Immune Intro & Innate Immunity

Question 1

Active Immunity

Answer 1

Induced by Natural Infection or by Vaccination

Question 2

Passive Immunity

Answer 2

Induced by transfer of Antibodies or Lymphocytes from immune to non-immune individual.

Natural example: from mother to fetus and newborn

Question 3

Immunity

Answer 3

Individuals previously infected develop resistance (immunity).

Naive individuals remain vulnerable.

Question 4

Effective Characteristics of Adaptive Immunity

Answer 4

Specificity

Diversity

Memory: more effective, more intense, faster response with repeated exposure

Clonal expansion

Specialization: the response is made specific and optimal to the type of microbe

Contraction and Homeostasis: Self-limited response. Down regulation after antigen declines and infection control with rapid reactivation upon later encounter with the antigen.

Non-reactivity to Self

Question 5

Neutrophils

Answer 5

PMNs.

Type of Phagocyte.

*Most abundant leukocyte in blood.

Short-lived (hours to day). *Die after a few hours.
(4-5 hours when fighting; 4-5 days when not fighting)

First cell to arrive in Acute inflammation. (Increase in neutrophils = Left shift = Increased number of bands.)

*Main initial fighting cell to most infections, particularly bacterial and fungal.

Much larger than RBCs

Segmented nuclei with 3 lobes: polymorphonuclear with Purple cytoplasmic granules = granulocytes,
Which are lysosomes that merge with ingested microbes to form phagosomes.

Are only fighters/killers. Are not sentinels.

Engulf and kill microbes with Reactive Oxygen intermediates generated within lysosomes.

Can eat microbes in blood circulation or in tissues.

Bone Marrow increases its production during infection. Colony-Stimulating Factors (CSF) are secreted by different BM cells in response to infection.

Question 6

Eosinophils

Answer 6

Type of Phagocyte

Segmented nuclei with 2 lobes

Bright red granules

Seen in Allergic and Parasitic responses

Question 7

Basophils

Answer 7

Type of Phagocyte

Least numerous phagocyte in blood.
Similar function to Mast cells in tissues.

Thick Blue cytoplasmic granules containing a variety of Vasoactive Amines. (e.g., histamine; vasodilation)

Question 8

Mast Cells

Answer 8

Type of Phagocyte

Strategically placed around Blood Vessels in Connective tissues, such as dermis and submucosa of Respiratory and GI tracts.

Large cells with granules filled with Vasoactive Amines, such as Histamine.

Participate in Allergic Reactions.

Question 9

Monocytes / Macrophages

Answer 9

Type of phagocyte.

*Reside in all Connective tissues with the same function as monocytes and are the first Sentinel to sense infection or cell damage.

Monocytes form in Bone Marrow and circulate in blood.
They leave the blood when attracted by infected or damaged cells, transforming into Tissue Macrophages (aka histiocytes). (Mononuclear Phagocyte System)

Engulf microbes and Secrete Cytokines:
Interleukin-1: Activates Endothelial cells to become “Leaky.”
Tumor Necrosis Factor (TNF): Activates Neutrophils
Interleukin-12: Activates NK cells

Can act as an APC to lymphocytes by carrying microbes from epithelia to local lymph nodes.

Can be in tissues for many months. Major innate immune cell.

Are mononuclear. Have kidney bean-shaped nucleus.

Is most efficient Phagocyte. Are both Sentinels and Killers.

Question 10

Dendritic Cells

Answer 10

Type of phagocyte. And are Sentinels.

Have long, cytoplasmic processes.
Difficult to identify in tissues.

1) Like macrophages, they are Sentinels that engulf microbes and respond by producing Cytokines that Recruit Leukocytes and Innate responses.
2) Also Function as Professional APC, capturing and transporting antigen to regional lymph nodes to lymphocytes.

Located in epithelia: skin (Langerhans cells), GI, respiratory, and other mucosa.

Also in lymph nodes as Follicular Dendritic Cells (FDC), which are different from regular Dendritic cells.

Question 11

Follicular Dendritic Cells

Answer 11

Type of phagocyte.

Display antigen to B cells to facilitate Humoral responses.

Do not present antigens to T cells. Thus, they are different from other dendritic cells.

Located in the Germinal centers of Follicles in peripheral lymphoid organs.

Question 12

Lymphocytes

Answer 12

Are slightly smaller than neutrophils.

