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7: Host Defenses

Reading assignment: Belk’s Biology p 478-479 (non-specific defenses) and p 479-487 (specfic defenses)

Note: (Warning?) Host defenses are complicated and amazing, worthy of a course alone. It is easy to be overwhelmed by the amount of info in this section.

  • At the start of non-specific defenses and specific immunity , a 2-3 page “overview” is used to introduce the topic and to provide a “roadmap” of the most important concepts.
  • The overview is followed by study guide questions.
  • Following the study guide questions are expanded notes for those who wish to explore more details.

Overview host defenses

1. Nonspecific defenses –first line of defenses

  1. Surface defenses: skin, mucous membranes
  2. Interior defenses: inflammation, phagocytosis, fever, complement, interferon, NK cells

2. Specific defenses-second line of defenses if non-specific defenses fail

  1. Humoral: antibody mediated defenses against extracellular pathogens and toxins
  2. CMI: cell mediated immunity against intracellular pathogens

Overview: Host non-specific defenses against microbial pathogens

I. Nonspecific surface defenses: skin and mucous membranes

            A. Skin:

                        1. tightly packed cells prevent movement of microbes into deeper layers

                                2. dry: inhibits microbial growth

                                3. low pH; fatty acids inhibits microbial growth

                                4. keratin: increases strength of skin cells

                                5. normal microbiota: competition for attachment sites/nutrients, production of  inhibitory bacteriocins

                                6. lysozyme: enzyme which breaks down bacterial cell walls

                                7. Any damage to skin (cuts, bites, burns) greatly decreases protective function of skin, may lead to infection

B. Mucous Membranes: delicate, moist cells lining gastrointestinal tract, respiratory, genital, urinary tracts, covered by sticky blanket of mucous. Control movement of substances into and out body.

                        1. mucin: “sticky blanket” traps invading microbes

                                2. flushing action of fluids, solid materials, removes microbes

                                3. lysozyme breaks down bacterial cell walls

                                4. normal microbiota: competition, bacteriocins, lactic acid inhibits pathogens

                                5. mucociliary escalator/ apparatus of respiratory tract, oviducts

                                                -cold, smoking inhibits function of respiratory mucociliary escalator

                                                -chronic infection of oviducts causes scarring, Pelvic Inflammatory Disease (PID), ectopic pregnancy, infertility

Overview: Host non-specific defenses against microbial pathogens, continued

IV. Nonspecific interior defenses

            A. Inflammation

                        1. inflammatory mediators


                                    -increase capillary permeability

                                    -pain receptors

                        2. signs/symptoms





                        3. advantages/disadvantages

            B. Phagocytosis

                        1. “professional” phagocytic cells: monocytes/macrophages and neutrophils

                        2. steps in phagocytosis


                                                -contact and attachment to pathogen

                                                -phagosome formation

                                                -lysosome fusion with phagosome

                                                -hydrolytic enzymes destroy pathogen

                                                -myeloperoxidase activated with production of  reactive oxygen intermediates, superoxide radicals, hydrogen peroxide,                                                                                                            hypochlorous chloride


                        3. evasion of phagocytic killing by pathogens: capsules, leukocidins, escape from phagosome, natural resistance to hydrolytic enzymes

            C. Fever

                        1. pyrogens cause fever

                                    -exogenous pyrogen: e.g. bacterial ex lipid A of endotoxin

                                    -endogenous pyrogen; WBC release interleukin-1 (IL-1)

                                                -triggers anterior hypothalamus to synthesize prostaglandinns

                                                -cause thermostat “resetting”-->fever

                        2. advantages/disadvantages

D. Complement

The “complement system” is a number of host proteins which normally circulate in an inactive state. Microbial substances can activate the complement system.

What are advantages of the complement system? Activated complement proteins can:

            1. act as inflammatory mediators (increase blood flow, phagocytosis, etc)

            2. act as “chemotaxins”, help guide phagocytes to invading microbes.

