymphocytes serve as recognition cells in specific immune responses • Use blood mainly for transportation and leave circulatory system to enter tissues where they

function

Plasma Cells • Differentiate from a particular set of lymphocytes – B lymphocytes • Not blood plasma; they are found in tissues • Major function is to synthesize and secrete antibodies

Macrophages • Found in all organs and tissues • Derived from monocytes that pass through walls of blood vessels to enter the tissues and

transform into macrophages • Found in places where they will encounter targets (skin and surfaces of respiratory and digestive

system tubes)

Dendritic cells • Macrophage-‐like • Found in all tissues

Mast Cells • Found throughout connective tissues • Not found in blood • Very similar to basophils • Contain large number of cytosolic vesicles

Cytokines • Protein messengers that function in nonspecific and

specific immune defenses • Produced by cells • Chemical communication network that allows

different immune system cells to talk to one another • There is normally a cascade of cytokines, in which

one cytokine stimulates the release of another and so on; this is essential for the precise timing of the functions of the immune system

Neutrophils, monocytes, macrophages, and dendritic cells have a variety of activities, but are particularly important to secrete inflammatory mediators and to function as phagocytes. (Refer to table 18-‐1 on pg. 649) Non-specific immune defenses; innate immune responses

• In this type of response, cells recognize a general property of the invader – recognize the lipid and carbohydrate system on microbial cell walls

• The non-‐specific response includes 3 mechanisms: body surface, inflammation, and interferons

• Defenses at the body surface o Skin glands secrete antimicrobial chemicals that destroy bacterial cell walls o Mucus secreted by epithelial linings of respiratory and gastrointestinal tracts also have

antimicrobial chemicals as well as a sticky factor, prevents bacteria from entering blood

• Inflammation (response to injury) o Coupled with specific responses to amplify inflammation o Key mediator is cells that function as phagocytes (Neutrophils, macrophages, and dendritic

cells) Fixed-‐tissue macrophage are the first to respond, then neutrophil, the dendritic cell Note: that these phagocytes, after eating the bacteria carry the contents back to

secondary lymph nodes, where a specific immune responses will take place o As well as cells with inflammatory mediators (basophils, mast cells, eosinophils)

In an example of an injury: 1. Vasodilation and increased permeability to proteins

a. Increased blood flow to inflamed area (redness and heat) b. Increased permeability to protein ensures that plasma proteins that are involved in

inflammation are able to reach injured site. Causes net flow of plasma into IF and development of edema (swelling)

2. Chemotaxis – process of Neutrophils moving out of blood across capillaries and into inflamed area a. Cells in injured area release chemoattractants (or chemotaxins), which act on Neutrophils

to promote the binding of Neutrophils to endothelial cells inside the blood vessel

b. Diapedesis – neutrophil is inserted into space b/w two endothelial cells and is released in IF

c. Once in IF, neutrophils migrate toward site of damage (chemotaxis), due to the release of chemoattractants

d. Neutrophil swallows bacteria, kills it and dies in the process. Result of dead cells is pus; pus being an indicator of an innate immune response

(Phagocytosis) Trigger for phagocytosis is interaction of phagocyte receptors with lipids and carbs on

microbial wall Opsonin – chemical that binds phagocyte to microbe Phagosome – microbe-‐containing sac formed after phagocyte engulfs microbe Phagosome fuses with lysosome = phagolysosome Lysosomal enzymes break down microbe’s macromolecules (with enzymes and oxygen

derivatives i.e. nitric oxide and hydrogen peroxide) 3. Complement – family of plasma proteins for extracellular killing of microbes without use of

phagocytosis a. Upon activation, cascade occurs i.e. first complement protein activates second protein,

which activates third complement protein and so on b. 5 active complement proteins for membrane attack complex (MAC). MAC embeds itself in

microbe membrane and forms pores that make it leaky, causing water and salt to enter and disrupt normal functions

c. C3b acts as an opsonin d. (Antibodies are required to activate the C1 protein and start the cascade via classical

complement pathway, but antibodies are secreted by lymphocytes and lymphocytes are not involved in nonspecific inflammation. Therefore an alternate pathway is used). Alternate pathway is rarely used, which means that complement systems have a minor role in inflammation.

