z-MicroBio Final – Ch. 29, 30

Virulence of Microorganisms -vs- Resistance of Host
Acquired or Induced Immunity (Resistance)
Humoral Immunity: mediated by antibodies
Cellular Immunity: mediated by cells (T cells)
Natural Resistance: (Innate immunity)
Cells: macrophages, etc.
Mechanical barriers: skin & mucous membranes
Chemical factors: intereferons, fatty acids on skin
Microbial factors
Innate Immunity (Natural Resistance) Non-Specific
Mechanical Barriers, Chemical factors, Microbial factors, Cellular factors
Mechanical Barriers
Skin: very few microorganisms penetrate
Mucous membranes: mucous traps many airborne bacteria; coughing/sneezing expel a portion of the microbe-laden mucous into the atmosphere, the rest are carried into the stomach were high acidity and digestive enzymes kill many microbes
Found in nasal-pharyngeal region and the upper respiratory tract and intestinal epithelium[areas open to outside]
Much more susceptible to penetration than the unbroken skin
Chemical or physical damage of mucous membranes increases the chance of infection and disease
Tears: flushing of eyes (contains lysozymes)
Respiratory tract: ciliated epithelium lines walls; cilia, beating upward, push bacteria, mucous and other particulate material upward to where it is expelled in saliva & nasal secretions
Chemical factors
Fatty acids: attack gram-negative on skin surface
Bile salts in gall bladder, liver, intestines inhibit gram-positive
Lysozyme in tears and saliva; more effective against gram-positive
Phagocytins in leucocytes (wbc)
Complement: microbial cell membranes and some cancer cells can lose physical integrity and lyse in presence of complement.
Interferon: active against viruses
Microbial factors
Normal flora of the body provides protection by competition with pathogens; ex: gram + making fatty acids on the skin
If damaged by antibiotics, the individual may get a fungal infection
Cellular factors
Phagocytic cells may engulf microorganisms and destroy it.
On rare occasions, pathogens break through the physical and chemical defenses. One of the first cells that they encounter are class of cells called phagocytes (literally: cell that eats)
The primary function of phagocytes is to engulf and destroy pathogens.
In some cases, phagocytes can act as antigen-presenting cells (APCs) and generate a peptide antigen (s) that will activate specific immune responses
3 Main: Neutrophils (polymorphonuclear leukocytes), monocytes, macrophages
Neutrophils: actively motile granulocytes containing large #s of lysosomes
Major Immune Cell Types
a. Nucleated cell in lower left is neutrophil
segmented nucleus
granular cytoplasm
a. Monocyte: slightly above and to right

B. Circulating lymphocyte: no visible cytoplasm.

Phagocytes employ a variety of chemical processes designed to destroy the pathogen: (hydrogen peroxide, lysozyme, proteases, phosphatases, nucleases, lipases)
Not to be outdone, some pathogens have mechanisms to kill or avoid phagocytes
Ex: Staphylococcus aureus: Carotenoids which quench singlet oxygen and prevent killing[also Staph. Has coagulase that walls it off and prevents phagocytosis.
Macobacterium tuberculosis: grow and persist in the macrophage

Induced Immunity
Requires prior contact or exposure to antigen
Humoral Immunity
Cellular Immunity
Humoral Immunity
mediated by antibodies produced by B-lymphocytes that differentiate into plasma cells that produce antibody.

B cells (lymphocytes) -> plasma cells -> Ab

Cellular Immunity
mediated by cells; Ex. Cytotoxic T-cells

T Cells (lymphocytes) -> Activated T-Lymphocytes

CD4: T-helper (TH1 and TH2): helps or induces an immune response; Associated with MHC Class II
CD8: T-cytotoxic cells; associated with MHC class I
Characteristics of specific immunity
Memory: Capacity to respond more quickly and vigorously after exposure to an antigen
Tolerance: acquired inability to make an immune response to certain antigens