            3. act as opsonins (coat pathogen to make pathogen “sticky”; consequently phagocytic cells can attach easier, increases phagocytic killing of pathogen)

            4. MAC=membrane attack complex, complement protein form holes in   membrane/envelopes of pathogens

E. Interferon: trigger production of “antiviral proteins” which inhibit viral replication in host cells

Study guide questions non-specific host defenses

1. Non-specific defenses of the host are the first lines of defense against invading pathogens. There are 2 lines of non-specific defenses listed below. Provide specific examples of each

                a. non-specific surface defenses:

                b. non-specific interior defenses:

2. What role does your normal microbiota, the “commensal” microbes which colonize your skin and mucous membranes play in non-specific surface defenses?

                -how can taking “broad spectrum” antibiotics lower your non-specific surface    defenses?

3. How could a pathogen “defeat” or “pass through” non-specific surface defenses?

Non-specific interior defenses

4.  -What is inflammation? 

                -What causes inflammation?

                -What are 4 signs of inflammation?

5. What are the advantages of inflammation?

6. Are there disadvantages to chronic inflammation?

7. What is phagocytosis?

                a. Which leukocytes/white blood cells/WBC are called “professional phagocytes”?

                b. Which of the phagocytes is the “first responder”?

                c. describe the steps involved in the phagocytic killing of a microbe

8. What is fever?

                a. what triggers fever production? Examples of pyrogens?

b. what are the advantages of fever production

c. What are “antipyretics”?

9. What is the complement system?

                a. How is the complement system “activated”?

b. What are 3-4 advantages of complement activation?

                                -what is/are:

                                i. inflammatory mediators

                                ii. chemotactic factors

                                iii. opsonins

                                iv. MAC/ “Membrane Attack Complex”

10. What are interferons?

a. What triggers interferon  production?

b. How do interferons protect cells against viral infections?

11. Why does smoking increase your risk for respiratory tract infections?

12.. Why will people frequently suffer from “yeast/Candida albicans” infections of mouth, vagina or anus following “broad spectrum” antibiotic therapy?

13. . Why will many people develop “bacterial cystitis” following urinary catheterization?

Why may bacterial cystitis lead to kidney infections?


expanded notes!

Non-specific defenses of hosts against microbial pathogens: expanded notes

Overview of non-specific host defenses

1. Always present/active (“constitutive”)

2. Active against a wide-range of potential pathogens= “non-specific”

3. First lines of defense against invading microbes

4.         2 levels of nonspecific defenses

                        a. surface defenses

                                    i. skin

                                    ii. mucous membranes

                        b. interior defenses

                                    i. inflammation

                                    ii. phagocytosis

                                    iii. fever

                                    iv. complement activation

                                    v. interferons

Surface non-specific defenses

1. skin: made up of multiple layers of tough keratinized epithelial cells held closely together by “tight junctions” and “intercellular cement” rich in hyaluronic acid. Features of skin which contributes to its ability to protect host from invading microbes include the following:

  • physical barrier: multiple layers of skin cells create a barrier, deeper layers of skin cells cemented tightly together (keratin produced by skin cells is a very strong protein thus adds to strength of barrier), consequently microbes can’t invade deeper into more delicate tissues
  • dry: many microbes require water rich environments to grow
  • acidic: fatty acids inhibit many microbes
  • sweat:  salt in sweat inhibits many microbes
  • normal microbiota (skin mutualists and commensals which grow on skin without causing harm to host). Normal microbiota block attachment of invading pathogens, compete for living space and nutrients. Some normal microbiota produce inhibitory waste products e.g. lactic acid, which decrease growth of potential pathogens
  • sloughing skin: top layers of skin continuously sloughed, “falling off”. Pathogens attached to these layers are thus lost from host.

2. mucous membranes: delicate linings of mouth, gastrointestinal tract, respiratory tract, genital tract, urinary tract, eyes. Mucous membranes are made up of non-keratinized epithelial cells and mucous producing cells. Cells of the mucous membranes are covered in a moist, sticky mucous blanket. Features of mucous membranes which contribute to their ability to prevent colonization by potential pathogens include:

a. sticky mucous blanket: the mucous blanket acts as a “trap” for invading pathogens, and physically prevents the ability of some pathogens from reaching    and binding to underlying host cells. Normally the mucous blanket is continually removed from the host, thus expelling the trapped pathogens before they can   cause serious disease

b. ciliated cells of upper respiratory tract and oviducts and the “mucociliary escalator”:  mucous membranes of respiratory tract and oviducts contain “ciliated epithelial cells”. Ciliated cells have tiny hair like protrusions (cilia) which can move in wavelike motions. When these cilia beat in unison, they can move substances across the surface of the ciliated cells. The ciliated cells can move mucous blankets out of the host or create currents to direct ovulated eggs toward the uterus. Damage to or loss of ciliated cells occurs in the respiratory tract as a consequence of smoking or viral infections; consequently there is a much higher risk of bacterial pneumonia. Likewise when women experience long-term genital tract infections, the ciliated cells of their oviducts are destroyed. Consequently the women can experience ectopic pregnancies, sterility and PID or Pelvic Inflammatory Disease.      

