Immunology Exam 1

Topics 1-3

Immune System

• definition

• pathogenesis

• Protects multicellular organisms from pathogens,
  disease-causing organisms that invade a host’s tissues/cells

• Pathogenesis: means by which a pathogen causes disease
  - A condition of an organism that impairs normal physiological function

Immune System Composition

Cells

• Identify/attack pathogens, secrete anti-pathogen molecules

Tissues

• Form barriers, secrete hormones & promote development of immune cells

Molecules

• Dissolve in body fluids, bind and neutralize pathogens, signal cells

What are the four classes of pathogens?

1. bacteria

2. viruses

3. fungi

4. helminth (worms)

Pathogen class: Bacteria

• cell type

• pathogenesis

• notes

• complications

• Type: unicellular prokaryote

• Pathogenesis: colonize tissues and/or release toxins

• Notes: Many foreign molecules make for easy recognition

• Complications: hard to distinguish normal flora from pathogens.

Pathogen class: Virus

• type

• pathogenesis

• notes

• complications

• Type: non-cellular parasite

• Pathogenesis: insert DNA/RNA into host cells to create more viral particles.

• Notes: Immune system must destroy both viral particles and infected host cells

• Complications: many are enveloped with host cell membranes and molecules.

Pathogen class: Fungi

• type

• pathogenesis

• notes

• complications

• Type: multicellular eukaryote

• Pathogenesis: colonize tissues and digest them

• Notes: Hyphae are resilient and infections often persist.

• Complications: Physically tough hyphae and eukaryotic cell structure.

Pathogen class: Helminth (worms)

• type

• pathogenesis

• notes

• complications

• Type: multicellular eukaryote

• Pathogenesis: attach to vessels or gut and absorb nutrients

• Notes: animal cell composition is similar to host

• Complications: macroscopic structure; cannot be phagocytosed.

Leukocytes

• what are they? what do they respond to? are they specific?

• do they travel through the body?

• what are lymphocytes?

• The major cells used to combat pathogens are leukocytes (aka white blood cells) → a group of several types of cells, each with a specific role

• Many leukocytes “patrol” body tissues in search of foreign entities and react to their presence with a generalized “attack” response

• Lymphocytes are a subtype of leukocyte that recognize and react to specific pathogens and generate a focused (adaptive) immune response designed for total suppression/clearance of the pathogen

Innate Immune System

• what does it involve?

• when is it present in the body?

• PAMPs? what recognizes them?

• PRRs

• Incorporates physical barriers, chemical deterrents (anti-microbial enzymes) and cells/molecules that seek and destroy non-self entities they encounter

• Innate immune system components are generally always present, regardless of whether an active infection is occurring → a “perimeter defense”

• Pathogen Associated Molecular Patterns (PAMPs) are non-mammalian molecules commonly found in broad groups of pathogens

• Many leukocytes possess Pattern Recognition Receptors (PRRs) that bind PAMPs and trigger an attack

What are examples of PAMPs?

• Lipopolysaccharide (LPS) on a bacteria

• peptidoglycan

• Mannose polymers → bacteria, fungi, helminths

Adaptive Immune System

• Acts only in response to an infection; sometimes called acquired immunity

• Does not respond immediately like innate immunity → takes several days, but response is more precise, long-lasting and completely clears pathogen

• The adaptive immune system responds to antigens (Ag) → species/strain-specific molecules (usually proteins) on a particular pathogen

• Two types of lymphocyte (subtype of leukocyte) respond to antigens and promote adaptive immunity

Antigens and Antibodies

• Antigens (Ag) are usually macromolecules (proteins, polysaccharides, lipids, nucleic acids) found on the cell/capsid surface or secreted by the pathogen

• Antigens are specific/unique to a given pathogen

• Antibodies (Ab) are Y-shaped proteins that bind to specific antigens; the “tips” of the Y-shape are Ag-binding sites

• This“tags” the pathogen for recognition by other leukocytes & other immune system elements

• As more pathogen is encountered, more Ab is produced

• Ab can “smother” a pathogen (blocks receptors/channels), while tagging it as a target

