Examination 4: Principles of Disease, Pathogenicity, and Immunology Study Guide

Principles of Disease and Epidemiology

• Pathology: The scientific study of disease.

• Etiology: The study of the cause of a disease.

• Pathogenesis: The manner in which a disease develops.

• Invasion or Colonization: Termed infection, referring to the invasion of the body by pathogens.

• Disease: An abnormal state in which the body is not performing normal functions; typically occurs when an infection results in a change of health.

• Pathogen: A microorganism that causes disease. This may involve one or multiple organisms.

Koch’s Postulates and Exceptions

Koch’s postulates are used to prove the etiology of an infectious disease:

• Postulate 1: The same pathogen must be present in every case of the disease.

• Postulate 2: The pathogen must be isolated from the diseased host and grown in pure culture.

• Postulate 3: The pathogen from the pure culture must cause the disease when it is inoculated into a healthy, susceptible laboratory animal.

• Postulate 4: The pathogen must be isolated from the inoculated animal and shown to be the original organism.

Exceptions to Koch’s Postulates:

• Growth Requirements: Some microbes cannot grow on synthetic media (e.g., Treponema pallidum, the cause of syphilis).

• Isolation Issues: Some microbes cannot be easily isolated from the host. Tissues may need to be transplanted (e.g., legionellosis) or incubated in alternative hosts like embryonated eggs.

• Multiple Diseases: Certain pathogens can cause several different diseases.

Classifying and Measuring Infectious Diseases

• Symptoms: Changes in body function felt by a patient (subjective).

• Signs: Objective changes in the body that can be measured or observed.

• Syndrome: A specific group of signs and symptoms that consistently accompany a disease.

• Communicable Disease: An infectious agent transmitted directly or indirectly from one person to another.

• Contagious Disease: A disease that is very easily communicated (e.g., measles).

• Noncommunicable Disease: A disease that is not spread from host to host.

Occurrence of Disease

• Incidence: The number of people in a population who develop a disease during a particular time period.

• Prevalence: The number of people in a population who develop the disease at a specific time, regardless of when it started (includes old and new cases).

• Sporadic Disease: Occurs only occasionally.

• Endemic Disease: Constantly present in a population.

• Epidemic Disease: Acquired by many people in a relatively short period.

• Pandemic Disease: An epidemic occurs worldwide (e.g., influenza).

Severity and Duration

• Acute Disease: Develops rapidly but lasts a short time.

• Chronic Disease: Develops slowly and continues for long periods.

• Subacute Disease: Intermediate between acute and chronic.

• Latent Disease: The causative agent remains inactive for a time but then becomes active to produce symptoms.

• Herd Immunity: A state where many people in a population possess immunity, protecting the few who do not.

Disease Transmission and Reservoirs

Diseases require a Reservoir of Infection (a continual source of pathogens).

• Human Reservoirs: The human body serves as a reservoir. Carriers are people harboring pathogens (e.g., HIV, hepatitis) without showing signs of illness.

• Animal Reservoirs: Zoonoses are diseases occurring primarily in wild and domestic animals that transmit to humans (e.g., rabies, Lyme disease).

• Nonliving Reservoirs: Soil and water.

Selected Zoonoses Examples

• Viral:     

• * Influenza: Reservoirs include swine and birds

• * Rabies: Reservoirs include bats, skunks, foxes, dogs, and raccoons.     

• * West Nile Encephalitis: Reservoirs include horses and birds; transmitted by Aedes and Culex mosquitoes.

• Bacterial:     

• * Anthrax (Bacillus anthracis): Reservoirs in domestic livestock.     

• * Plague (Yersinia pestis): Reservoirs in rodents; transmitted by flea bites.     

• * Lyme Disease (Borrelia burgdorferi): Reservoirs in field mice; transmitted by tick bites.     

• * Salmonellosis (Salmonella enterica): Reservoirs in poultry and reptiles.

• Fungal/Protozoan/Helminthic:     

• * Ringworm: Domestic mammals.     

• * Malaria (Plasmodium spp.): Monkeys; transmitted by Anopheles mosquitoes.     

• * Trichinellosis (Trichinella spiralis): Pigs, bears; transmitted via undercooked meat.

Methods of Transmission

• Contact Transmission:     

• * Direct: Close association between infected and susceptible host.     

• * Congenital: Mother to fetus/newborn at birth.

• * Indirect: Spread via a fomite (nonliving object like a syringe or cup).     

• * Droplet: Airborne droplets traveling less than $1\,\text{meter}$.

• Vehicle Transmission: Transmission by an inanimate reservoir such as water, food, or air.