Are mononuclear.

3 major classes:

• -B cells: produce Antibodies and mediate Humoral immunity.
• -T cells: produce Cytokines and mediate Cellular immunity.
• -Natural Killer (NK) cells: are part of Innate Immune system.

Other Types of Lymphocytes:
Special types of lymphocytes participate in early host defense (true sentinels). They express so magically rearranged receptors with very limited diversity:
–Intraepithelial delta gamma T cells
–NK-T cells in epithelia and lymph nodes.
–B-1 cells in peritoneal cavity

Question 13

B lymphocytes

Answer 13

B Lymphocytes

Recognize Soluble or Cell surface Extracellular antigen.

Plasma cells are fully differentiated B cells that secrete large quantities of Ig or Antibody.

Express CD 19 and CD 20 and Ig on their cell surfaces.

Ig serves as a specific antigen receptor.

Question 14

T lymphocytes

Answer 14

Recognize Non-soluble Intracellular antigen displayed by MHC molecules on host APCs.

Express CD3 and T cell receptors (TCR) that recognize specific small peptide antigens.

Two main classes:
–Helper CD4+ T cells: recognize antigen presented on the cell surface of APC.
Produce cytokines to help B cells produce Antibody and to help Phagocytes digest microbes.
Restricted: recognize antigen presented on host Class 2 MHC molecules.

–Cytotoxic CD8+ T cells: recognize antigen on the cell surface of Infected Host cells (intracellular epithelial or mesenchymal cells).
Directly kill infected cells.
Restricted to recognize antigen presented on Host Class 1 MHC molecules.

Question 15

Natural Killer (NK) Cells

Answer 15

Function: **Recognize and Kill Infected or Damaged cells and lyse them.
*Secrete IFN-gamma to Activate Macrophages.

• Contain **Cytoplasmic Granules.
• Express specific CD markers.
• *Lack antigen receptor.
• *Have surface Fc Receptors that recognize cells coated with Ig (Antibodies).

Cytoplasmic Granules:

• -Released upon NK cell activation.
• -Contain Perforin (perforates target cell membrane) and Granzyme B (activates apoptotic genes in target cell).

NK cells and Macrophages work together/cooperate to decrease Intracellular microbes by decreasing Reservoir of infected host cells:
–Activated Macrophages produce IL-12, which Activates NK cells to produce IFN-gamma, which reactivates Macrophages to become efficient killers *(NK-Macrophage Loop)

• *NK cells have Surface Inhibitory Receptors for Class 1 MHC molecules.
• -Precludes them from targeting Normal Host cells.
• -Allows them to target Tumor cells or Virus-infected cells, both of which may Down Regulate Class 1 MHC expression.

Cytokines released by infected Macrophages and Dendritic cells stimulates NK cell activity.
–Major NK cell activating cytokines are IL-12 (and IL-15 and type 1 IFN).

Antibody-Dependent Cellular Toxicity (ADCC):
–**NK cells bind and lyse Antibody-coated Host Target cells.

–They have Fc-gammaRIII Receptor (CD16), which binds to IgG bound to Host target cells and *Promotes ADCC.
(Marker for NK cells)

–*How often Infected cells express surface molecules recognized by Antibody is Unknown.
Recognize changes on cell surface of Infected cells and kill them by a process similar to CTLs.

(Recognize Host cells Under Stress by Cellular Damage due to Infection or Tumor transformation.

Do not have antigen receptors. Do not recognize specific antigen.
Instead, recognize molecules associated with Cellular Stress).

Question 16

MHC

Answer 16

Major histocompatibility complex.
In humans, the various alleles are called Human Leukocyte Antigens (HLA).

Function to keep immune responses limited to Non-self (foreign) antigen.

Class 1 MHC:

• -Consists of A, B, and C gene loci.
• -Found on All Nucleated Cells.

Class 2 MHC:

• -Consists of DR, DQ, and DP.
• -Found on APCs (Dendritic cells, Macrophages), Endothelial cells, and B cells (can act as an effective APC to activated Helper T cells during B cell activation).

One allele of each Class 1 and Class 2 genes is inherited from each parent. The most clinically relevant loci are A, B, and DR.

MHC molecules are very polymorphous = many alleles present in human population.