4. Tissue Repair

• Interferons (family of antiviral proteins) o Family of cytokines inhibit viral replication inside host cells o In the presence of virus, cells secrete interferons and release them into ECF o They bind to all cells whether infected or not, and triggers the synthesis of dozens of

different antiviral proteins by the cell o These proteins interfere with ability of viruses to replicate o Not specific!

Specific Immune Defenses Overview

• Antigens – Immunogen, foreign molecule that can trigger a specific immune response i.e. proteins or polysaccharides ex. Protein coats on viruses, specific proteins on foreign cells, cancer cells, or transplanted cells, and toxins.

1. Stage 1: a. Each lymphocyte has its own unique receptor that can bind to a specific antigen; the

binding is the physiochemical meaning of the word recognize. Each lymphocyte is specific for just one type of antigen.

b. Progeny of a specific antigen-‐stimulated lymphocyte are called clones. 2. Stage 2:

a. Binding of antigen to receptor – lymphocyte activation b. Upon binding, lymphocyte undergoes cell division – clonal expansion c. Some lymphocytes will act as effector lymphocytes to carry out attack

d. Some lymphocytes will be set aside as memory cells – to recognize antigen if it returns 3. Stage 3:

a. Effector lymphocytes launch attack against all antigens of the kind that initiated the immune response

b. Takes only 1-‐2 antigens to initiate the specific immune response c. B-Cells – type of lymphocyte that differentiate into plasma cells, which secrete antibodies

into the blood d. Antibodies – guide other molecules and cells to perform the actual attack e. Cytotoxic T-cell – directly attack and kill cells bearing antigen f. Once antigens have ceased, most cells involved in attack die via apoptosis, so as to prevent

immune defense becoming excessive and harmful to self g. Memory cells persist

Lymphoid Organs and lymphocyte origins

• Lymphatic system comprises of a network of lymphatic vessels; the lymph nodes are the only lymphoid organs found within the vessels

• Primary lymphoid organs o Stem cells from yolk sac and fetal liver o Bone marrow o Thymus gland

Upper part of chest Supply mature lymphocytes to secondary lymphoid organs via blood

o Not the place where lymphocytes undergo activation during immune response • Secondary lymphoid organs

o Lymph nodes Lymph nodes are scattered along vessels Lymph is the fluid flowing along lymphatic vessels (it is IF that has entered the

lymphatic capillaries and is routed to the large lymphatic vessels that drain into systemic veins). Lymph is the site where lymphocytes in lymph nodes encounter the antigens that activate them

o Spleen 2nd largest lymphoid organ Left part of abdominal cavity b/w stomach and diaphragm The Spleen is to the circulating blood what the lymph nodes are to the lymph Contains lymphocytes, macrophages, and dendritic cells Site for destruction of old erythrocytes

o Mucosa-associated lymphoid tissue (MALT) Contain IgA

o Note that lymphocytes in secondary organs are not the same cells that originated in the primary lymphoid organs

o All lymphocytes are descended from ancestors that matured in the bone marrow or thymus but may not themselves have arisen in those organs

• Lymphocytes from all secondary lymphoid organs are constantly drained by the lymphatic vessels and directed into the blood. (Note: all lymphoid organs, not just lymph nodes are drained by lymphatic vessels and are carried to the blood)

• Similarly, lymphocytes are pushing through endothelium of venules to enter IF where they move into lymphatic vessels to lymph nodes to take up residence

• There is a constant cycle

• All lymphocytes derive from the bone marrow (refer to figure 18-‐8 pg. 657)*

o B-‐cells mature in bone marrow and then are carried by the blood to the secondary lymphoid organs

o Some cells leave BM in immature state and mature in thymus i.e. T lymphocytes or T-‐cells o Both B and T cells undergo cell division in secondary lymphoid organs o Natural killer cell (NKC) arise in BM

Functions of B cells and T cells

• B cells differentiate into plasma cells, which secrete antibodies • Antibodies combine with antigens and guide an attack that eliminates the antigens or the cells

bearing them • Linus Pauling’s hypothesis to antibody formation (1940) was based on the idea that the antigen

was a model, to which a protein wrapped around to form an antibody (wrong). • Ehrlich (1990) theorized that a cell under threat grew additional side-‐chains to bind the toxin, and

that these additional side chains broke off to become the antibodies that are circulated through the body. He referred to them as “magic bullets”.