-normal microbiota: some mucous membranes (ex mouth, intestine, vagina, nose)are colonized by microbial mutualists and commensals which usually do not             cause harm to the host . As with skin microbiota, these normal inhabitants can block attachment of pathogens to host cells, compete for nutrients and produce waste products which can inhibit the growth of many microbial pathogens.

 -taking “broad spectrum antibiotics”, those which kill a wide-range of bacteria both beneficial and pathogenic, can cause lower one’s natural defenses against potential pathogens. e.g. women taking antibiotics for bacterial bladder infections/cystitis frequently develop “secondary” vaginal yeast infections with the yeast Candida albicans as a consequence of the killing of their “good” normal microbiota.

-“flushing action”- the movement of liquids or solids over the surfaces of mucous membranes helps to wash/flush away potential pathogens. Examples include movement of food/water through gastrointestinal tract, defecation, urination, lacrimation/”crying”, movement of nasal secretions/mucous through respiratory tract, movement of mucous secretions through genital tract 

-additional chemicals found in body secretions: for example lysozyme (breaks down bacterial cell walls); bile (emulsifies microbes, has a detergent-like action on lipid rich  membranes/envelopes)

Interior non-specific defenses

How can  pathogens cross surface defenses?

Pathogens may bypass surface defenses when skin/mucous membranes are damaged (e.g. insect bites, wounds, burns, surgery). Microbes may also evolve strategies to avoid these surface defenses.

What happens if pathogens cross surface defenses?

Second line of non-specific defenses are activated, the interior (‘inside”) defenses: inflammation, phagocytosis by neutrophils and macrophages; fever; complement  activation; interferon production


1. Inflammation: body’s response to any kind of damage including invasion by microbial pathogens

            a. classical signs of inflammation= redness-heat-swelling-pain

                        -caused by release of substances called “inflammatory mediators” ( for example histamine)

            b. The inflammatory mediators trigger the following changes (which in turn cause the classical signs of inflammation)

               - “vasodilation”: blood vessel diameter increases so more blood flows to site of injury. Increased blood flow causes area to appear red and increases temperature/heat

              -“increased capillary permeability”: vessels become “leakier” so fluid escapes from blood into surrounding tissues, causing the injured area to swell

            c. pain: inflammatory mediators may act directly on pain receptors and/or  swelling can contribute to sensation of pain

            d. What does inflammation accomplish?

d. What does inflammation accomplish?

            -By increasing blood flow to an injured area, inflammation increases                                           delivery of protective white blood cells/leukocytes called “phagocytes” .                                      First to arrive are phagocytic neutrophils, then later phagocytic                                                            macrophages.

            -The phagocytes will “eat”/phagocytize invading pathogens and kill them                                     hopefully before they can cause disease.

-The inflammatory mediators set-up a chemical concentration gradient                            which helps guide the phagocytes to “ground zero”, the site of pathogen           invasion

  • The phagocytes use the chemical concentration gradient to guide their movements in a process called “chemotaxis

            -Increased temperature may increase the killing activity of phagocytes

            -Increased blood flow also increases deliver of nutrients, oxygen and other defensive substances to the injured site including complement and antibodies

-Vessel leakiness eases the ability of the phagocytes to leave the blood vessels (a process called emigration) and migrate into the invaded tissues

-Leaky vessels also increase delivery of substances which produce “fibrin clots” around the invading pathogens which will reduce their ability to spread into neighboring tissues

e. Are there any disadvantages to inflammation?

            -chronic/long term inflammation can actually harm the host. Why?