Question 17

Tissues of Immune System

Answer 17

Central: Where immune cells are matured: Bone marrow and Thymus
Peripheral: Where Adaptive immune responses begin: lymph nodes, spleen, cutaneous and mucosal-associated lymphoid tissue (MALT)

Organized to optimize interactions between antigen, APC, and lymphocytes by a concentrating effect.

Question 18

Bone Marrow

Answer 18

Where all blood cells are generated: WBC, RBC, and Platelets.

All blood cells are derived from a common ancestor, the hematopoietic stem cell.

Bone Marrow stem cells are CD34+ and give rise to Lymphoid, Myeloid, and Erythroid cell lines.

Question 19

Thymus

Answer 19

“Finishing school” where T cells learn to differentiate Self from Non-self.
Very active in fetal life and childhood until adolescence when declines (atrophy).

T cells circulate throughout lymphatics and blood vessels from all organs to recognize antigen.

Only a small fraction of total body’s T cells react to any specific antigen.

*Can find antigen within one day.

Question 20

Lymph nodes

Answer 20

Contain a variety of immune cells: T cells, B cells, Plasma cells, NK cells, Macrophages, and Dendritic cells.

Nodes gather fluid draining from peripheral tissues called lymph.

Lymph carries antigen escorted from tissues inside Macrophages or Dendritic cells. Also, free antigen subsequently captured by resident lymph node Follicular Dendritic Cells and Macrophages.

Dendritic cells pick up antigen from epithelia, enter node via afferent lymphatics, the. Migrate to cortex and paracortex. B cells are in the cortex. T cells are in the paracortex.

Question 21

GALT / MALT

Answer 21

MALT of the GI tract.

GALT is the largest lymphoid organ in the body.

Lymphocytes are present in the mucosa and submucosa of the GI tract.

Microfold (M-cells) are special APC within intestinal epithelium.

Question 22

Spleen

Answer 22

Has an extensive lymphoid population, most of which is in the “White pulp” that cuffs around the Arterioles.

Additional lymphocytes, Macrophages, and granulocytes are found in the “Red pulp.”

Spleen acts as a big blood Filter to contain microbial invasion in the blood.

**Spleen contains large number of Phagocytes that clear Opsonized bacteria.

**Splenectomy predispose to Infections by certain bacteria.

Question 23

Mononuclear Phagocyte System

Answer 23

Some tissues have specialized cells with an APC or Macrophage-like function and are Permanent tissue residents.

These cells participate in immune surveillance. Examples:

• -Kupffer cells: Line hepatic sinusoids in the Liver.
• -Macrophages near bone marrow sinusoids in the Bone Marrow.
• -Macrophages in splenic sinusoids in the Spleen.
• -Microglial cells of the CNS.

Question 24

Peripheral Blood Proteins

Answer 24

Other molecules of the immune system include proteins that circulate in the blood.

2 broad categories:

• -Albumin: the predominant blood protein; provides most of the oncotic pressure.
• -Globulins: include Ig and other associated proteins; subdivided into Alpha, Beta, and Gamma globulins. Gamma Globulins = Ig.

Question 25

Alpha Globulins

Answer 25

Proteins acting as Carriers of substances, some in high enough concentration to be measured in serum.

Major one in alpha 1 fraction is Alpha-1-Antitrypsin, a protease inhibitor.

Alpha 2 fraction includes Ceruloplasmin, which carries copper, and Haptoglobin, which binds to free hemoglobin.

Alpha-2-Macroglobulin is another protease inhibitor found in the alpha 2 fraction.

Question 25

Gamma Globulins: Poly and Monoclonal Ig

Answer 25

Globular proteins acting as Antibodies produced by B cells and Plasma cells.

Consist of IgG, IgA, and IgM. And very small quantities of IgD and IgE.

Normally, antibody is produced in response to a variety of antigen or Polyclonal Response, so the Peak is Broad.

A tall, narrow peak suggests a Monoclonal “Spike” of abnormal Ig, which is very often associated with Plasma Cell Tumors.

Question 26

Beta Globulins

Answer 26

Proteins acting as Carriers, some of which are in high enough concentration to be measured in serum.

Includes Transferrin, which carries iron,
and Beta-2 Microglobulin, which links with the Class 1 MHC molecules to support their function.

The Complement protein C3 may produce a small peak in this fraction.

Question 27

C-Reactive Protein (CRP)

Answer 27

Innate immunity.