Antigen Recognition and Effector Response

• Antigens are unique to a given pathogen; this is critical so ach pathogen can be identified specifically and so the effector response that follows can be tailored to best eliminate said pathogen

• the adaptive immune system must also ignore normal flora

Adaptive Immunity Overview

• responds to specific pathogens

• has 4 main characteristics

Antigen Specificity

• subtle differences between antigens are distinguishable; Ab often will not bind to an Ag with even a single aa difference

• Pathogens that mutate rapidly can easily change alleles by one codon

Diversity

• Billions of types of producible Ab; each binds a given Ag

• contrasts strongly with generic PAMPs

Immunological Memory

• memory lymphocytes are retained and respond quickly to repeat infections

Self/Non-Self Recognition

• Lymphocytes that produce Ab that bind self-molecules as Ag are destroyed (this is critical so the IS doesn’t attack host)

Innate vs Adaptive Immunity

Toxins as Antigens

• Antigens are often on the surface of a pathogen, but many secreted toxins (microbial or animal) are also antigens

• Antivenins are made from blood serum from animals that have generated large concentrations of antibodies that bind and neutralize

• Serum sickness: other animal proteins in the antivenin are antigens that trigger an immune response

• tetanus, diphtheria, and cholera cause disease by releasing toxins

Basic Ab Structure

• Antibodies are Y-shaped molecules consisting of four polypeptide chains (2 heavy, 2 light) that are bound together by disulfide bonds

• Antibodies are hetero-tetramers w/ Ag Fab binding sites at the tips of the “Y”

• The binding site of each Ab has a specific amino acid sequence that makes it chemically compatible with a specific Ag, like enzyme/substrate binding

• Fc region of Ab “stem” can bind to receptors on leukocytes, triggering effector responses

Lymphocyte Overview

• two types

B-lymphocytes

• mature in the bone marrow

• React to antigens and secrete antibodies

T-lymphocytes

• mature in the thymus gland

two types

• Helper - secrete cytokines

• Killer - induce apoptosis in infected cells

Clonal Selection & Expansion

• through “controlled mutation” of receptor genes, each lymphocyte in the immune system creates a receptor that recognizes a different Ag

• upon infection, the specific lymphocyte binds Ag and undergoes mitosis for several days, resulting in an “army” of lymphocytes against that Ag

• This process is called clonal selection because all the daughter lymphocytes produced are genetically identical (recognize same Ag) as the original

Clonal Selection in B-Cells

• B-cells have an Ag-receptor called the B-Cell Receptor (BCR)

• When Ag binds, the B-cell proliferates and daughter cells differentiate into plasma cells that secrete Ab into lymph and bloodstream

• BCR and Ab have the same structure/gene, but BCR is membrane-bound

• A smaller number of memory cells are dormant and remain after infection

Clonal Selection in Helper T-Cells

• Helper T-cells (T_H) have an Ag-receptor called the T-Cell Receptor (TCR)

• When Ag binds, T-cells proliferate and begin secreting cytokines

• T-cells cannot bind Ag alone; it must be presented by a leukocyte that has phagocytosed the Ag and “presents” Ag particles on its surface

• This Ag-Presentation is important because it prevents T-cells from reacting to Ag that hasn’t been confirmed by another cell

Cytotoxic T-Cells

• Killer T-cells (T_C) respond to the cytokines released by T_H cells and convert into active cytotoxic T-lymphocytes (CTLs)

• CTLs identify and destroy virally-infected host cells

• Virally-infected cells display viral Ag particles on their cell surface

• CTL bind to the Ag complex on infected cells and induce apoptosis to prevent the spread of virus

Humoral and Cell-Mediated Immunity

Adaptive immunity results from B-cell induced humoral immunity because it and T-cell driven cell-mediated immunity

Immunological Memory

• When the immune system detects an Ag, large concentrations of Ab are produced against the Ag and cytokines mobilize leukocytes

• Memory cells remain dormant in the body after infection but are rapidly activated generated upon re-infection

• This secondary immune response generally eliminates the pathogen before any symptoms develop; thus it’s true immunity

• The immunological memory can eventually fade a memory cells undergo apoptosis after years of dormancy → Why we need booster shots

• Vaccines are solutions of non-infectious Ag that induce an immune response (and memory) without causing a disease state