                        -“sloppy feeding” phagocytes can dump hydrolytic enzymes onto host cells, causing host cell damage

                        -sufficient damage can cause loss of normal cells and replacement by fibrous connective tissue (“scar tissue”) with  resulting loss of function of the tissue/organ

-inflammation can cause abnormal function of tissues/organs

2. Phagocytosis: literally means “cell eating”

            -Many cells in the host can carry out “endocytosis’, bringing into the cell   substances from outside in a membrane bound vesicle

                        -endocytosis involving liquids is called “pinocytosis”/cell drinking

                        -endocytosis involving large particles or other cells is called “phagocytosis”/cell eating

-only some cells in the body are called “professional phagocytes”. The function of professional phagocytes is to phagocytize (“eat”) and destroy invading pathogens (they may also play a role in triggering the specific immune responses we will discuss later)

Non-specific interior defenses: Phagocytosis, continued

Professional phagocytes include:

a. neutrophils: the “first responders” to  microbial invasion., short-lived.

b. monocytes-macrophages: slower, but longer lived; often called the “garbage trucks” of the body. Also important in helping trigger specific immune responses

c. both neutrophils and macrophages are called “leukocytes” or white blood cells or WBC’s. They do not contain hemoglobin as do erythrocytes or red blood cells/RBC’s and thus appear “white”.


There are 3 other types of leukocytes (lymphocytes, eosinophils and basophils) which we will discuss later

Steps in phagocytosis

1. Chemotaxis: chemical gradients  guide phagocytic cells to the site of microbial invasion

2. Attachment: phagocyte surface receptors must bind to surface molecules on  pathogen

3. Phagosome formation: phagocyte pseudopodia (“false feet”, extensions of cytoplasmic membrane) wrap around pathogen and fuse, forming membrane bound vesicle around pathogen. This vesicle is called the phagosome (literally “eating body”). The pathogen is now within phagocyte.

-proton pumps in the phagosome membrane pump hydrogen ions into the phagosome, acidifying the inside

-the hydrogen ions/protons will help activate the hydrolytic  enzymes in the next step….

4. Lysosome fusion with phagosome: A second membrane bound vesicle filled with hydrolytic enzymes then fuses with the phagosome, creating a “phagolysosome”. The hydrolytic enzymes are activated by the hydrogen ions.The hydrolytic enzymes start to “digest” the pathogen, breaking down the proteins, carbohydrates, lipids and nucleic acids; the pathogen is thus killed.

                        -nutrients will be absorbed by the phagocyte

5. Respiratory/oxidative burst triggered by activation of myeloperoxidase system: enzymes in the phagolysosome also generate superoxide anions, hydrogen peroxide and other toxic substances which will increase the likelihood of the pathogen’s destruction

6. Waste material is dumped via a process called “exocytosis”

Non-specific interior defenses, phagocytosis continued       

Note: many pathogens have evolved ways to avoid/”outwit” one or more of the steps involved in phagocytosis, more later. Some pathogens require phagocytosis to successfully invade the host!


3. Fever production

Substances which trigger fever production are called “pyrogens” (literally “fire-makers”)


Microbial substances can act as pyrogens (exogenous pyrogens; exogenous=comes from outside). Microbial substances can cause host cells to release substances called endogenous pyrogens (endogenous: from within) such as Interleukin-1 IL-1. IL-1 travels to the brain’s hypothalamus and triggers a re-setting of the body’s thermostat. Consequently the brain triggers a number of adaptations to increase core body temperature e.g. shivering, vasoconstriction, behavioral changes (seeking warmth, dressing in more layers, covering self with blankets)


Why will increased body temperature help the host when invade by a pathogen?

Increased body temp has shown to :

            -increase killing rate by phagocytes

            -increase chemical killing of pathogens

            -may decrease replication of some pathogens

            -decrease iron absorption from intestine (some pathogens require iron)

Antipyretics are substance which reduce fevers; examples acetaminophen, aspirin, ibuprofen


4. Complement activation1. inactive proteins synthesized by liver

                        2. 2 ways to activate complement

                                    a. alternative pathway: contact with bacterial surface e.g. LPS                                                    

                                    b. classicial pathway: antigen-antibody complexes activate (later;                                                                         specific defenses)

                                    c. both pathways lead to formation of

                                                -C3a: inflammatory mediator

                                                -C3b: opsonin

                                                -and activation of the terminal pathway

                        3. Terminal pathway: results in formation of:

                                    a. C5a: inflammatory mediator

                                    b. ”MAC”: membrane attack complex

                                                -creates pores in bacterial outer membrane/ any cell                                                                      membrane-> osmotic lysis

                        4. cascade: series of reactions in which product of one reaction activates                                      the next reaction.