CRP binds to Polysaccharides on the surfaces of many Bacteria, Fungi, and Protozoa.

*Once complexed with Microbial Polysaccharides, CRP becomes an Activator of Complement!

• Binds to Microbial Phosphorylcholine, coating them for Phagocytosis.
• Macrophages have CRP receptors.

Also binds to phosphorylcholine, phosphatidylcholine such as lecithins, and polyanions such as nucleus acids.

Question 28

Mannose Binding Lectin (MBL)

Answer 28

Innate immunity.

Is homologous to collagen and has carbohydrate binding domains.

MBL recognizes Microbial Carbohydrates, coating them with Complement proteins for Phagocytosis (Lectin Pathway).

Question 29

Acute Phase Response

Answer 29

Circulating levels of CRP, MBL, and other related proteins increase rapidly after Acute Inflammation.

Question 30

Innate Immunity

Answer 30

= Body’s First Line of Defense

Prevents/Eradicates most infections before Adaptive immunity becomes active.

Recognizes Microbes and Damaged Host cells.
Does not recognize foreign, non-microbial molecules.

Innate immunity is Always Ready; may act as second signal to alert Adaptive immunity to respond.
Adaptive immunity needs to be stimulated before becomes effective.

Conversely, Adaptive immunity often stimulates mechanisms of Innate immunity to eradicate infections (cross-talk).

Question 31

PAMPs and PAMP Receptors

Answer 31

Innate immunity recognizes Molecules shared by Microbes that are Not Present on Host Mammalian cells, such as:

• -Bacterial Lipopolysaccharide (LPS) aka Endotoxin
• -Bacterial Terminal Mannose Residues
• -Viral Double-Stranded RNA
• -Microbial Unmethylated CpG rich DNA

^These molecules are called Pathogen-Associated Molecular Patterns (PAMPs) and are shared by many microbes.

Receptors recognizing them are called PAMP Receptors.

*Innate immunity is so effective because it attacks these Vital Microbial Structures, not present on mammalian cells.

Question 32

Innate Immunity Cellular Receptors

Answer 32

Innate Receptors are Non-Clonal (same on each cell) and are encoded by Germline genes. They recognize less than 1000 microbial molecular patterns.

In contrast, Lymphocyte Receptors are Clonal (each cell has a unique receptor different from other cells) and are encoded by Genetic Recombination.
They recognize millions/billion different antigen.

Innate Immunity Lacks Memory:
Responds always with the Same Magnitude and Same Speed to repeated encounters.

In contrast, Adaptive immunity responds more efficiently to successive encounters (faster and stronger). *Memory function is a defining characteristic of Adaptive immunity.

Types of Innate Immunity Receptors:

PAMP Receptors

DAMP Receptors

Toll-like Receptors (TLR)

NOD-like Receptors (NLRs)

Other Cell Surface receptors:

• -N-formyl methionine: Beginning bacterial peptides
• -*Terminal Mannose Residues

Cytoplasmic Receptors for Viral Nucleic Acids or Bacterial Peptides.

Receptors for cytokines and other molecules produced in response to microbes, such as:

• -*Chemokines: Migration toward microbes
• -*Interferon gamma (IFN-y): Enhances Phagocyte microbial killing.
• -*Antibodies and *Complement: Attach to Microbes.

Question 33

DAMPs and DAMP Receptors

Answer 33

Innate immunity also recognizes molecules released from Damaged or Dead cells.

Damaged-Associated Molecular Patterns

Innate immunity does not react against Host because:

• -Innate receptors recognize Molecules Not Expressed on Mammalian cells
• -Host Mammalian cells express Surface Regulatory Inhibitors (microbes don’t)

Question 34

Toll-Like Receptors

Answer 34

Innate receptors are expressed by many cell types other than innate cells, such as Lymphocytes, Epithelial cells, and Endothelial cells.

TLRs are homologous to the Drosophila’s Toll protein.
They are expressed in Plasma and Endosome membranes.