• Some vaccinations/infections provide lifelong immunity (chickenpox) while other pathogens (cold virus, sleeping sickness) resist immune memory

• Length of memory is related to the pathogen and severity of infection

• Weak infections produce fewer memory cells

• Some pathogens have adapted strategies to evade or “out-mutate” immunity

Active Immunity

• the person/animal’s immune system develops their own immune cells and antibodies in response to a pathogen

• the patient is challenged with Ag, generating lymphocytes & memory cells

• Vaccines and infection generate active immunity

Passive Immunity

• A subject is provided antibodies against a specific antigen directly (don’t produce any themselves)

• Antisera (injectable Ab solutions) or antivenins are emergency treatments of infections of poisonings (e.g. tetanus) or for immunocompromised patients

• Mothers provide maternal antibodies through placenta and breast milk

• Passive immunity does not generate immunological memory because the host immune system is not generating the response (no memory cells)

Immune Response and Memory

• Due to clonal expansion and the production of memory cells, a greater amount of antibody will be produced (and with shorter lag time) on the second exposure to a given antigen

• Lymphocytes produced through clonal expansion are specific to a given Ag, so they can’t help fight off a different pathogen

Blood cells

• where are they derived from?

• what is the major distinguishing step for HSCs?

• do HSCs ever run out?

• all blood cells derive from hematopoietic stem cells (HSCs) in bone marrow, and differentiate into erythrocytes and leukocytes through hematopoiesis

• the myeloid/lymphoid split is the major distinguishing step for HSCs

• HSCs are ever-renewing through mitosis

Myeloid progenitors

• what do they differentiate into?

• what are erythrocytes?

• what are megakaryocytes?

• does either cell have a direct role in immunity?

• myeloid progenitors can differentiate into erythrocytes (RBCs) and megakaryocytes

• erythrocytes are non-nucleated cells that passively transport O₂ bound to hemoglobin

• megakaryocytes respond to wounds by blebbing off non-nucleated platelets (aka thrombocytes) that form blood clots

• neither of cell type has a direct role in immunity

Myeloid lineage leukocytes

• what do they function as? where?

• what do they respond to?

• what can they detect and trigger?

• function in the innate immune, as “first responders” to
  infection, recognizing and responding to PAMPs with their PRRs

• However, each also has receptors that detect Ab bound to Ag and trigger aggressive effector responses

• So these function in both innate & adaptive immunity

Mononuclear Phagocytes

• how do these cells begin? what do they become?

• what happens when they convert?

• morphology?

• these cells begin as monocytes, circulating in blood for ~8 hours, then differentiating into amoeboid macrophages that “crawl” through tissues

• When monocytes convert to macrophages, they become 5-10X bigger, gain more organelles, longer pseudopods and increased phagocytic capabilities

• Macrophages can be immobile/fixed in a particular tissue or “wandering”

• Morphology = mononuclear with lots of pseudopods; 5-10% of blood cells

What are the differences in morphology between monocytes and macrophages?

Monocytes

• bean-shaped nucleus

• small pseudopods

• a few lysosomes

• phagosomes

Macrophage

• round nucleus

• aggressive, large pseudopods

• many lysosomes

• larger? phagosomes

• phagolysosome

Macrophages

• what is primary role?

• what triggers phagocytosis?

• Fc receptors?

• how do cytokines effect activity?

• primary role is phagocytosis of pathogens, triggered by contact of pathogen with MØ pseudopods

• MØ PRRs can bind to PAMPs, triggering phagocytosis as part of innate immune system response

• MØ also have Fc receptors that bind Ab bound to Ag and induce phagocytosis more effectively than PRRs

• Cytokines greatly increase MØ activity, aggressiveness, and pseudopod action.

Macrophages and Antigen Presentation

• When MØ ingest pathogens, they fuse the phagosome/endosome with a lysosome to digest pathogen molecules

• Remaining peptide fragments are bound into a Major Histocompatibility Complex (MHC) receptor and “displayed” to T-cells to initiate adaptive immunity

• MØ displaying MHC/Ag are called Ag-Presenting Cells or APCs

• APC MØs become more activated after engulfing pathogen and/or in response to cytokines

• Increase phagocytic activity, secrete inflammatory molecules, produce antimicrobial substances and secrete chemokines that attract more leukocytes

Are there many types of macrophages?

yes, they are given specific names based on function and tissue location

Granulocytes

A group of four cell types characterized by cytoplasmic granules of enzymes

• inflammatory molecules and hormones that are released in response to infection.

