BI 233 Immunology Test

What are the lines of defense for the immune system

1. physical and chemical barriers

2. innate immune response - phagocytosis, inflammation, fever

3. adaptive immune response - B, T cells, immunological memory

Neutrophils

aka polymorphonuclear leukocytes (PMN), first phagocytes to arrive at injury sites, short-lived, highly mobile, can phagocytize multiple microorganisms before destroying themselves and nearby pathogens via respiratory (oxidative) burst

Monocytes

arrive to injury 10 hours to days after neutrophils, differentiate into macrophages upon entering tissue

Eosinophils

microphage, white blood cell, response to parasites, worms, allergies and can lead to inflammation, mostly parasites, phagocytic of antibody-labeled pathogens, parasites, secrete antihistamines

Basophils

Secrete histamine and heparin (anticlotting), microphage, circulating cell that releases histamine and responds to allergies primarily

Lymphocytes

from lymphoid cell lineage, defend against specific pathogens, 3 types - NK cells, B and T cells, responsible for bodys immune defense

Cardinal signs of inflammation

1. edema

2. redness

3. heat

4. pain

Edema

Increased capillary hydrostatic from vasodilation and increased interstitial osmotic pressure, decreased capillary osmotic pressure from protein leakage from endothelial separation, note: increases viscosity, slows flow for margination

Redness

vasodilation increases blood flow, from NO too

Heat

increased blood flow and blood is hotter than body temperature, boosts enzymatic and phagocytic activity, creation of fever from IL-I

Pain

chemical/mechanical stimulation, arachidonic acid/COX-2 sensitizes receptors, chemical stimulation of free dendritic nerve endings and swelling putting mechanical pressure on same nerve endings

Sentinel Cells

Neutrophils

Monocytes

Macrophages

Dendritic Cells

Innate lymphoid cells (ILCs)

Macrophages

primary phagocytes, selective, generally survive phagocytosis, ingest up to 100 microorganisms, specialized WBC, detect, engulf, and destroy pathogens, coordinate immune responses, are differentiated monocytes

Dendritic Cells

• primary antigen-presenting cells (APCs), present antigens to and stimulate lymphocytes for response

  • Follicular dendritic cells (FDC) - found in lymphoid follicles (lymph nodes, spleen), display intact antigen for recognition by B cells - via Fc and CR1, do not express MHC-II, are stromal in origin, unique, mesenchymal-derived immune-system cells located in the B-cell follicles of secondary lymphoid organs like lymph nodes and the spleen. They form extensive, dense three-dimensional networks critical for capturing antigens, organizing lymphoid microarchitecture, and driving B-cell maturation

  • Conventional (classical) dendritic cells (cDCs) - peripheral dendritic cells, immature DCs reside in peripheral tissues as sentinels, constantly sampling antigens on MHC, upon activation, they mature, migrate to lymph nodes, and present processed antigens on MHC molecules to T cells, initiating adaptive immune responses

Innate lymphoid cells (ILCs)

produce signature cytokines and provide a rapid, first line of defense at barrier surfaces such as the skin, gut, and lungs

Macrophages vs peripheral dendritic cells

• Peripheral Dendritic cells

  • Morphology - dendrites, allows them to probe environment and interact with lots of T cells

  • Function - initiate adaptive immunity - capture antigens, process them, and migrate to lymph nodes to present to naive T cells, less phagocytic - rely on micropinocytosis, present to and activate naive T cells, migrate to lymph nodes

• Macrophages - 

  • Morphology - larger, amoeboid

  • Function - tissue integrity and innate defense - clear dead cells and debris at site of infection, phagocytic - can destroy large numbers of microbes, present and interact with pre-activated effector T cells, stay in tissues

• macrophages act as front-line, heavy-duty phagocytes that clear pathogens and clean up debris, while dendritic cells act as specialized sentinels that capture antigens and migrate to lymph nodes to activate the adaptive immune system 

selectin-dependent vs integrin-dependent phase

• Part of margination

• Selectin-dependent (initial tethering and rolling) - the low-affinity, fast on/off bonds of selectins allow the neutrophil to continuously roll, enabling it to detect chemotactic signals on the endothelial surface