• Vectors: Animals (mostly arthropods like fleas, ticks, mosquitoes) that carry pathogens.     

• * Mechanical: Passive transport on the insect's feet.     

• * Biological: Pathogen reproduces in the vector (active process, e.g., bites).

Healthcare-Associated Infections (HAIs)

• Also known as Nosocomial Infections.

• Affect $1$ in $25$ hospital patients.

• Result in approximately $2\,\text{million}$ infections and over $70,000$ deaths per year.

• Factors: Microbes in the hospital, compromised hosts (weakened by AIDS, burns, or trauma), and the chain of transmission (fomites, person-to-person).

Epidemiology and CDC

Epidemiology is the science studying when and where diseases occur and how they are transmitted.

• Descriptive Epidemiology: Collecting data describing occurrence (Snow).     

• * Retrospective: Backtracking to source.     

• * Prospective: Following disease-free populations.

• Analytical Epidemiology: Analysis of a disease to find probable cause (Nightingale).

• Experimental Epidemiology: Hypothesis testing using control and test groups (Semmelweis).

• CDC (Center for Disease Control and Prevention): Collects and analyzes information on notifiable infectious diseases.     

• * Morbidity: Incidence of specific notifiable diseases.     

• * Mortality: Number of deaths from notifiable diseases.     

• * MMWR: Morbidity and Mortality Weekly Report.

Microbial Mechanisms of Pathogenicity

Virulence Factors: Extracellular Enzymes (Exoenzymes)

Pathogens secrete enzymes to dissolve structural chemicals and invade tissues:

• Hyaluronidase: Hydrolyzes hyaluronic acid in connective tissues.

• Collagenase: Breaks down collagen in muscles/connective tissue (e.g., Clostridium).

• Coagulase: Coagulates fibrinogen in the blood to form a protective clot around the microbe.

• Kinases: (e.g., Fibrinolysin) Break down fibrin clots, allowing the bacteria to spread.

• IgA Proteases: Digest antibodies intended to destroy the bacterium.

Evading Host Defenses

• Capsules: Protect against phagocytosis (e.g., Streptococcus pneumoniae).

• Cell Wall Components:     

• * M protein: Found in Streptococcus pyogenes; mediates adhesion and resists phagocytosis.     

• * Opa: Protein in Neisseria gonorrhoeae used for attachment.     

• * Mycolic Acid: Found in Mycobacteria; resists digestion by phagocytes.

• Antigenic Variation: Pathogens alter surface markers (antigens) so antibodies no longer recognize them (Influenza virus, Trypanosoma brucei).

Damaging Host Cells

1. Using Nutrients: Pathogens produce siderophores (iron-transporting proteins) to scavenge iron from the host.

2. Direct Damage: Pathogens metabolize and multiply within cells, causing them to rupture. Some bacteria like E. coli or Salmonella induce host cells to take them in via phagocytosis and then exit via reverse phagocytosis.

3. Production of Toxins: Poisonous substances.     * Toxigenicity: Capacity to produce toxins.     * Toxemia: Toxins in the blood.

4. * Intoxications: Disease caused by toxin presence, not necessarily microbial growth.

Exotoxins vs. Endotoxins

Exotoxins

• Mostly from Gram-positive bacteria.

• Proteins (enzymes) secreted during log phase or following lysis.

• Extremely lethal ($1\,\text{mg}$ of botulinum toxin can kill $1\,\text{million}$ guinea pigs).

• Antitoxins: Antibodies against specific exotoxins.

• Toxoids: Inactivated exotoxins used for vaccines (Tetanus, Diphtheria).

Types of Exotoxins:

1. A-B Toxins: Consist of an Active (A) enzyme component and a Binding (B) component. The toxin enters via receptor-mediated endocytosis; A inhibits protein synthesis.

2. Membrane-disrupting Toxins: Cause lysis by forming channels (e.g., S. aureus) or disrupting phospholipids (e.g., C. perfringens).     

3. * Leukocidins: Kill white blood cells.     

4. * Hemolysins/Streptolysins: Kill red blood cells.

5. Superantigens: Intense immune response; provoke T cells to release massive cytokines, causing fever, nausea, and shock (e.g., Toxic Shock Syndrome).

6. Genotoxins: Damage DNA, leading to mutations and cancer (e.g., Helicobacter).

Endotoxins

• Located within the cell wall (Lipid A of LPS) of Gram-negative bacteria.

• Released when the bacteria die and cell walls lyse.

• Effect: Fever (Pyrogenic response). Macrophages release IL-1 and TNF-alpha, which travel to the hypothalamus. Hypothalamus produces prostaglandins to reset the "thermostat" higher.