Neutrophils

• how common are they?

• morphology?

• how do they engage with pathogens?

• what do they respond to?

• line of defense?

• lifespan?

• Most numerous leukocyte (50%-70%) and numbers increase quickly in response to infection (leukocytosis → high “white blood cell count”)

• Morphology = multi-lobed nucleus (3-5 lobes) with granules & pseudopods

• Engage pathogens via rapid and efficient phagocytosis and exocytosis of antimicrobial compounds, including lysozyme

• Highly responsive to chemokines released by other leukocytes or by damaged tissues; neutrophils arrive at a damaged site within minutes

• Simple, efficient and disposable, neutrophils are the first line of defense and use “swarm tactics” against pathogens

• Very short lifespan (1-2 days) but produced in high numbers

Eosinophils

• common?

• what are they designed to fight?

• morphology?

• phagocytic activity?

• allergies?

• present in low numbers (1%-3%); motile leukocytes designed to fight multicellular parasites (helminths) by releasing digestive enzymes onto them

• Morphology: bilobed nucleus & granules

• Very minor phagocytic activity; not often employed

• Cause of allergies in areas free of worm parasites

Basophils

• function?

• what do they respond to?

• regulatory?

• Morphology?

• common?

• Release granules of inflammatory chemicals, serotonin & histamine, which increases vasodilation & blood flow to an area (recruits more leukocytes)

• Responds in particular to helminths and ectoparasites (ticks); in their
  absence reacts to pollen & other allergens

• Seems to be more of a regulatory cell, designed to attract & direct other leukocytes

• MORPHOLOGY = Comma-shaped nucleus and granules, no pseudopods

• Least common leukocyte (0.5-1.0% of total WBCs)

Mast Cells

• function?

• function of the granules?

• what do they cause? what can they aid in?

• Morphology?

• Circulate in the blood as undifferentiated cells, then enter the tissues and “implant” in a fixed location

• Have cytoplasmic granules containing histamine, which are released in response to either chemical signals or physical stimulation (nerve signals)

• Cause anaphylaxis, itchiness, hives, etc.

• Also aid in wound healing and other processes

• Morphology: mononuclear, very high concentrations of granules, pseudopods (sensory, not phagocytic)

Lymphoid Cells

• how many types stem from these?

• how common are they?

• three types of cells that stem from a common progenitor

• collectively 20%-40% of blood WBCs and 99% of lymph WBCs

• 1 trillion cells circulating in average adult human - lymphoid cells = 1% of cells in human body

B-Cells

• where do they originate? when do they get their receptor assigned?

• defining feature?

• Naïve B-cell function?

• plasma cell function?

• memory cells?

• B-cells mature in the bone marrow; assigned their specific B-cell receptor

• Defining feature = BCR (membrane-bound Ab, 150,000 per cell) can only be detectable chemically, not visibly

• Naïve B-cells bind Ag, clonally expand via mitosis, and differentiate into plasma cells and memory cells

• Plasma cells secrete 1000 Ab/sec, but have no membrane-bound Ab (live for 1-2 weeks, then apoptosis)

• Memory cells are morphologically indistinguishable from naïve B-cells

B-cell and T-cell Morphology

• size? why?

• cost-saver?

• what phase are they stuck in?

• Naïve B-cells and T-cells are morphologically indistinguishable from each other, but are often called small lymphocytes because of their small size (~6 um)

• This size is due to low # of organelles and generally low infrastructure (cell is mostly nucleus)

• A cost-saving mechanism → no need to develop many organelles because naïve cells are dormant until they bind Ag

• Naïve cells are stuck in G₀ phase

Lymphocyte Differentiation

• what happens when small lymphocytes encounter Ag?

• what do lymphoblasts do? What happens when they stop dividing?

• what do effector cells have more of?