Innate immunity
results from interactions between pathogen-associated molecular patterns (PAMPS) found as cell surface components and pattern recognition molecules (PRMs) found on phagocytes.
PRMs first observed in Drosophila, called Toll receptors
Toll-like receptors on human phagocytes recognize specific PAMPS (Ex. TLR-4, a PRM on human phagocytes recognizes & responds to interactions with LPS, a PAMP in outer membrane of gram negative.
Interaction of a PAMP with the phagocyte PRM triggers a trans-membrane signal that results in transcription of a number of cell proteins in phagocyte. This transcriptional activation leads to production of toxic oxygen compounds that lead to pathogen death. See diagram: H2O2; O2-; HOCl; NO; OH.; 1O2
Adaptive Immunity
Adaptive immunity: due to antigen specific T cells, resulting in two distinct effector pathways
II. Antibody mediated immunity: results from soluble antigen-specific antibody proteins, products of antigen stimulated B lymphocytes.
III. Cell-mediated immunity: mediated by antigen specific T cells
Antibody mediated (humoral immunity) results from soluble antigen-specific antibody proteins, products of antigen-stimulated B lymphocytes
Cell mediated (cellular immunity) mediated by antigen specific T cells.
Origins of Immune Response
Lymphoid precursor can generate T & B cells
Myeloid precursor can generate
Monocytes which in turn develop into macrophages or dendritic cells that are phagocyte cells involved in antigen uptake and display.
Granulocytes: including neutrophils, mast cells, esinophils, and basophils. Neutrophils are also phagocytes. The others release their granule contents in reponse to pathogens, pathogen products, or damaged host cells.
Acquired Immunity
Acquired immunity (a state of altered responsiveness to a specific substance induced by prior contact with the “foreign” substance)
Anamnestic: has memory; repeated contact with antigen amplifies response
Usually protein or complex polysaccharides (molecular complexity)
Nucleic acids, simple polysacchrides, & lipids are poor immunogens, because composed of repeating monomers.
High molecular weight: >10,000
Appropriate physical form: Large, complex macromolecules in insoluble or aggregate form (example proteins precipitated by heating) are good immunogens. Insoluble readily taken up by phagocytes, leading to immune response.
Foreign to responder
Note: Epitopes recognized by antibody
Antibody: Plasma synthesized in humoral response which are capable of combining with the provoking antigen
5 classes
Fab & FC portions
substance capable of eliciting an immune response. An antigen may or may not be an immunogen
Antigens are substances that react with antibodies or TCRs (T cell receptors)
Low molecular weight substance which can not act as an immunogen by itself.
When attached to high molecular weight materials, an antibody response is made to the carrier (the high molecular weight material), as well as to the hapten.
Next time hapten enters it acts as antigen and can react with those antibodies made to its epitope (s).
Antigen Binding by Antibodies and T-Cells
Antibody or TCR does not interact with the antigenic macromolecule as a whole, but with distinct portions of the molecule called the epitope.
Ab specificity is sensitive enough to distinguish between two very similar epitopes; Ex. Glucose from Galactose which only differ in the orientation of a -OH group
Antigen Binding by T-Cells
While antibodies generally recognize epitopes expressed on macromolecular surfaces, T-Cell receptors (TCR’s) recognize determinates only after the immunogens have been partially degraded.
The degraded or “processed” antigen is presented to T cells on the surface of specialized anti-presenting cells (APCs) or target cells.
Antigen Presentation to T-Cell
major histocompatibility complex
Function as antigen presenting molecules and interact with both antigen and TCR
T cell cannot recognize foreign antigen unless it is presented in context of an MHC protein.
Two Classes
Class I: found on surface of all nucleated cells; proteins made and assembled in ER. Protein antigens manufactured within the cell [ex from viruses or tumors] are degraded in proteosome in the cytoplasm, transported across ER through a pore formed by TAP proteins, bind to Class I, transported to cell surface to bind with TCR [CD8 co-receptor makes binding stronger]; then, Tc cell releases cytokines and cytotoxins to kill target cell.
Class II: found only on surface of B lymphocytes,macrophages and dendritic cells, all dedicated APCs
MHC II proteins;produced in ER and assembled with a blocking protein (Ii, invariant chain); goes to phagolysozome; Ii and foreign proteins [imported from outside the cell by endocytosis] are digested. Class II then binds to digested proteins, complex tranported out of cell to bind to TCR & CD4 coreceptor on TH cell,. TH release cytokines that act on other cells to activate immune response.
T Cells
T-Cytotoxic Cells (Tc): destroys cells that display antigens embedded in MHC class-I molecules
Uses perforin to lyse the infected cells
Uses granzymes (proteins) that cause apoptosis
THelper cells(TH1 and TH2)
Macrophages: central role as APCs: bind, process, and present AG to TH cells.
T-inflammatory (TH1)cells, are activated by antigens presented on macrophages in the context of MHC Class II protein.
TH1 cells then produce cytokines that stimulate macrophages to take up and kill certain foreign cells by themselves
Activated macrophages can kill intracellular bacteria that would normally multiply
TH2 cells: key role in B cell activation & AB production.
Antibodies (Immunoglobulins
Antibodies, or immunoglobulins (Ig) are protein molecules that able to combine with antigenic determinates.
Found in the serum and in other body fluids such as gastric secretions and milk
Serum containing antigen-specific antibody is often called anti-serum
Comprised of 5 major classes on the basis of their physical, chemical, and immunological properties; IgG, IgA, IgM, IgD and IgE
About 80% of the serum Igs are IgG proteins
All immunoglobulin classes have VH (variable heavy) and VL (variable light) domains that bind antigens.
All immunoglobulin classes have Constant domains. It is the difference in the amino acid sequence of the constant domain that defines the class of immunoglobulin molecule; gamma (), alpha (), mu (), delta () or epilison ()
Most common of the circulating antibodies
Composed of 4 polypeptide chains.
Interchain disulfide bridges (S-S) connect the individual chains
