Microbiology Study Guide: Infection, Immunity, and Adaptive Responses

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Chapter 14: Infection, Infectious Diseases, and Epidemiology

Stages of Infection and Disease

The progression of infectious disease follows a series of stages, each characterized by specific clinical and microbiological features.

Incubation Period: Time between exposure to pathogen and onset of symptoms.
Prodromal Period: Early, mild symptoms appear.
Illness: Disease is most severe; characteristic signs and symptoms are present.
Decline: Symptoms begin to subside as immune response or treatment reduces pathogen numbers.
Convalescence: Recovery and return to normal health.

Pathogen vs Microbe

Microbe: Any microscopic organism, including bacteria, viruses, fungi, and protozoa.
Pathogen: A microbe capable of causing disease in a host.
Example: Escherichia coli is a microbe; certain strains are pathogens.

Epidemiology and Etiology

Epidemiology: Study of the distribution, determinants, and control of diseases in populations.
Etiology: Study of the cause or origin of disease.
Example: Identifying the etiological agent of tuberculosis as Mycobacterium tuberculosis.

Symbiotic Relationship Terminology

Mutualism: Both organisms benefit (e.g., gut flora).
Commensalism: One benefits, other unaffected (e.g., skin bacteria).
Parasitism: One benefits, other harmed (e.g., pathogenic bacteria).

Modes of Transmission

Contact Transmission: Direct (person-to-person), indirect (via fomites), or droplet.
Vehicle Transmission: Via air, water, or food.
Vector Transmission: Via animals, typically arthropods (e.g., mosquitoes).

Microbiome

Microbiome: The collection of microorganisms living in and on the human body, contributing to health and disease.
Example: Gut microbiome aids digestion and immune function.

Contamination, Infection, and Disease

Contamination: Presence of microbes on surfaces or tissues.
Infection: Invasion and multiplication of microbes in host tissues.
Disease: Disruption of normal body functions due to infection.

Sporadic, Endemic, Epidemic, Pandemic

Sporadic: Occurs occasionally and irregularly.
Endemic: Constantly present in a population.
Epidemic: Sudden increase in cases above expected levels.
Pandemic: Epidemic that spreads across countries or continents.

Koch's Postulates

1. The suspected pathogen must be present in every case of the disease.
2. The pathogen must be isolated and grown in pure culture.
3. The cultured pathogen must cause disease when introduced into a healthy host.
4. The same pathogen must be re-isolated from the experimentally infected host.

Signs vs Symptoms

Signs: Objective, measurable indicators (e.g., fever, rash).
Symptoms: Subjective experiences reported by patient (e.g., pain, fatigue).

Virulence and Pathogenicity

Virulence: Degree of pathogenicity; how severe the disease caused by a pathogen is.
Pathogenicity: Ability of a microbe to cause disease.

Chapter 15: Innate Immunity

Innate Immunity

Innate immunity is the body's first line of defense against pathogens, providing immediate, non-specific protection.

Characteristics: Present at birth, rapid response, no memory.
Components: Physical barriers, chemical barriers, cellular defenses.

First and Second Lines of Defense

First Line: Skin, mucous membranes, secretions (e.g., lysozyme).
Second Line: Phagocytes, inflammation, fever, antimicrobial proteins.

Microbial Antagonism

Definition: Normal microbiota compete with pathogens, preventing their colonization.
Example: Gut flora outcompete pathogens for nutrients.

Leukocytes Involved in Innate Immunity

Neutrophils: Phagocytosis, most abundant.
Macrophages: Phagocytosis, antigen presentation.
Natural Killer (NK) Cells: Destroy infected or abnormal cells.
Eosinophils: Combat parasites.
Basophils: Release histamine, involved in inflammation.

Pathogen Associated Molecular Patterns (PAMPs)

PAMPs: Molecules found on pathogens (e.g., LPS, peptidoglycan) recognized by host immune cells.
Function: Trigger innate immune responses.

Fever and Inflammation

Fever: Elevated body temperature, inhibits pathogen growth, enhances immune activity.
Inflammation: Localized response to injury or infection; characterized by redness, heat, swelling, pain.

Steps of Phagocytosis (in order)

1. Chemotaxis: Phagocyte moves toward pathogen.
2. Adherence: Phagocyte attaches to pathogen.
3. Ingestion: Pathogen is engulfed into a phagosome.
4. Digestion: Phagosome fuses with lysosome; enzymes digest pathogen.
5. Exocytosis: Debris expelled from cell.

Endotoxins vs Exotoxins

Type	Source	Properties
Endotoxins	Gram-negative bacteria (LPS)	Released upon cell death; less specific; causes fever
Exotoxins	Gram-positive and Gram-negative bacteria	Secreted; highly specific; potent effects

Endogenous vs Exogenous Antigens

Endogenous Antigens: Generated within cells (e.g., viral proteins).
Exogenous Antigens: Enter body from outside (e.g., bacteria, pollen).

Chapter 16: Adaptive Immunity

MHC I and II

MHC I: Found on all nucleated cells; presents endogenous antigens to cytotoxic T cells.
MHC II: Found on antigen-presenting cells; presents exogenous antigens to helper T cells.

T Cells and B Cells

T Cells: Develop in thymus; mediate cellular immunity.
B Cells: Develop in bone marrow; mediate humoral immunity (antibody production).

Antigen Presenting Cells (APCs)

Function: Process and present antigens to T cells.
Types: Dendritic cells, macrophages, B cells.

Antibody Classes and Functions

Class	Function	Characteristics
IgG	Main antibody in blood; neutralization, opsonization	Crosses placenta; long-term immunity
IgM	First antibody produced; agglutination	Pentamer; activates complement
IgA	Protects mucosal surfaces	Dimer; found in secretions
IgE	Allergic responses; defense against parasites	Binds to mast cells and basophils
IgD	B cell receptor	Found on B cell surface

Apoptosis

Definition: Programmed cell death; eliminates damaged or unnecessary cells.
Role: Maintains immune system balance.

Primary vs Secondary Immune Responses

Primary Response: First exposure; slower, lower antibody levels.
Secondary Response: Subsequent exposure; faster, higher antibody levels due to memory cells.

Antibody Functions

Opsonization: Coating pathogens to enhance phagocytosis.
Neutralization: Blocking pathogen attachment or toxin activity.
Complement Activation: Triggering complement cascade to lyse pathogens.
Inflammation: Promoting inflammatory response.
Agglutination: Clumping pathogens for easier clearance.
Antibody-Dependent Cellular Cytotoxicity (ADCC): Targeting infected cells for destruction by immune cells.

Cytokines

Cytokines: Signaling proteins that regulate immune responses.
Examples: Interleukins, interferons, tumor necrosis factor.

Clonal Expansion, Deletion, and Selection

Clonal Expansion: Proliferation of lymphocytes after activation.
Clonal Deletion: Removal of self-reactive lymphocytes.
Clonal Selection: Activation of lymphocytes specific to an antigen.

Lymphocyte Functions

Cell Type	Function
Cytotoxic T Cells (CD8+)	Destroy infected or cancerous cells
B Cells	Produce antibodies
Helper T Cells (CD4+)	Activate B cells and other T cells
Regulatory T Cells Kill infected or abnormal cells without antigen specificity	Suppress immune responses
Natural Killer Cells