• when small lymphocytes encounter Ag, they reenter the cell cycle and grow into a larger lymphoblast (~15 um) that has more extensive organelles

• the lymphoblast divides many times and some cells cease dividing, becoming effector cells (plasma cell, or cytokine-secreting effector T-cell)

• Effector cells have more cytoplasm and organelles to produce large amounts of protein (Ab or cytokines)

• image applies to both B-cells and T-cells

T-cells

• difference between naïve T-cells and B-cells?

• where do T-cells mature? where is the TCR assigned?

• How do TCRs bind?

• How many MHC classes are there?

• Naïve T-cells are similar to B-cells in many ways except that they have the TCR instead of membrane-bound Ab

• A T-cell matures in the thymus gland and is assigned its Ag-specific TCR

• TCR can only bind Ag peptide fragments presented in an MHC receptor by an Ag-Presenting Cell (APC)

• There are 2 classes of MHC receptors, one for T_H cells and one for T_C cells.

MHC Class I

• expressed by (nearly) all animal cells.

• When a cell becomes infected with a virus, it displays viral Ag bound by MHC-I as a “warning”.

• TC cells detect these Ag/MHC-I complexes

MHC Class II

• Expressed only by antigen presenting cells (APCs).

• when certain leukocytes engulfs a pathogen, it displays Ag bound by MHC-II on its surface and is called an APC

T-Cells

• what do T_H cells bind to and do?

• what do T_C cells bind to and do?

• which glycoproteins do each type of T-cell use? what are their functions?

• T_H cells bind Ag/MHC-II on APCs, then start secreting cytokines

• T_C cells bind to Ag/MHC-I on virally infected cells; then differentiate into CTLs and initiate apoptosis in infected cells

• T_H cells have the glycoprotein CD4 that enables them to recognize Ag bound to MHC-II only; T_C cells have CD8 that recognizes only MHC-I

T-Cell Subtypes

• what are T_H cell subtypes typically involved in? what do they secrete?

• What do regulatory T-cells secrete? what are they important for?

• T_C cells have only one major type, but T_H cells have multiple subtypes

• Several subtypes are involved in response to specific types of pathogens & secrete types of pro-inflammatory cytokines suited to fighting a certain pathogen

• Regulatory T-cells secrete anti-inflammatory cytokines and are important in ended an immune response or preventing an improper immune reaction

Ag-Presenting Cells

• what 3 types of cells can be APCs?

• What do B-cells present to?

• what do MØs present to?

• Only three types of cells can be APCs: Macrophages (MØs), B-cells & Dendritic Cells (DCs)

• B-cells bind Ag & present it to T-cells before they undergo expansion & differentiation into plasma cells

• MØs present to T-cells after phagocytosis, DCs after pinocytosis

• Most Ag is presented in MHC-II to a T_H cell, but viral Ag is presented in MHC-I to “kickstart” T_C expansion & differentiation into CTLs

Lymphocyte Cell Surface Molecules

• are the surface proteins the same at every stage?

• what are the surface proteins denoted as?

• what functions do CD proteins serve?

• are they specific to one cell type?

• Lymphocytes of a particular type, at a given stage of development, have specific glycoproteins on their cell surface

• These proteins are denoted CD for cluster of differentiation (old term)

• Each CD protein has a specific cell function (often signaling)

• Some CD proteins are specific to one cell type, others are found on several

Natural Killer Cells

• do they bind specific Ag?

• what is their function?

• what are the two methods of detection?

• what are natural killer t-cells?

• Although lymphoid cells, these do not bind specific Ag and are part of the innate immune system

• NK cells roam & perform “surveillance” to detect abnormal cells (virally) infected or cancerous) & destroy them

• Two methods of detection:

1. Detect abnormalities on the cell’s surface (lack of normal receptors like MHC-I, or tumor-specific markers)

2. Bind antiviral or anti-tumor antibodies on the cell surface

• Morphology = large, granular lymphocyte, pseudopods for “touching” cell surfaces

• Natural Killer T-cells (NKT) are a subgroup with TCRs that detect lipid and glycolipid Ag

Dendritic Cells

• what kind of extensions do they have?

• where are they found? what is their function?

• how do they handle pathogens?