  • P-selectin - exposed after cytokine stimulation - TNF-⍺, IL-1

  • PSGL-1 on neutrophil binds to P-selectin

• Integrin-dependent (firm adhesion, crawling, transmigration) - as the neutrophil rolls, it encounters chemokines presented by the endothelial cells, these trigger signaling in the neutrophil, prompting its integrins to shift to a high-affinity state. This integrin binding to ICAM halts the neutrophil, causing it to flatten and spread. The neutrophil then actively crawls to locate weak spots in the endothelial junctions

  • β₂Integrin - binds endothelial ICAM - prompts firm attachment and diapedesis

  • Chemokines like IL-8 activate leukocyte integrins, converting them to a high-affinity state for firm adhesion

• Diapedesis - once firmly adhered, the neutrophil squeezes through or between the endothelial cells

What chemokine does the neutrophil diapedesis in response to

CXCL8

What are possible sources of chemotaxic signals

• Bacterial byproducts

• Host-derived cytokines and chemokines

• Complement proteins

• Mast cell degranulation

Non-oxidative vs oxidative killing of pathogens

• Oxidative (respiratory) burst - produced superoxide radicals

• Non-oxidative - lysosomes fuse, releasing enzymes

What membrane receptors do phagocytes possess? What do these receptors bind to?

• PRRs - pattern recognition receptors - detect pathogen-associated molecular patterns (PAMP) on microbes or damage-associated molecular patterns (DAMPs) from damaged cells

  • TLRs - toll-like receptors - PRRs on plasma membranes or intracellular endosomal membranes that recognize bacterial LPS, viral RNA, etc.

• Fc receptors - binds antibody constant region

• Complement receptors - like CR1 that binds C3b, bind to complement proteins that have attached to pathogen surface

What is the function of the natural killer cell?

Lymphocytes that recognize and kill abnormal cells, non-phagocytic, preferentially kill cells with reduced MHC-I expression and increased stress-induced activating ligands

Type-1 vs Type-2 interferon

• Type 1 primarily drive early antiviral innate immunity, antiviral produced by infected cells, binds receptors on surrounding cells, inducing antiviral protein synthesis that inhibit viral replication

• Type 2 bridges innate and adaptive immunity by heavily activating macrophages and NK cells against infected/malignant cells and regulating long-term immune responses, immune modulating

What initiates the activation of the classical pathway?

Complement binds antigen-bound IgG/IgM, starts with C1

C1 complex binds to the Fc region of IgG or IgM antibodies attached to a target antigen

What activates the alternate pathway?

• C3 spontaneously divides, if in presence of pathogen C3b does opsonization - binds to microbe and phagocytizes receptors

• Activated directly on microbial surfaces independent of antibody

Which of the following are vasodilators of vascular smooth muscle?

a. norepinephrine

b. nitric oxide

c. bradykinin

d. thromboxane A2

e. histamine

f. endothelin

B, C, E

• NO - endogenous vasodilator produced by endothelial cells, calcium in from histamine response causes activation eNOS, leads to stimulation NO that diffuses to vsm and increases cGMP causing vasodilation

• Bradykinin - inflammatory mediator binds to receptors on endothelial cells, stimulating them to release NO

• Histamine - part of NO

What are the biochemical pathways by which the individual vasodilators exert their effects?

• Histamine release from mast cells, platelets, and basophils results in activation of H1 receptors on endothelial cells. H1 receptor activation opens Ca2+ channels on the endothelial membrane. Increased Ca2+ influx activates eNOS, leading to NO synthesis. NO diffuses into VSM to increase cGMP levels which activates myosin light chain phosphatase and inactivates MLCK to produce VSM relaxation

• Nitric oxide - released from endothelial cells; Nitric oxide relaxes vascular smooth muscle through cGMP-mediated signaling, leading to vasodilation

  • TNF-α, released by macrophages stimulates iNOS

• Bradykinin : Stimulates vasodilation by increasing NO synthesis

What chemical allows for contraction of the endothelial cells so that neutrophils can move through the intracellular clefts?