• Antibody-mediated responses = humoral responses o Antibodies in the blood o Major defense against bacteria, viruses, and microbes

• T cells: o Cytotoxic T cells – contain CD8 in plasma membranes; CD8+; attack cells; travel to location

of target, bind to them, and directly kill them via secreted chemicals; o Helper T cells – contain CD4; CD4+; combine with antigen and undergo activation; once

activated, secrete cytokines that act on B cells and Cytotoxic t cells that also bound to antigen; stimulate activity of B and Cytotoxic T cells

Lymphocyte Receptors

• Immunoglobulins – family of proteins that make up B cell receptors and antibodies • The B cell receptors and antibodies are very similar in shape • However, the B cell receptors are not antibodies, since only secreted immunoglobulins are

antibodies • Composed of two long chains – heavy chains • Two short chains – light chains • Five major Classes – determined by the Heavy (H) chain (δ delta, µ mu, α alpha, ε epsilon) • Two Types – determined by the Light (L) chain κ kappa, λ lambda • Each chain has 2 identical H and 2 identical L chains • Interchain and intrachain disulfide bonds are present between and within all chains • “Fragments” prepared by laboratory cleavage result in: • Fab = antigen binding fragment (at the “hypervariable region”)

• Fc = crystalizable fragment determines the biological activity of the Ig Class. Stem of lower part of structure

o E.g., IgM: complement binding (pentameric structure) o IgG: placental transfer, complement binding o IgA: secretory properties (MALT) (dimeric structure) o IgE: Mastocytophilic properties

• Refer to 18-‐10 pg. 659 for immunoglobulin structure • 5 classes of immunoglobulins contains up to millions of unique immunoglobulins • The body arms itself with millions of small clones of different B cells in order to ensure that

specific receptors exist for the vast number of different antigens the body might encounter during its lifetime

• To explain the fact that there are millions of different antigen binding regions and only 200 genes that code for immunoglobulins DNA of genes that code for antigen binding site are cut into small segments and randomly rearranged along the gene, then rejoined to form new DNA molecules

T-‐Cell Receptors • Two chained receptors that have specific regions that differ

from one T-‐cell clone to another • Receptors remain embedded in T-‐cell membrane and are

not secreted like immunoglobulins • Multiple DNA rearrangements also occur for T-‐Cell

maturation • T-Cell receptor cannot combine with antigen unless the

antigen is first complexed with certain of the body’s own plasma membrane proteins

• Self membrane proteins are collectively known as major histocompatibility complex (MHC)- or Human Leukocyte Antigens (HLA)

• Two classes: Class I MHC proteins are found on all cell surfaces; combine with CD8 of Cytotoxic T-‐cell; MHC I (HLA-‐A, HLA-‐B, HLA-‐C)

• Large alpha chain, small beta-‐2 chain • Class II MHC proteins are found only on surface of

macrophages, B cells, and dendritic cells; combine with CD4 of helper T-‐cell; MHC II (HLA-‐DP, HLA-‐DQ, HLA-‐DR)

• Alpha and beta chains are the same size • Antigen presenting cells (APC) – cells that have the antigen on plasma membrane to attach to T-‐

cells; the APC cells for TH cells are the phagocytes from the nonspecific response: macrophages, dendritic cells (and not neutrophils, remember that they die when they consume an antigen), as well as the B cells that recognize the antigens in the secondary lymph nodes

• Helper T-‐cells require class II MHC; macrophages, dendritic cells, and B cells are the only APC’s to helper T-‐cells

• Antigen/microbe is phagocytized, broken down into smaller peptide fragments called epitopes, attach to MHC proteins, transported to cell surface where it is displayed in the plasma membrane; this is antigen-specific

• Nonspecific interactions occur b/w pairs of non-antigenic proteins on the helper T-‐cell and APC – costimulus

• Both antigen specific and nonspecific protein bindings (costimulus) causes APC to release cytokines interleukin 1 (IL – 1) and tumor necrosis factor (TNF) which act as paracrine agents on attached helper T cell

• Cytotoxic T-‐cells require class I MHC; all nucleated cells; major function is the destruction of any of the body’s own cells that have become cancerous or infected by virus;

• Key point: antigens that complex with MHC proteins arise within the body cells, because virus uses the materials within the host cell to produce viral proteins. The viral proteins are considered antigens to the body

• Oncogenes code for proteins that aren’t found within the body • Antigenic proteins are hydrolyzed by cytosolic enzymes (proteasomes) into peptide fragments

which are transported into the endoplasmic reticulum, where they are complexed with the host cell’s class I MHC proteins and then shuttled by exocytosis to the plasma membrane, where a Cytotoxic T-‐cell specific for the complex can bind to it