• what happens when they engulf pathogens?

• where do they present Ag?

• Have long, membranous extensions, similar to nerve cell dendrites; very flexible and able to extend/retract

• Found in the tissues of major organs, filtering/sampling blood & lymph as it flows through tissues

• Engulf pathogens by phagocytosis or pinocytosis (can “drink” its own volume in fluids every hour)

• When they engulf pathogens, dendritic cells become APCs

• As APCs, they move from tissues into the bloodstream

• Move to the lymphoid tissues (e.g. lymph nodes) to present Ag to T-cells

Hematopoiesis

• DCs can derive from myeloid or lymphoid progenitors

• Subtle differences btw myeloid and lymphoid DCs

Immune System Organs

• what are the primary lymphoid organs? the secondary?

• what are their functions?

• why are the blood and lymphatic system important?

PRIMARY LYMPHOID ORGANS: are sites where lymphocytes mature and gain their specific Ag-receptors

• bone marrow

• thymus

SECONDARY LYMPHOID ORGANS: provide sites where lymphocytes and Ag are forced together

• lymph nodes

• spleen

• mucosal lymphoid tissue

• The blood stream and lymphatic system are critical for moving cells & Ag to and from these organs

Bone Marrow

• what are stromal cells? function?

• what are osteoblasts? function?

• where do differentiated cells go?

• what is unique about immune cell cytoskeletons?

• Stromal cells express soluble cytokines & membrane-bound proteins that promote HSC proliferation & differentiation (a stem cell niche)

• Osteoblasts in the endosteal niche produce bone tissue and also direct the mitosis & differentiation of HSCs

• As cells differentiate, they migrate into the vascular niche and enter blood vessels

• Immune cells have very pliable cytoskeletons to extravasate in and out of blood vessels; cell adhesion molecules (CAMs) assist

Bone Marrow

• what are reticular cells? function?

• what are sympathetic neurons?

• where do all leukocytes (except T-cells) mature?

• how does maturation occur?

• How do B-cells develop? where do they mature?

• Reticular cells form connections between osteoblasts of the endosteal niche and endothelial cells lining blood vessels

• Sympathetic neurons stimulate/regulate release of blood cells

• All leukocytes, except T-cells, fully mature in bone marrow

• Maturation occurs on a gradient as cells proceed towards blood vessels

• B-cells are assigned their specific Ag-receptor (BCR) as they develop & self-reactive B-cells are destroyed

• B-cell maturation occurs in the bone marrow for humans and mice, but not other animals

• Fetal spleen and gut lymphoid tissue are common sites

Thymus Gland

• what is the thymus

• what are T-cells produced from?

• what are lobules made of?

• what are thymocytes? where are they? why do they diminish?

• Bi-lobed, surrounded by a capsule and divided into lobules

• T-cells are produced from HSCs in the bone marrow, but mature here

• Each lobule has an outer cortex, containing many immature T-cells (thymocytes), and an inner medulla, which contains sparse thymocytes

• Thymocytes arrive in the cortex and then migrate inward to the medulla, departing via medullary blood vessels

• Thymocyte numbers diminish due to negative selection that destroys those that are potentially self-reactive

Thymus Gland

• what happens as T-cells mature?

• what do cortical and medullary thymic epithelial cells do?

• what happens to self-reactive thymocytes?

• what do Hassall’s corpuscles do?

• As T-cells mature in the thymus, they begin expressing their Ag-specific TCR

• Cortical and medullary thymic epithelial cells (TECs) & dendritic cells interact with thymocytes, promoting development & testing them for self-reactivity

• Self-reactive thymocytes undergo apoptosis and macrophages engulf the resulting apoptotic bodies

• Surviving thymocytes development into fully mature T-cells

• Hassall’s corpuscles secrete cytokines to aid development and possibly test thymocytes for self-tolerance

T-Cell Maturation Overview

• 95% of thymocytes fail selection, either because the TCR doesn’t bind Ag/MHC properly or because it cross-reacts with self-Ag

• The thymus atrophies with age and T-cell production slows (at age 65, only 2% T-cell production)

• The thymus is one of several glands (e.g. tonsils, appendix) once thought to be vestigial & useless because function wasn’t understood