A functional IgG molecule consists of two antigen binding sites. IgG is therefore bivalent and can bind two identical epitopes
Crosses the placental barrier
Usually found as an aggregate of five immunoglobulin molecules attached by at least one “J” chain
Every heavy chain of IgM contains a fourth constant domain (CH4)
1st class of immunoglobulin made in a typical immune response to a bacterial infection
10 binding sites
Low affinity but high avidity
Present in the serum in the monomeric form, but in secretions it is a dimer.
colostrum, gastrointestinal secretions, and mucus secretions of respiratory and genitourinary tracts
Found in serum in very small amounts.
Mediates Immediate-type hypersensitivities (allergies)
Like IgM, IgE has a fourth constant domain
Constant region functions to bind IgE to MAST cells surfaces (more on MAST cells later)
Present in serum in low concentrations
Has no know function
Abundant on surfaces of B cells and plays a role along with monomeric IgM in binding antigen to B cells
Introduction of antigen
Antigens are spread via lymphatic and blood circulatory systems to neighboring secondary lymphoid organs (lymph nodes, spleen, or mucosal-associated lympoid tissue (MALT).
Intervenously injected antigen travels via blood to spleen, where antibodies are formed.
Subcutaneously, intradermally, topically, or intraperitoneally introduced antigens are carried by lymphatic system to the nearest lymph node.
Antigens introduced to mucosal surfaces (ex. Mouth) are delivered to the GALT lining the intestinal tract resulting in antigen-specific IgA antibody production in the gut.
Primary Response
Following initial antigen introduction, each antigen-stimulated B cell multiplies and differentiates to form both antibody-secreting plasma cells, and memory cells
Plasma cells are short-lived (< 1 week) but produce large amounts of IgM antibody in this primary antibody response.
Following a latent period, specific antibodies show up in blood, followed by a gradual increase in antibody titer (quantity), and then a slow fall in the primary response.
Secondary Response
Memory cells generated by the initial exposure to antigen may live for several years.
Upon re-exposure to the immunizing agent memory cells need no T cell activation; they quickly transform to plasma cells and begin producing IgG
The antibody titer rises rapidly to a level 10-100 greater than the titer following the initial exposure
The rise in antibody titer is referred to as the secondary antibody response
The secondary response is characterized by a switch from IgM to IgG production (class switching)
Over time titer slowly decreases but a later exposure can cause another secondary response. This forms the basis for booster shots
Summary of Antibody Production
Two Divisions of Immune Response
B lymphocytes differentiate into plasma cells and produce antibody. Mediated by B lymphocytes (mature in bone marrow)
Ex. Chickens – Remove Bursa & they fail to produce antibodies and become susceptible to bacterial or extrinsic invaders
Ex. Humans – Burton’s Agammaglobulinaemia- Children fail to produce antibody. Humans do not have a bursa, but instead have GALT (gut associated lymphoid tissue) and MALT (mucosal cells that line external surfaces)
In both of the above examples, cellular immunity remains intact and they are more susceptible to bacterial infections rather than viral or intrinsic.
Mediated by cells; involve lymphocyte processing by thymus and involve activated T cells.
Where Ab mediated (humoral) recognizes substances that are outside host cells (extrinsic), cell-mediated immune response is effective in recognizing modified host cells; thus it is important in controlling infections in which pathogens can reproduce within living cells (ex. Viruses, some bacteria such as rickettsia & chlamydia, some parasitic protozoans like typanosomes.
Also important in surveillance/destruction of malignant
Cell mediated immune responses also include:
Delayed hypersensitivity in response to intracellular bacteria like Mycobacterium tuberculosis and fungal infections such as Histoplasma capsulatum.
Cytotoxic T lymphocyte responses to virally infected cells
Response to tumor cells and tissue grafts by natural killer cells
Cellular – experiments and nature
Thymectimized mice-
Susceptible to viral infections or intrinsic parasites like M. tuberculosis
Increased rate of tumor production
Failure to reject transplanted tissue
Di George’s Syndrome (humans)
Infants lacking a functioning thymus gland (equivalent to thymectimized mice above) a, b, & c above apply
25,000 times greater chance of malignancy but not susceptible to bacterial “extrinsic” infection
drugs or if chemicals are present (pollutants, drugs, radiation, UV, cigarette smoking, insecticides, etc), or in old age
Attenuated strains have lost virulence
Often they retain immunogenicity; therefore, may be used for production of vaccines; especially viral vaccines: measles, mumps, e.t.c
Laboratory cultivation typically results in a decrease in virulence of pathogens or even a complete loss
Nosocomial Infections
Many hospital patients with noninfectious diseases (ex. cancer and heart disease) acquire microbial infections that produce disease because they are compromised.
Such health-care associated infections are called nosocomial infections.
Procedures like catheterization, hypodermic injection, spinal puncture, biopsy and surgury unintentionally introduce microbes into patient.
Control of Microorganisms
Physical Agents
Most widely used is heat
Chemical Agents
Called Antimicrobial Agents
Kill or Inhibit Growth
Sterilization: treatment that frees the treated object of all living organisms, including viruses.
Death: defined as irreversible loss of ability to reproduce when inoculated into an appropriate medium or host.
An autoclave permits application of steam heat under pressure at temperatures above the boiling point of water, killing endospores.
At 15 lbs per sq inch above normal atmospheric pressure water boils at 121oC. At that temperature for 15 minutes, endospores are killed if volume of liquid not too large. Ex. it takes 30 min for 2000mL flask.
Pasteurization does not sterilize liquids but reduces microbial load, killing most pathogens and inhibiting the growth of spoilage microorganisms.
Originally 30 min at 62o C
Now: Flash Pasteurization: 72o C, 15 sec.
Ultraviolet Light
Most effective at 260 nm
Ionizing Radiation
Higher energy and shorter wavelength
Membrane Filter Sterilization
Antimicrobial Agents