Natural Killer Cells

• Similar to Cytotoxic T-‐cells • Not antigen specific • Can attack virus-‐infected cells or cancer cells without recognizing a specific antigen • Do not have T-‐cell receptors nor the immunoglobulin receptors of B cells • Its surface receptors are unknown to scientists • Although they are non-‐specific they are involved in the specific immune response because: • Their activity is enhanced by certain antibodies and by cytokines secreted by helper T cells

Development of Immune Tolerance

• How is it that with the huge diversity of lymphocyte receptors, the lymphocytes don’t attack a person’s own proteins?

• Clonal deletion – T cells with receptors that are capable of binding self proteins are destroyed by apoptosis (programmed cell death)

• Clonal inactivation – occurs not in the thymus but in the periphery and causes potentially self-‐reacting T cells to become nonresponsive

• During fetal and early postnatal life, there is a lack in costimulus proteins • As a result, the helper T cell fails to become activated by antigen and also dies or becomes

inactivated forever (This marks the end of the framework of the specific immune responses) **Antibody-Mediated Immune Responses: Defenses Against Bacteria, Extracellular Viruses, and Toxins – a Detailed Summary 1. Antigen Recognition and Lymphocyte Activation

• Bacteria penetrates one of body’s linings and enter the IF • The bacteria is then acted upon by a nonspecific

response o Through inflammation, the attempt at

phagocytizing the bacterium/virus with macrophages, dendritic cells, and neutrophils will either completely eliminate bacterium in that area or partially; all phagocytes that have consumed the bacteria (except neutrophils, which die in the process) will return to the secondary lymph nodes

o If nonspecific response is too slow then a specific response occurs

• If a nonspecific response does not suffice, intact bacteria will survive long enough to make it into the bloodstream, circulating the body and eventually reaching the secondary lymph nodes

• In the secondary lymph nodes, a B cell, using its immunoglobulin receptors, recognizes the bacterial surface antigen and binds the bacterium

o Note that B cell may also phagocytized the antigen to become an APC for a TH cell • For a B cell to produce plasma cells and plasma cells to produce antibody, the B cells must be

activated o Mainly triggered by signals from cytokines released by helper T cells nearby antigen-‐

bound B cells • For helper T cells to react against bacteria and release cytokines, they must bind to a class II MHC

protein on the APC o The APC are the phagocytes that consumed

the bacteria during the nonspecific response as well as the B cell. If it were a fixed tissue macrophage that has phagocytized one of the bacteria, carried it to the lymph nodes, hydrolyzed its proteins into peptide fragments (epitopes) complexed them with class II MHC proteins and displayed the complex on the plasma membrane surface, a helper T cell specific for the complex will bind to it, beginning activation of helper T cell

o For a dendritic cell, the TH2 cell binds to the dendrite’s MHC II complex as well the costimulus proteins:

The costimulus proteins are B7 on dendritic cell and CD28 on TH2 cell; when these bind, the complex releases IL-‐1 and TNF

• Macrophage will help TH cell by providing costimulus via nonantigenic membrane proteins as well as the secretion of IL – 1 and TNF

o IL – 1 and TNF are paracrine agents (substance released by cells and affects adjacent cells) that act on helper T-‐cell, which ultimately secretes cytokine interleukin 2 (IL – 2) and to express the receptor for IL – 2

o IL – 2 acts as an autocrine agent (chemical messenger) that helps helper T cells proliferate (cell multiplication) as well as a paracrine agent

• The helper T cell divides; daughter cells secrete IL – 2 and other cytokines

• According to Vander 11: these cytokines provide additional signals to activate nearby antigen-‐bound B cells to proliferate and differentiate into memory cells and plasma cells, which secrete antibodies

• According to Gold: o TH2 bind to the MHC II complex (as well as

the B7–CD28 costimulus complex); these helper cells bind to B cells via a CD40

complex and provides IL-‐4, 5, 6 (not IL-‐2) to stimulate B-‐cells to proliferate and differentiate into plasma cells

o Whereas the TH1, which attaches to cytotoxic T cells releases IL-‐2 to stimulate Tc cells • In this case we used a macrophage as an APC, but B cells can do the job just as well

o B cell will gain maximal exposure to cytokines and differentiate much more rapidly 2. Antibody Secretion

