Innate Immunity

Immunity

• Ability to ward off a disease/ disease causing organism/ foreign material (pollen etc)

• Susceptibility: lack of resistance to a disease• INNATE IMMUNITY

– At birth– First and Second line of defense– Non-specific– No Memory

• ADAPTIVE IMMUNITY– Third line of defense– Specific– Memory response (to previous disease/ vaccination)

First line of defense

• Intact skin• Mucous membranes and their secretions• Normal microbiota

Second line of defense Third line of defense

• Specialized lymphocytes: T cells and B cells• Antibodies

• Phagocytes, such as neutrophils, eosinophils, dendritic cells, and macrophages• Inflammation• Fever• Antimicrobial substances

Figure 16.1 An overview of the body’s defenses.

Physical Barriers: First Line of Defense

• Skin– Epidermis

• Outer layer – thin• Layers of tightly packed epithelial cells• Outermost layers dead

– Keratin (Protein)– Sloughs off routinely

– Dermis• Inner layer – thick• Connective tissue

Bacteria enter through Skin ONLY when skin is damaged

Top layersof epidermiswith keratin

Epidermis

Dermis

Figure 16.2 A section through human skin.

Mucous Membranes

• Line respiratory, gastrointestinal, urogenital• Epithelial cells with inner connective tissue

– Goblet cells • Secrete mucus (glycoproteins)

– Prevents dessication, traps microbes, enzymes to kill microbes

• Mucus-coated nose hairs: Trap microbes• Ciliary escalator

– Ciliated cells of Lower Respiratory Tract– Mucus-coated dust/microbes pushed outward– Sneezing/ Coughing speed up process

• Epiglottis– Covering over larynx: prevents food and microbes getting in

Other Physical/Chemical Barriers• Tears

– Lacrymal Glands: Under upper eyelid– Constantly wash over eye; drain into lacrymal ducts– Lysozyme (breaks peptidoglycan walls of bacterial cell walls)

• Urine– Flushing mechanism prevents colonization– Contains uric acid, urea (makes pH ~6.5), lysozyme

• Saliva– Salivary Glands– Constantly washes over teeth and mouth– Contains Lysozyme

• Peristalsis– Movement of food by coordinated contractions– Defecation– Vomiting in response to microbial toxins

Lacrimal glands

Upper eyelid

Lacrimal canal

Nasolacrimalduct

Nose

Figure 16.3 The lacrimal apparatus.

Chemical Factors• Sebum: oils – protective layer on skin

– Low pH: unsatd. Fatty acids – inhibit path. Bacteria• Sweat

– Regulates body temperature– Flushes microbes, waste– Lysozyme

• Gastric Juice: HCl, mucus and enzymes– Low pH 1.5-3– Kills most microbes and toxins (except those of S. aureus and

Botulinum)• Some bacteria enter protected by food• H. pylori neutralizes acid and makes a niche for itself in stomach

• Vaginal Secretions: low pH

Normal Flora

• Competition/ Exclusion– Compete for nutrition – Alter environment to prevent pathogen growth

• Commensal– Where one benefits (normal flora), the other is

unaffected (host)– Opportunistic pathogens

• E. coli, S. aureus, S. epidermidis

Second Line of Defense

• Formed Elements in the Blood• Lymphatic System• Phagocytes• Inflammation• Fever• Antimicrobial Substances

Formed Element of Blood

• Platelets (for clotting)• Erythrocytes (Red Blood Cells)• Leukocytes (White Blood Cells)

– Granulocytes (visible granules)• Neutrophils• Basophils• Eosinophils

– Agranulocytes (granules not visible)• Monocytes• Dendritic Cells• Lymphocytes

Insert Table 16.1If possible, break into multiple slides

Table 16.1 Formed Elements in Blood (Part 1 of 2)

Insert Table 16.1If possible, break into multiple slides

Table 16.1 Formed Elements in Blood (Part 2 of 2)

• Percentage of each type of white cell in a sample of 100 white blood cells

Neutrophils 60–70%

Basophils 0.5–1%

Eosinophils 2–4%

Monocytes 3–8%

Lymphocytes 20–25%

Differential White Cell Count

The Lymphatic System• lymph : fluid• Lymph vessels • Lymph tissue (contain a large number of lymphocytes

T cells and B cells)• Red bone marrow• Lymph nodes: sites of activation for T cells and B cells • Tonsils/ Peyer’s patch• Spleen: monitor blood for microbes and secreted

products (toxins)• Thymus: T cell maturation

Rightlymphaticduct

Rightsubclavian vein

Leftsubclavianvein

Thoracic(leftlymphatic)duct

Tonsil

Thymus

Lymphatic vessel

Large intestine

Redbone marrow

Heart

Thoracic ductSpleen

Small intestine

Peyer’s patch

Lymph node

(a) Components of lymphatic system

Figure 16.5a The lymphatic system.