Antimicrobial agent is a natural or synthetic chemical that kills or inhibits growth of microorganisms.

Kills: cidal (viricidal, fugicidal, bactericidal)

lysis: bacteriolytic

Inhibits growth: static (bacteristatic)

MIC Two classes: Invitro and Invivo In vitro: Disinfectants: chemicals that kill microorganisms but not necessarily spores Sanitizers: reduce to safe levels Antiseptics or germicides: kill or inhibit growth but nontoxic enough to be applied to human tissue.

Agar Zone Diffusion
Zones of inhibition
Tube Dilution Test
MIC: minimum inhibitory concentration: in lab
Bactericidal Tests
transfer to medium without agent and test for growth
Phenol Coefficient Test
Greatest dilution that kills microorganism at 10 minutes but not 5 to greatest dilution of phenol that kills microorganism in 10 but not 5 minutes.
chemical substance produced by one microorganism that kills or inhibits growth of another. (Ex Penicillin G)
Effective against gram +: gram – impermeable
Synthetic Antimicrobial Agents
other penicillins: modify R group.
Ideal Properties
Selective Toxicity
Toxic to Microorganisms but not harmful to man and higher animals.
Diffuses to site of infection
Very few toxic side effects
Allergic reactions not common
Antifungal Drugs
Antifungal agents (Table 20.6) fall into a wide variety of chemical categories. Because fungi are Eukarya, selective toxicity is hard to achieve, but some effective chemotherapeutic agents are available.

Leave a Reply

Your email address will not be published. Required fields are marked *