• Septic Shock: Phagocytosis of Gram-negatives causes macrophages to release TNF, which increases blood vessel permeability, leading to a life-threatening drop in blood pressure.

• DIC: Disseminated intravascular coagulation; obstruction of capillaries by blood-clotting enzymes.

• LAL Test: Limulus amebocyte lysate test uses horseshoe crab blood to detect endotoxin (clotting/gel precipitate indicates a positive test).

Innate Immune Response (Non-specific)

First Line of Defense: Surface Barriers

• Physical Factors:     

• * Skin: Epidermis provides a barrier; shedding removes microbes. High acidity and sebum inhibit growth.     

• * Mucous Membranes: Trap microbes (mucus); Ciliary escalator moves them out of the respiratory tract.     

• * Lacrimal Apparatus: Tears wash microbes away; contain lysozyme.     

• * Saliva/Urine/Vaginal Secretions: Dilute and wash; contain lysozyme; acidity discourages growth.

• Chemical Factors: Sebum (acidic film), gastric juice ($\text{high acidity}$), and lysozyme in perspiration.

Second Line of Defense: Internal Defenses

1. Cells:

• Neutrophils: Phagocytic; dominate early infection. Use NETosis (Neutrophil Extracellular Traps)—fibers of DNA/enzymes—to trap/kill microbes.

• Macrophages: Derived from monocytes. Most voracious phagocytes.     

• * Fixed: Resident in certain organs (e.g., Kupffer cells in liver).     

• * Free-wandering: Migrate through tissues.

• Natural Killer (NK) Cells: Granular lymphocytes. Recognize "missing-self" (lack of MHC Class I). Induce cytolysis via perforins (channels) and granzymes (protein-digesting enzymes).

2. Phagocytosis Process:

1. Chemotaxis: Movement toward chemical signals (cytokines, complement peptides).

2. Adherence: Attachment via Toll-like receptors (TLRs) binding to Pathogen-associated molecular patterns (PAMPs). Enhanced by opsonization (coating with C3b or antibodies).

3. Ingestion: Formation of a phagosome.

4. Digestion: Phagosome fuses with a lysosome to form a phagolysosome.

3. Inflammation:

• Cardinal signs: Redness, swelling (edema), pain, heat, impaired function.

• Vasodilation: Triggered by Histamine (mast cells), Kinins (attract neutrophils), Prostaglandins (intensify effects), and Leukotrienes (increase permeability).

• Process: Margination (phagocytes stick to endothelium) followed by Diapedesis (squeezing between endothelial cells).

4. Fever:

• Systemic response triggered by pyrogens (e.g., IL-1) acting on the hypothalamus.

• Complications: Acidosis, dehydration, tachycardia, seizures, and death at $44^\circ\text{C}$ ($112\text{--}114^\circ\text{F}$).

5. Complement System: Group of over $30$ plasma proteins (C1-C9). Activated by three pathways:

• Classical: Antigen-antibody complex.

• Alternative: Spontaneous action on microbe surface.

• Lectin: Lectin binds to mannose on microbe.

Outcomes of Complement:

• Cytolysis: Formation of the Membrane Attack Complex (MAC) ($C5b + C6 + C7 + C8 + C9$) creating pores and causing bursting.

• Opsonization: $C3b$ coats microbes.

• Inflammation: $C3a$ and $C5a$ trigger histamine release from mast cells.

6. Interferons (IFNs):

• IFN-alpha/beta: Antiviral; induce neighboring cells to produce Antiviral Proteins (AVPs) to block viral replication.

• IFN-gamma: Secreted by lymphocytes; activates macrophages.

7. Antimicrobial Peptides (AMPs): Short peptides ($12\text{--}50$ amino acids) that inhibit cell wall synthesis, form pores, or destroy DNA/RNA (e.g., dermicidin, defensins).

Adaptive Immunity (Specific)

Characteristics

• Specific: Targets specific foreign substances.

• Systemic: Not restricted to the infection site.

• Memory: Stronger secondary response.

Humoral Immunity (B Cell Mediated)

• Fights invaders outside cells (bacteria, toxins, viruses in fluids).

• B cells: Mature in red bone marrow.

• Antigens: Substances provoking immune response. Recognizable parts are epitopes.

• Haptens: Incomplete antigens; require a carrier protein to become immunogenic.

• Antibodies (Immunoglobulins, Ig):     

• * Structure: 4 polypeptide chains (2 heavy, 2 light) in a Y-shape; variable (V) regions bind epitopes; constant (C) regions (Fc stem) determine class.     