• Histamine - Activates H1 receptors on endothelial cells leads to ↑ Ca 2+ and MLCK activation. This results in increased myosin activation, endothelial cell contraction and enlarged intracellular clefts. Thus, increasing capillary permeability

  • Increased permeability -> ↑ OPif and ↓ OPc -> edema

  • Decrease blood plasma -> increased localized viscosity (allows for increased margination)

• Substance P - Stimulates mast cell release of histamine -> increases vessel permeability

• Leukotrienes - Stimulate contraction of endothelial cells

What is an epitope?

Antigenic determinant, specific, localized region on the surface of an antigen that is recognized and bound by an immune cell receptor or antibody

T/F: Haptens allow for immunogenicity

F - Small molecules with reactivity but little or no immunogenicity unless bound to a larger carrier protein

Immunocompetence vs immunogenicity

• Immunocompetence - before infection occurs, there already exist naive lymphocytes with receptors capable of recognizing vast repertoire of antigens

• Immunogenicity - ability to stimulate proliferation of specific lymphocytes or antibody production

Primary vs secondary response

• Primary - first exposure to a pathogen - delay of ~4 days before detectable antibody production, peak antibody levels in ~10 days, IgM antibodies are first antibodies produced by the follicular B cells in the primary response

• Secondary - second exposure - faster response - 1-2 days, higher magnitude, longer duration, antibodies display higher affinity and can undergo class switching (IgM -> IgG, IgA, IgE)

How long does it take to reach peak antibody levels during a second exposure to an antigen?

1-2 days, higher magnitude, longer duration

What is the difference between active and passive immunity (also artificial and natural)?

• Active - individual produces their own antibodies, delayed effect but long duration

  • Natural - following infection

  • Artificial - following vaccination (live-attenuated, inactivated, subunit, conjugate, mRNA-based)

• Passive - transfer of preformed antibodies, immediate effect but short duration

  • Natural - maternal IgG via placenta or IgA in breast milk

  • Artificial - injection of immune globulins

A nursing mother affords her infant what type of immunity?

Passive natural

What Igs typically serve the function of the BCR on the naive B cell?

IgD - BCR on naive B cells, role in activation

IgM

5 classes of antibodies

• IgG: Most abundant; dominant in secondary responses; crosses placenta

• IgA: Found in mucosal secretions (saliva, tears, milk, respiratory/GI tract); exists as a dimer

• IgM: Typically the first antibody secreted in primary immune response; pentamer in circulation; also serves as the BCR on naïve B cells

• IgD: BCR on naïve B cells; role in activation

• IgE: Binds Fc receptors on mast cells and basophils; triggers histamine release in allergies; important in parasite defense.

What antibody can cross from maternal circulation to fetal circulation?

IgG - most abundant, dominant in secondary responses, crosses placenta

What antibody plays the role of our plasma agglutinins and are involved in transfusion reactions?

IgM - typically, the first antibody secreted in primary immune response, pentamer in circulation, also serves as the BCR on naive B cells

CD8+ cells are colloquially known as

Tc cells

CD4+ cells are colloquially known as

Th cells

Positive vs negative selection during T cell development. Where does it occur?

• Occurs in the thymus, immature T cells migrate from bone marrow to thymus for selection

• Ensures T cells can recognize foreign invaders but not attack the body's own healthy tissues. 95% of developing T cells die during selection. Survivors are immunocompetent naive T cells that recognize self-MHC but not self-antigens

  • Positive - T cells must recognize self-MHC molecules

  • Negative - T cells strongly binding self-antigen-MHC complexes undergo apoptosis

Do MHC I express endogenous or exogenous protein

Endogenous

Do MHC II express endogenous or exogenous protein

Exogenous

Which MHC class is loaded via peptide entry via TAP 1 channels

MHC I

Which MHC class is loaded via fusion with a phagolysosome

MHC II

The first signal needed for activation of the TH0 cell is:

• Recognize antigen–MHC-II complexes on APCs

• Require co-stimulatory signals (e.g., B7–CD28)

