Immunology and Epidemiology: Study Guide for Microbiology Exam #3

Innate Immune Responses

Overview of Innate Immunity

Innate immune responses are the body's first line of defense against pathogens, providing rapid, non-specific protection. These mechanisms are present from birth and do not require prior exposure to a pathogen.

• Mechanical Defenses: Physical barriers such as skin and mucous membranes prevent pathogen entry.

• Physical Defenses: Actions like coughing, sneezing, and the movement of cilia help expel pathogens.

• Chemical Defenses: Substances like stomach acid, lysozyme in saliva and tears, and antimicrobial peptides destroy pathogens.

• Biochemical Defenses: The microbiome competes with pathogens for resources, producing substances that inhibit pathogen growth.

Innate Immune Defenses Associated with Specific Tissues:

• Skin: Keratinized cells, sebum, and sweat

• Respiratory tract: Mucus, cilia, and coughing

• Gastrointestinal tract: Acidic pH, digestive enzymes

Key Innate Immune Components

• Microbiome: Normal flora outcompete pathogens and produce antimicrobial compounds.

• Complement System: A group of proteins that enhance phagocytosis, lyse pathogens, and promote inflammation.

• Toll-like Receptors (TLRs): Recognize pathogen-associated molecular patterns (PAMPs) and activate immune responses.

• Interferons: Proteins released by cells in response to viral infection, inhibiting viral replication and activating immune cells.

Phagocytosis

Phagocytosis is the process by which certain cells engulf and digest pathogens.

• Phagocytic Cells: Neutrophils, macrophages, dendritic cells

• Steps of Phagocytosis: Chemotaxis, adherence, ingestion, digestion, exocytosis

Complement Pathway

The complement pathway is a cascade of protein activation leading to pathogen destruction.

• Main Functions: Opsonization, inflammation, cell lysis

• Outcomes: Enhanced phagocytosis, recruitment of immune cells, direct killing of pathogens

Routes of Infection and Epidemiology

Transmission Routes

Pathogens can be transmitted via various routes, influencing disease spread and epidemiology.

• Respiratory Route: COVID-19, Influenza, Whooping cough, Diphtheria, Pneumonia

• Oral-Fecal Route: Listeriosis, Salmonella, Botulism, Cholera

• Fomite Transmission: Via contaminated objects

• Airborne Transmission: Via aerosols

• Vector Transmission: Via insects (e.g., mosquitoes)

• Parenteral Transmission: Via breaks in skin (e.g., injections, bites)

Epidemiological Terms

• Endemic: Disease constantly present in a population

• Epidemic: Sudden increase in disease cases

• Pandemic: Epidemic that spreads across countries or continents

• Zoonosis: Disease transmitted from animals to humans

Virulence Factors

• Virulence: Degree of pathogenicity

• Pathogenicity: Ability to cause disease

• Fitness: Ability of a pathogen to survive and reproduce

• Examples: Toxins, adhesion factors, immune evasion mechanisms

Adaptive Immune Responses

Overview of Adaptive Immunity

Adaptive immunity is specific and develops after exposure to antigens. It involves two main types of responses: humoral (antibody-mediated) and cell-mediated.

• Humoral (Antibody-Mediated) Response: B-cells produce antibodies that neutralize pathogens.

• Cell-Mediated Response: T-cells recognize and destroy infected cells.

Immune Cell Types and Functions

• B-cells: Mature in bone marrow; produce antibodies

• T-cells: Mature in thymus; include helper (Th), cytotoxic (Tc), and regulatory (Treg) cells

• Antigen Presenting Cells (APCs): Dendritic cells, macrophages, B-cells

• Phagocytes: Neutrophils, macrophages, dendritic cells

Antibody Classes and Functions

Antibodies (immunoglobulins) are produced by plasma cells and have distinct classes with specialized functions.

Class	Main Function	Location
IgG	Long-term immunity, opsonization	Blood, tissues
IgM	First response, agglutination	Blood
IgA	Mucosal immunity	Mucous membranes, secretions
IgE	Allergic responses, defense against parasites	Blood, tissues
IgD	B-cell receptor	B-cell surface

Primary vs Secondary Antibody Responses

• Primary Response: First exposure, slower, mainly IgM

• Secondary Response: Subsequent exposure, faster, mainly IgG

• Duration and Strength: Secondary response is stronger and longer-lasting

Antigen Density and Antibody Response

• High antigen density can enhance antibody production

• Low antigen density may result in weaker responses

VDJ Recombination

VDJ recombination is a genetic mechanism in B and T cells that creates diverse antigen receptors, allowing the immune system to recognize a vast array of pathogens.

Immune Cell Types and Antigen Presentation

MHC Molecules and Antigen Presentation

• MHC-I: Presents endogenous antigens to cytotoxic T cells (Tc)

• MHC-II: Presents exogenous antigens to helper T cells (Th)

Exogenous vs Endogenous Pathogens

• Exogenous: Extracellular pathogens, presented via MHC-II

• Endogenous: Intracellular pathogens, presented via MHC-I

Pathogen Evolution and Immune Evasion

Mechanisms of Immune Evasion

• Antigenic Variation: Changing surface antigens to avoid detection

• Phase Variation: Switching expression of genes

• Antigenic Drift: Gradual mutations in viral genes

• Antigenic Shift: Sudden reassortment of viral genes

• Phase Shift: (Additional info: Refers to changes in gene expression phase)

Examples of Pathogen Immune Evasion

• HIV: High mutation rate, attacks immune cells

• Influenza: Antigenic drift and shift

• Coronaviruses: Mutations in spike protein, immune modulation

• Staphylococcus aureus: Protein A binds antibodies, inhibits phagocytosis

Immune System Countermeasures

• Generation of diverse antibodies (VDJ recombination)

• Memory cells for rapid response

Hypersensitivity and Allergic Responses

Types of Hypersensitivity

• Type I: Immediate, IgE-mediated (allergies)

• Type II: Cytotoxic, antibody-mediated

• Type III: Immune complex-mediated

• Type IV: Delayed, cell-mediated

Diagnosis, Prevention, and Treatment

• Diagnosis: Allergy testing, symptom assessment

• Prevention: Avoidance of allergens

• Treatment: Antihistamines, immunotherapy

Cells Associated with Allergic Responses

• Mast cells, basophils, eosinophils

Koch's Postulates and Disease Causation

Koch's Postulates

• 1. Pathogen must be found in all cases of the disease

• 2. Pathogen must be isolated and grown in pure culture

• 3. Pathogen must cause disease when introduced into a healthy host

• 4. Pathogen must be re-isolated from the experimentally infected host

Alternative Methods

• Molecular techniques (PCR, sequencing)

• Genetic manipulation (knockout studies)

Signs vs Symptoms of Disease

• Signs: Objective, measurable indicators (e.g., fever, rash)

• Symptoms: Subjective experiences reported by patient (e.g., pain, fatigue)

Summary Table: Immune Response Types

Response Type	Key Cells	Key Molecules	Pathogen Type
Innate	Phagocytes, NK cells	Complement, TLRs, Interferons	All
Adaptive - Humoral	B-cells, plasma cells	Antibodies (IgG, IgM, etc.)	Extracellular
Adaptive - Cell-mediated	T-cells (Th, Tc, Treg)	Cytokines, MHC	Intracellular

Additional info:

• VDJ recombination allows for immense diversity in antigen recognition.

• Pathogen fitness includes survival, reproduction, and transmission efficiency.

• Interpretation of epidemiological graphs requires understanding incidence, prevalence, and transmission dynamics.