• * IgG: $80\%$ of serum antibodies; crosses placenta; enhances phagocytosis.     

• * IgM: Pentamer; first produced in response to infection; effective at agglutination.     

• * IgA: Dimer in secretions (tears, mucus, milk); localized protection.     

• * IgD: B cell surface; initiates immune response.     

• * IgE: Allergic reactions and lysis of parasitic worms.

Antibody Action Mechanisms:

• Agglutination: Clumping particles to reduce infectious units.

• Opsonization: Coating for phagocytosis.

• Neutralization: Blocking attachment of toxins/viruses.

• ADCC (Antibody-Dependent Cell-Mediated Cytotoxicity): For large parasites (e.g., flukes); eosinophils/NK cells release lytic enzymes/perforin externally.

• Activation of Complement.

Cellular Immunity (T Cell Mediated)

• Fights intracellular pathogens (viruses, cancer).

• T cells: Mature in the thymus.

• T-Cell Receptors (TCR): Bind antigens only when presented on MHC proteins by Antigen-Presenting Cells (APCs) (Dendritic cells, Macrophages, B cells).

• CD4+ Cells: Become T Helper (TH) or Regulatory T cells. Bind MHC Class II.     

• * TH1: Activate macrophages and CTLs.     

• * TH2: Produce IL-4; activate B cells for IgE; eosinophils.     

• * TH17: Recruit neutrophils/inflammation.     

• * Regulatory T (Treg): Suppress self-reactive lymphocytes to prevent autoimmunity.

• CD8+ Cells: Become Cytotoxic T Lymphocytes (CTL). Bind MHC Class I.     

• * Destroy target cells via perforins and granzymes; induce apoptosis (programmed cell death) often via the CD95 (Fas) signaling pathway.

Types of Acquired Immunity

• Naturally Acquired Active: Infection.

• Naturally Acquired Passive: Antibodies from mother (placenta/milk).

• Artificially Acquired Active: Vaccination.

• Artificially Acquired Passive: Injection of preformed antibodies (gamma-globulin).

Practical Applications of Immunity

Vaccines

• Live Attenuated: Weakened microbes; lifelong immunity (humoral and cellular).

• Inactivated (Killed): Dead microbes; safer; requires boosters.

• Subunit: Only antigenic fragments.     

• * Recombinant: Produced by genetic engineering (e.g., Hepatitis B).     

• * VLP: Resemble viruses but lack DNA/RNA (e.g., HPV).     

• * Toxoids: Inactivated toxins (Tetanus).

• Conjugated: Polysaccharides fused to protein (e.g., Hib) to help children's immune systems respond.

• Nucleic Acid: DNA (plasmids) or RNA (SARS-CoV2) delivered via liposomes; cells translate the antigen.

• Herd Immunity: Requires $85\text{--}90\%$ population coverage.

Diagnostic Immunology

• Sensitivity: Positive test for true positives.

• Specificity: Negative test for true negatives.

• Monoclonal Antibodies (Mabs): Derived from a single hybridoma clone (Hybridoma = B cell + Myeloma). Used in pregnancy tests (HCG detection).

• Precipitation Reactions: Soluble antigens + antibodies form a lattice (cloudy ring).

• Agglutination Reactions: Particulate antigens; Titer is the most dilute concentration showing a reaction.

• ELISA (Enzyme-linked immunosorbent assay):     * Direct: Detects antigens (Antibody adsorbed to well).     

• * Indirect: Detects antibodies (Antigen adsorbed to well).

Hypersensitivity

• Type I (Anaphylactic): <30\,\text{min}. IgE binds to mast cells/basophils. Histamine release leads to shock (systemic) or hay fever/asthma (local). Treated with epinephrine.

• Type II (Cytotoxic): $5\text{--}12\,\text{hours}$. IgM/IgG + Complement. ABO transfusion reactions and Hemolytic Disease of the Newborn (Rh- mother vs Rh+ fetus). Drug-induced examples: Thrombocytopenic purpura (platelets).

• Type III (Immune Complex): $3\text{--}8\,\text{hours}$. Antigen-antibody complexes deposit in vessel walls/basement membranes. Arthus reaction, serum sickness.

• Type IV (Delayed Cell-Mediated): $24\text{--}48\,\text{hours}$. CTLs and macrophages. TB skin test and allergic contact dermatitis (poison ivy, nickel).

Hygiene Hypothesis: Limiting pathogen exposure may lower immune tolerance, increasing allergies/asthma. Treatments for inflammatory diseases include whipworm eggs or fecal transplants.