Phagocytosis

• Greek: Phagos (eat), cyte (cell)• Ingestion of a substance/ microbe by a cell• Phagocytes

– Cells that perform phagocytosis– Leukocytes and/or derivatives

SEM of a neutrophil phagocytosing

Aspergillus spores

Pseudopods

Bacterium

Macrophage

Figure 16.6 A macrophage engulfing rod-shaped bacteria.

Phagocytes

• Neutrophils: early during infection– First phagocytes at site of infection

• Monocytes– Morph into Macrophages when infection

progresses– Fixed v/s Wandering Macrophages

• Non-motile; Specifically present in tissues/ organs– Lymph nodes, bone marrow, spleen, liver

• Roaming through tissue, gather at site of inflammation

Phagocytosis: Mechanism

• Chemotaxis: attracted to site of infection– Cytokines (released from other WBCs)– Cell damage– Microbial products

• Adherence: attachment to microbial surface– Toll-like receptors (TLRs)– Pathogen associated Molecular Patterns (PAMPs)– Opsonins: proteins that coat microbe

• Ingestion: pseudopodia engulf microbe into phagosome• Digestion: fusion of phagosome with lysosome

– Enzymes digest microbe– Residual body excreted

Mechanism of Phagocytosis

PAMPs and TLRsTLRs in the plasma membrane of phagocytes attach to components commonly found on pathogens (PAMPs)

• LPS of gram (-) outer membrane• Peptidoglycan of gram (+) cell wall• DNA and RNA of viruses• Fungal and parasite components

Shown: Pattern Recognition Receptor of a TLR attaching to a pathogen’s PAMP

Inhibit adherence: M protein, capsules

Streptococcus pyogenes, S. pneumoniae

Kill phagocytes: Leukocidins Staphylococcus aureus

Lyse phagocytes: Membrane attack complex

Listeria monocytogenes

Escape phagosome Shigella, Rickettsia

Prevent phagosome–lysosome fusion

HIV, Mycobacterium tuberculosis

Survive in phagolysosome Coxiella burnettii

Microbial Evasion of Phagocytosis

Inflammation

• Damage to tissue: heat, infection, chemical• Four signs of inflammation:

– Heat (calor), swelling (tumor), redness (rubor), pain (dolor)– Loss of function in some cases

• Acute: intense– Infecting agent removed in short time– S. aureus (boil)

• Chronic: less intense, more destructive– Infecting agent cannot be removed– TB lesion in lungs

InflammationPurpose of inflammation:1. Destroy infectious agent

- Remove it and its byproducts from the body2. If #1 is impossible, confine the infectious agent and

byproducts; keep from spreading3. Repair or replace damaged tissue

Steps in the inflammatory response:4. Vasodilation & increased blood vessel permeability5. Phagocyte migration & phagocytosis6. Tissue repair

InflammationStage 1: Vasodilation & increased vessel permeability• Histamine

• released from injured cell granules (basophils, mast cells)• Kinins

• In plasma; attract phagocytic granulocytes to injured site• Prostaglandins

• From damaged cells • intensify the effects of histamine and kinins

• Leukotrienes• Damaged basophils, mast cells • increase vessel permeability; attach phagocytes to pathogens

• Cytokines • activated fixed macrophages• increase vasodilation and permeability

Clotting factors enter infection site; clot prevents spreadPUS: dead cells and body fluids; ABCESS: cavity after tissue breakdown

Histamine Vasodilation, increased permeability of blood vessels

Kinins Vasodilation, increased permeability of blood vessels

Prostaglandins Intensify histamine and kinin effect

Leukotrienes Increased permeability of blood vessels, phagocytic attachment

Chemicals Released by Damaged Cells

InflammationStage 2: Phagocyte migration and phagocytosis

• Phagocytes appear on the scene within ~1 hourMargination = cytokines alter blood vessel lining, cause phagocytes to stick to vessel walls at inflammation site

- Traverse vessel walls to get into affected area (= diapedesis), phagocytize invading microbes

• Granulocytes are first on scene; die off rapidly• Macrophages enter at a later stage

- larger and more phagocytic- Phagocytize destroyed tissue, granulocytes, remnants of

invaders

Inflammation

Margination

Diapedesis

Macrophage

Neutrophil

InflammationStage 3: Tissue repair• Dead or damaged cells are replaced in affected tissues• Repair capacity depends on tissue type

Stroma = supportive connecting tissue- Ex) capsule around the liver that encloses and protects

it; not involved in liver functionsParechyma = functioning portion of tissue- Ex) Hepatocyte cells of liver that perform the liver’s

functions

If parenchymal cells are active in repair = perfect reonstruction; if stroma cells are more active = scar

Bacteria enteringon knife

Epidermis

Dermis

Subcutaneoustissue

(a) Tissue damage

Bacteria

Blood vessel

Nerve

Figure 16.8a-b The process of inflammation.