• Plasma cells produce thousands of antibodies per second before they die in a day or so

• Remember there are 5 major classes of antibodies o IgG (gamma globulin) and IgM –pentameric

provide the most support for specific immune defense against bacteria and viruses in ECF

o IgE – defense against multicellular parasites and mediate allergic responses

o IgA – dimeric secreted in linings of gastrointestinal, respiratory, and genitourinary tracts, secreted by mammary glands, primary antibodies in milk

o IgD – functions are unclear • Immunoglobulins play two roles

a. During initial recognition step, those on surface of B cells bind to antigen brought to them

b. Those secreted by plasma cells bind to bacteria bearing the same antigens, “marking” them as the targets to be attacked

3. ZE ATTAK: The Effects of Antibodies

• Bind to antigen on microbial surface • Link the microbe physically to the killing mechanisms – phagocytes (neutrophils and

macrophages), complement, or NK cells 1. Act as opsonins – chemicals that bind microbe to phagocyte

o Phagocyte has membrane receptors that bind to Fc portion of an antibody 2. Activation of complement system – using the classical complement pathway (only in specific

immune responses)

o C1 – binds to binding sites on Fc portion of an antibody that has combined with antigen; this triggers the complement cascade and the formation of the MAC, which kills the cells by making their membrane leaky

o Recall C3b also acts as opsonin to bind microbe to neutrophils and macrophages o Thus antibodies enhance phagocytosis directly and via complement C3b o C3a, C5a, and C5b act as cytokines for neutrophils and natural killer cells

3. Antibody-dependent cellular cytotoxicity o The killing of microbes is done with cytotoxic cells such as NK cells o But this process requires antibodies o This is the only exception to the generalization that the method by which NK cells identify

their targets are unclear 4. Neutralization of bacterial toxins and viruses

o Antibodies bind to toxins secreted by bacteria o Forms clumps and is then phagocytized

Active and Passive Humoral Immunity (Antibody-mediated response)

• Antibody production occurs slowly over several weeks following first contact with antigen • Any subsequent infection by the same invader elicits an immediate and considerable outpouring

of additional specific antibodies • Resistance build up to microorganisms as a result of previous exposure is known as active

immunity • Vaccine – small quantities of living or dead microbes, or harmless antigenic molecules derived

from the microorganism or its toxins o Exposure of body to agent results in an active immune response along with induction of

the memory cells required for rapid, effective response to possible future infection by that particular organism

• Passive immunity – direct transfer of antibodies from one person to another, recipient receives preformed antibodies

o Occurs b/w mother and fetus o IgG can move across the placenta o Breast-‐fed milk also provides the child with sufficient IgA

Defenses Against Virus-infected cells and Cancer cells

• The humoral response is a long-‐term defense against exogenous antigens – bacteria, viruses, and individual foreign molecules that enter the body and are encountered by immune system in the ECF

• Destroying infected cells releases the virus into ECF where they are neutralized by circulating antibody

o However if too many cells are infected and destroyed, organ malfunction occurs Role of Cytotoxic T cells • Virus-‐infected or cancer cell produces endogenous antigens which are presented with class I MHC

proteins • Cytotoxic T cells specific for the particular antigen can bind to the complex, but just as with B cells,

binding to antigen alone does not cause activation of the cytotoxic T cell; cytokines from adjacent helper T cells are needed

• Once activated by cytokines, cytotoxic T cells release by exocytosis secretory vesicles into extracellular space between itself and target cell to which it is bound

o Vesicles contain perforin – (pore-‐forming protein) similar to the MAC of complement molecules

o Causes cell to become leaky and die Role of NK Cells and Activated Macrophages • NK cells and macrophages also destroy such cells by secreting toxic chemicals • NK cells do not generally have specificity

o They must bind directly to antigen without the help of antibodies o The major signal for NK cells to proliferate and secrete toxic chemicals are IL-‐2 and

interferon-gamma secreted by the helper T cell o Remember that all cells can produce interferons but only helper T cells and NK cells can

produce interferon-gamma o Positive feedback for interferon-‐gamma cytokine that causes NK cells to produce more o IL – 2 and interferon gamma also activate macrophages – activated macrophages that kill

targets in a variety of mechanisms o View 18-‐7 pg. 669 for a brief table summary of all immune responses

Dr.gold Immunology Notes

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