  • The first signal is antigen recognition of peptides associated with MHC-II molecules on APC cells

    • This occurs as TCR binds to MHC –II along with CD4 coupling

TH1 vs TH2 vs TfH

• TH1: IL-12 promotes TH1 differentiation. TH1 produce IL-2 → activate macrophages, support CD8+ cells = cell mediated immune responses 

• TH2: IL-4 promotes TH2 differentiation. TH2 cells subsequently produce IL-4, and IL-5, which promote antibody responses, especially IgE and eosinophil activity = antibody mediated immune responses 

• IL-21/IL-6 contribute to Tfh differentiation and function. Tfh produce IL-21 → provide B cell help in germinal centers. 

Follicular vs conventional/classical dendritic cells

• cDCs are mobile, bone marrow derived sentinels that digest pathogens to present to T cells

• FDCs are stationary, non-bone marrow derived structural cells that present intact antigens directly to B cells

What are the three signals required to activate the B cell, and what cytokines secreted by the TfH cell assist with this activation

1. Antigen binds to naïve B cells receptor (BCR). Clustering (Signal 1) initiates intracellular signaling and the antigen is internalized, processed into peptides, and presented on MHC II molecules

2. Migration to the T-B Cell Border

   1. B cells and Tfh cells meet at the T–B border

   2. Tfh cells TCR recognizes the antigen–MHC II complex on the B cell

   3. Tfh cells deliver key molecular signals that drive B cell activation

      1. CD40L (Tfh) binding to CD40 (B cell), initiates intracellular signaling

         1. CD40L–CD40 provides (Signal 2)

      2. Cytokine IL-21 provide (Signal 3)

   4. Activated B cells proliferate and form:

      1. Memory B cells – long-lived, rapidly reactivated cells that persist in circulation and secondary lymphoid organs

         1. Can initiate an almost immediate response if they encounter the same antigen

      2. Long-lived plasma cells - Long-lived plasma cells migrate to the bone marrow and secrete highaffinity antibodies for months–years. Responsible for maintaining long-term serum antibody titers

3. Primary Response: Delay of ~4 days before detectable antibody production.

   1. Peak antibody levels in ~10 days

   2. IgM antibodies are the first antibodies produced by the follicular B cells in the primary response

4. Secondary Response: Faster (1–2 days), higher magnitude, longer duration

   1. Antibodies display higher affinity and can undergo class switching (IgM → IgG, IgA, or IgE).

What are the three signals required to activate the CD4+ cell?

• Recognize antigen–MHC-II complexes on APCs

• Require co-stimulatory signals (e.g., B7–CD28)

  • The first signal is antigen recognition of peptides associated with MHC-II molecules on APC cells

    • This occurs as TCR binds to MHC –II along with CD4 coupling

  • The second signal is provided by a co-stimulatory binding of surface molecules

    • B7 molecules on APC ligate the CD28 receptor on the Th0 (CD4+)

  • The third signal is provided by a cytokine milieu during activation that determines the CD4 subtype

What are the three signals required to activate the CD8+ cell?

• Antigen Presentation: TCR recognize antigen–MHC-I complex on DC cell (Signal 1) 

• Co-stimulation:  CD28 on CD8⁺ T cell binds to B7 (CD80) on DC cell (Signal 2) 

• Cytokine-Mediated Differentiation: IL-2 from TH1 (Signal 3) 

  • thus, maximal activation of killer T-cells requires presentation of antigen associated with both MHC-I and MHC-II molecules. 

What cytokines are needed for a CD4+ to develop into a TH1

IL-12, IFN-𝛄 - IL-12 promotes TH1 differentiation. TH1 produce IL-2 → activate macrophages, support CD8+ cells, cell mediated immune responses

What cytokines are needed for a CD4+ to develop into a TH2?

IL-4, IL-2 - IL-4 promotes TH2 differentiation. TH2 cells subsequently produce IL-4, and IL-5, which promote antibody responses, especially IgE and eosinophil activity

What cytokines are needed for a CD4+ to develop into a TfH?