5. amplification: each active product molecule activates many subsequent reactions so cascade expands/amplifies as it proceeds.

                        6. Results: inflammatory mediators, opsonins, chemotactic factors,                                                “MAC”, membrane attack complex



5. Interferons alpha and beta

Some viruses can trigger invaded host cells to produce alpha and beta interferon IFN. IFN’s trigger neighboring cells to make “AVPs”, antiviral proteins which can block replication of viruses.

Specific acquired immunity


Recall the body’s first line of defense are the non-specific defenses discussed earlier (skin, mucous membranes, inflammation, phagocytosis, complement activation and more).

-What happens if non-specific defenses are unable to halt infection by invading pathogens?


The body has a second, stronger line of defense called “specific, acquired” immunity. It is “specific” for the invading pathogen and “acquired” because it is only activated after the body is invaded by the pathogen (or after vaccination…more later).


Specific immunity has 2 branches:

1. humoral immunity= protection provided by antibodies or immunoglobulins

-humoral immunity offers protection against extracellular pathogens and toxins (toxins and pathogens found outside of host cells)

2. cell mediated immunity (“CMI”)= protection offered by T lymphocytes and activated macrophages

  • CMI offers protection against intracellular pathogens (ex viruses) (intracellular=within a cell)
  • CMI also offers protection against cancer cells, large eukaryotic pathogens such as worms and fungi
  • CMI is also responsible for transplant rejection

Specific Acquired immunity Overview; Humoral and Cell Mediated Immunity

  1. Antigens trigger specific immunity
  2. Humoral immunity is protection provided by antibodies/immunoglobulins. Antibody functions include:
    1. Neutralization: black attachment of pathogen or toxins to host cells
    2. Agglutination/clumping and precipitation
    3. Opsonization: coat pathogens to increase phagocytic killing
    4. Activation of MAC via complement
  3. Antibodies are synthesized by special B lymphocytes called plasma cells. Good antibody production requires T helper lymphocyte chemical “help”
  4. Primary immune responses are slow, low and short lived BUT memory cells are made agasint the pathogen antigens
  5. Secondary immune responses are faster, stronger and longer lasting than primary immune responses. Production of memory cells and secondary immune responses are the basis for vaccine protection against pathogens.
  6. Antibodies can protect against extracellular pathogens or toxins but cannot get inside cells. Cell Mediated Immunity  ‘CMI” is used to protect against intracellular pathogens
  7. CMI against viruses: cytotoxic T lymphocytes target and kill virus infected cells
  8. CMI against facultative intracellular parasites of phagocytic cells: special T helper lymphocytes can activate phagocytes to enable them to kill bacterial intracellular parasites.
  9. HIV destroys T helper lymphocytes, the most important cells of the immune system. Without T helpers, neither humoral nor cell mediated immunity can function properly. When T helper lymphocytes decrease sufficiently, a person becomes immunocompromised (AIDS) and often dies from low virulence “opportunistic pathogens”.

Study guide Specific acquired Immunity

  1. How is specific immunity different from non-specific immunity? Does specific immunity have any disadvantages when compared with non-specific defenses?
  2.  What is “immunological memory” and why is it important?
  3. What are antigens?
  4. What is humoral immunity?
  5. Describe 4 functions of antibodies
  6. Describe the structure of an antibody and different functional parts.
  7. Describe 5 antibody classes and functions (matching)
  8. Which cells make antibodies?
  9. Why are T helper lymphocytes important in  antibody production?
  10. Which 3 types of cells are involved in “B dependent” antibody production?
  11. What are special challenges of “B independent antigens”? How have these problems been overcome? What are conjugate vaccines? Examples?
  12. Briefly describe the function of T cytotoxic lymphocytes in virus infections
  13. Briefly describe role of T helper lymphocytes in activation of macrophages.
  14. Why may HIV infections “cripple” a person’s immune system? What is the consequence, why?