*TLR-4 recognizes Bacterial Endotoxin (LPS)

TLR-2: bacterial lipoglycans
TLR-3, 7, and 8: microbial dsRNA
TLR-5: flagellin (bacterial flagella protein)
TLR-9: bacterial unmethylated CpG DNA

Question 35

NOD-Like Receptors

Answer 35

NLRs are Cytoplasmic receptors with a NOL domain that **binds to a wide variety of Microbes and Damaged/Dead cell products, such as ATL, Uric Acid, K+, Cholesterol, Fatty Acids…

NLRP-3 binds to an adaptor protein and *triggers Caspase-1 Activation, *increases IL-1, and *results in Acute Inflammation and Fever.

*The Inflammasome = the NLRP-3 adaptor and caspase-1 complex.

Question 36

Components of Innate immunity

Answer 36

Epithelial Barriers

Cells in Circulation:
Phagocytes: Neutrophils, Monocytes, Macrophages
Dendritic cells
Natural Killer (NK) Lymphocytes
Other lymphocytes: delta gamma T cells, NK-T cells, B-1 cells

Plasma Proteins:
Complement
MBL
CRP

Question 37

Epithelial Barriers of Innate Immunity

Answer 37

Main microbe portals are skin, GI, respiratory, urinary and genital tract mucosa.

Intact functionality of these barriers (breathing, GI motility, urinary flow) helps prevent infections.

Chemical barrier: these epithelial cells may also locally produced antibiotics.

Immune barrier: Intraepithelial delta gamma T cells and NK-T cells are Sentinels against microbes attempting to breath the epithelia.

Question 38

Leukocyte Migration

Answer 38

Leukocytes attach to Endothelium via adhesion molecules and migrate toward infection site.
Multi-step process with *initial loose attachments followed by *form attachments to the Endothelium.

1) Microbe breaches epithelium and Resident Macrophages produce cytokines, such as TNF and IL-1, that induce Endothelial expression of Selectins. (E and P-selectins)
2) Leukocytes express Lectins that bind weakly to Selectins and detach by the mechanical force of blood flow.
3) Attachments reform downstream repeatedly, resulting in Rolling Over.
4) Inactive Leukocytes express Low-Affinity Integrins; but during Rolling Over, TNF and IL-1 activate Endothelium to express Chemokines.

Integrins (LFA-1); Ligand (ICAM-1)

5) Rolling Leukocytes exposed now to Chemokines to express High-Affinity Integrins that induce Endothelium to express Integrin Ligands, leading to Firm Adhesion.
6) Under the effect of the Chemokines, Leukocytes reorganize their cytoskeleton and increase their motility, migrating throughout the endothelium.

Sequence of Selectin-mediated Rolling, Integrin-mediated Adhesion, and Chemokine-mediated Motility.

Question 39

Phagocytosis

Answer 39

Phagocyte skill microbes with Reactive Oxygen Species (ROS) and Nitric Oxide (NO).

Lysosomes contain *Oxidase, which converts oxygen into Superoxide and Free Radicals.

Phagocytes produce *Nitric Oxide Synthase (NOS), which converts arginine to Nitric Oxide.

Microbial binding to Innate Receptors signals activation of these enzymes.

*Proteases break down microbial proteins.

Enzymes are bound within merged lysosomes and phagosomes (cytoplasmic granules), so that damage is restricted to microbes.

If these enzymes leak to extracellular spaces, *they injure host tissues.

*TNF secreted by Macrophages activates Neutrophils and *Enhances Inflammation.

Question 40

Macrophages and Dendritic cells

Answer 40

Are the first cells to detect microbes or damaged cells.

They are the main producers of cytokines, triggering inflammation.

Question 41

Second Signals

Answer 41

Second signals ensure lymphocytes respond to dangerous infection and not to harmless cells.

First signal is Antigen recognition by lymphocyte.
Second signal is provided by Innate immune response molecules produced in response to microbes.

**NK cell production of IFN-gamma, upon infection, stimulates Macrophages and Dendritic cells to produce second signals.

• Activated Dendritic cells and Macrophages, upon infection, express Costimulators that bind to Lymphocyte Receptors essential for Lymphocyte Activation.
• –Main Costimulators: B7, recognized by CD28 on T cells.
• –Microbial Activation of Complement generates C3d, recognized by CR2 on B cells.

T cell Activation requires Antigen, and B7 and CD28.
B cell Activation requires Antigen, and C3d and CR2.

Question 42

Adaptive Stimulation of Innate Immunity

Answer 42

Activates B cells and T cells stimulate Innate cells to kill pathogens.

Innate cells have Fc receptors and become activated by Antibodies (Ig).