Chemicals such as histamine, kinins, prostaglandins, leukotrienes, and cytokines (represented as bluedots) are released bydamaged cells.

(b) Vasodilation and increasedpermeability of blood vessels

Blood clot forms.

Abscess starts to form(orange area).

1

2

3

Insert Fig 16.8d

Scab

Blood clotRegenerated epidermis(parenchyma)

Regenerateddermis(stroma)

(d) Tissue repair

Figure 16.8d The process of inflammation.

Fever

Fever = abnormally high body temperature• a systemic response

Hypothalamus = brain region that controls body temp• Raises temp in response to cytokines by:

- blood vessel constriction- increased metabolism- shivering

• Maintained until cytokines (and infection) are eliminated• Heat loss by vasodilation and sweating

Drop in body temperature

Fever benefits:• Intensifies the effect of antiviral interferons• Increases production of transferrins

- decreases iron available to microbes• Increased speed of tissue repair

- speeds up all of the body’s reactions

Fever complications:• Increased metabolism effects:

- Tachycardia = rapid heart rate- Acidosis = increased acidity of blood/tissue- Seizures, delirium, coma- Death (temp above 112-114oF)

Fever

Antimicrobial Substances

Complement system = defensive system of >30 proteins produced in the liver that circulate the blood & tissues

• “Complements” the action of immune cells

Destroy microbes by:1. Cytolysis2. Inflammation3. Phagocytosis

• Act in a cascade with one reaction triggering another Activated by one of 3 possible pathways

The Complement Cascade

1. C3 splits into C3a and C3b

2. C3b coats the microbe to promote phagocyte attachment (opsonization)

3. C3b initiates formation of membrane attack complex (MAC) on invading cell

4. MAC causes cytolysis = bursting of invading cell due to inflow of extracellular fluid

5. C3a and C5a bind mast cells stimulate release of histamine increase blood vessel permeability• C5a also attracts phagocytes

Complement-induced cytolysis

before cytolysis after cytolysis

1. Antibodies bind antigens antigen-antibody complexes activate C1

2. Active C1 splits (activates) C2 and C4 into C2a, C2b, C4a, C4b

3. C2a and C4b combine and split C3 into fragments C3a and C3b Active fragments initiate the complement cascade

Complement activation: classical pathway

Steps:1. C3 combines with factor B, D and P

(complement proteins) on the surface of a microbe

2. C3 splits into C3a and C3b complement cascade

• No antibodies involved• Direct contact between

complement proteins and pathogen

Complement activation: alternative pathway

Lectins = proteins produced by the liver that bind carbohydratesMannose binding lectin (MBL) = binds mannose (in bacterial cell walls and some viruses)

Steps:1. MBL binds an invader2. Activates C2 and C43. C2a and C4b combine and

activate C3 complement cascade

Complement activation: The lectin pathway

InterferonsInterferons = class of cytokines produced by certain animal cells after viral stimulation

• Interfere with viral multiplication

Three types in humans:Alpha and Beta interferon = produced by infected host to induce antiviral protein synthesis in neighboring cells

• Oligoadenylate synthetase = degrades viral mRNA• Protein kinase = inhibits viral protein synthesis

Gamma interferon = produced by lymphocytes; induces neutrophils and macrophages to kill invaders; suppresses tumor cell proliferation

Interferons

Interferon complications:• Stable for only short time periods• Side effects of injection:

- Nausea, fatigue, vomiting, fever• Toxic in high concentrations

- heart, kidneys, liver, red bone marrow

Medical usage:• Limited or no effect on tumors in clinical trials• Alpha interferon some virus-associated disorders

• Kaposi’s sarcoma• Chronic Hepatitis B and C

Iron-binding proteinsHumans use iron in many ways:• component of cytochromes in the ETC• cofactor of many enzymes• component of hemoglobin

Iron-binding proteins = transport and store ironTransferrin = blood and tissue fluidsLactoferrin = milk, saliva, mucusFerritin = liver, spleen, red blood marrowHemoglobin = red blood cells

deprives pathogens of available iron!

Iron-binding proteinsSiderophores = proteins released into the medium by bacteria to capture iron from transport proteins• Forms iron-siderophore complex, recognized by

bacterial receptors and taken into cell• Splits iron from siderophore and utilizes it

Other mechanisms of obtaining iron:• Release toxins when iron is low

Kills host cells, releasing their ironEx) Strep pyogenes

• Hemolysins lysis of red blood cells• Hemoglobin broken down to capture iron

Antimicrobial PeptidesAntimicrobial peptides (AMPs) = short chains of amino acids synthesized on ribosomes• Synthesized by neutrophils when TLRs contact PAMPs

- Broad spectrum killing of bacteria, viruses, fungi- Attract other phagocytes- Sequester endotoxins

What makes them interesting?• Work together with other antimicrobials (synergy)• Stable over a wide range of pH• Microbes don’t develop resistance