IL-21/IL-6 - IL-21/IL-6 contribute to Tfh differentiation and function. Tfh produce IL-21 → provide B cell help in germinal centers.

Selectin-dependent phase

Part of margination, capture and rolling via leukocyte PSGL-1 binding endothelial Pselectin (stimulated by TNF-α, IL-8)

Integrin-dependent phase

part of margination, firm attachment and diapedesis via leukocyte β2 integrin binding endothelial ICAM (Chemokines such as IL-8 activate leukocyte integrins, converting them to a high-affinity state for firm adhesion)

diapedesis

Amoeboid emigration between endothelial cells through transient junctional openings.. - Histamine stimulates H1 receptors on endothelium, causing contraction and enlarged clefts. - IL-8 (CXCL8) binds neutrophil CXCR for acute-phase diapedesis (neutrophil-selective)

Type I IFN

(IFN-α and IFN-β): Antiviral produced by infected cells. Bind receptors on surrounding cells, inducing antiviral protein synthesis that inhibit viral replication

Type II IFN

(IFN-γ): Immune modulating. Activate macrophages/NK cells against infected/malignant cells

Function of C3a

inflammation

Function of C3b

opsonization

Opsonization

enhances via complement (C3b) and/or antibodies coating microbes, tag pathogens for phagocytosis

cytolysis

Form membrane attack complex (MAC), pores cause rupture

what complement proteins form the MAC complex

C5b–C9

What complement is a potent chemotactic factor for neutrophils and monocytes

C5a

Fever

• Triggered primarily by IL-1β, TNF-α, and IL-6 pyrogens released from activated macrophages

• Functions: enhances neutrophil activity, inhibits microbial growth, and amplifies interferon (IFN) activity

• IL-1β stimulates endothelial cells in the hypothalamus to produce prostaglandin E2 (PGE₂)

  • PGE₂ binds to receptors on hypothalamic neurons and raises the thermoregulatory set point

• Physiological response: The body perceives its current temperature as “too low” and triggers heat-generating mechanisms such as shivering → core body temperature rises until it matches the new set point, producing fever

Inflammasomes

complexes within cells that detect harmful stimuli and initiating inflammatory responses

act as intracellular sensors for two main types of danger signals:

• Pathogen-associated molecular patterns (PAMPs): Molecules from microbes (e.g., bacterial DNA, viral RNA)

• Damage-associated molecular patterns (DAMPs): Molecules released by damaged cells (e.g., ATP)

Inflammation

A chemical driven protective response that: limits microbial spread, recruits and mobilizes phagocytes, clears debris, and initiates tissue repair

histamine

release from mast cells, platelets, and basophils results in activation of H1 receptors on endothelial cells. H1 receptor activation opens Ca2+ channels on the endothelial membrane. Increased Ca2+ influx activates eNOS, leading to NO synthesis. NO diffuses into VSM to increase cGMP levels which activates myosin light-chain phosphatase and inactivates MLCK to produce VSM relaxation

Activates H1 receptors on endothelial cells leads to ↑ Ca 2+ and MLCK activation. This results in increased myosin activation, endothelial cell contraction and enlarged intracellular clefts. Thus, increasing capillary permeability o Increased permeability à ↑ OPif and ↓ OPc à edema o Decrease blood plasmaà increased localized viscosity (allows for increased margination)

Nitric oxide

released from endothelial cells; Nitric oxide relaxes vascular smooth muscle through cGMP-mediated signaling, leading to vasodilation

TNF-α, released by macrophages stimulates iNOS

Bradykinin

Stimulates vasodilation by increasing NO synthesis

substance P

Stimulates mast cell release of histamine → increases vessel permeability

Leukotrienes

Stimulate contraction of endothelial cells

Trained Immunity

Innate immune cells (such as macrophages and NK cells) can develop long-lasting, memory-like responses through epigenetic modifications. Epigenetic and metabolic reprogramming can produce prolonged enhanced innate responses that may persist through some cell divisions, allowing enhanced responses upon later exposures.

B cells

Humoral/Antibody-Mediated Immunity, AMI: Effective against extracellular pathogens

T cells

Cell-Mediated Immunity, CMI: Effective against intracellular pathogens (viruses, some bacteria), cancer cells, and transplanted tissues. T cells do not recognize free antigens in solution

Immunogenicity

Ability to stimulate proliferation of specific lymphocytes or antibody production

Reactivity

Ability to react (bind) with the antibodies or lymphocyte receptors produced against it

Antibody-Mediated (Humoral) Immunity

Involves B cells, which do not directly destroy pathogens. Instead, when activated, they differentiate into plasma cells. Antibodies bind to bacteria, bacterial toxins and viruses in body fluids inactivating them and marking them for destruction.

Destruction via phagocytosis or complement activation which leads to cell lysis

Effective against extracellular pathogens

Plasma Cells

Secrete large amounts of pathogen-specific antibodies which are carried in the blood and lymph

Activated B cells proliferate and form

• Memory B cells – long-lived, rapidly reactivated cells that persist in circulation and secondary lymphoid organs

  • Can initiate an almost immediate response if they encounter the same antigen

• Long-lived plasma cells - Long-lived plasma cells migrate to the bone marrow and secrete high-affinity antibodies for months–years. Responsible for maintaining long-term serum antibody titers.

Primary Response

Delay of ~4 days before detectable antibody production

Peak antibody levels in ~10 days

IgM antibodies are the first antibodies produced by the follicular B cells in the primary response

Secondary Response

Faster (1–2 days), higher magnitude, longer duration

Antibodies display higher affinity and can undergo class switching (IgM → IgG, IgA, or IgE)

Active Immunity

Individual produces their own antibodies. Delayed effect but long duration

Active natural immunity

Following infection

Active artificial immunity

Following vaccination (live-attenuated, inactivated, subunit, conjugate, or mRNA-based)

Passive immunity

Transfer of preformed antibodies. Immediate effect but short duration

Passive natural immunity

Natural: Maternal IgG via placenta or IgA in breast milk

Passive artificial immunity

Injection of immune globulins (antibodies)

Antibodies

Aka immunoglobulins, Ig - Proteins secreted by plasma cells in response to antigen

All antibodies from a single B cell clone share identical antigen-binding sites

Structure:

• Variable region: Forms the antigen-binding site (specificity)

• Constant region: Determines antibody class and effector function

Class-switched antibodies derived from the same B-cell clone retain the same antigen specificity variable region) while differing in constant region

Antibody neutralization

Block pathogen binding to host cells

Antibody complement activation

Classical pathway → cell lysis via membrane attack complex (MAC)

Antibody agglutination

Clumping of cells

Antibody precipitation

Cross-linking of soluble antigens (e.g., bacterial toxins)

Cell-Mediated Immunity

Involves T lymphocytes that directly kill infected cells or regulate other immune cells like cytotoxic T Cells (CD8+

Upon recognition and co-stimulation, the CD8+ cell proliferates and in ~7 days a single activated Tc cell can produce a few thousand Killer T cells

Mechanisms of killing:

• Perforin (produce pores) + Granzymes (fragment DNA): Induce apoptosis

• Fas–FasL pathway: Activates apoptosis signaling

Regulatory T Cells (Tregs)

Subset of CD4+ cells that suppress immune responses to maintain tolerance and prevent autoimmunity. Many Tregs produce suppressive cytokines such as IL-10 and TGF-β

Antibody Functions

Do not directly destroy invading microbes but rather tag them, inactivating them and marking them for destruction. Form antigen-antibody complexes

• Neutralization: Block pathogen binding to host cells

• Opsonization: Tag pathogens for phagocytosis

• Complement activation: Classical pathway → cell lysis via membrane attack complex (MAC)

• Agglutination: Clumping of cells

• Precipitation: Cross-linking of soluble antigens (e.g., bacterial toxins)

B cell activation signal 1

Antigen binds to naïve B cells receptor (BCR) → Clustering (Signal 1)

B cell activation signal 2

CD40L (Tfh) binding to CD40 (B cell), initiates intracellular signaling => CD40L–CD40 provides Signal 2