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 the pathogen and onset of symptoms.
Prodromal Period: Early, mild symptoms appear.
Illness: Most severe symptoms; pathogen is actively multiplying.
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: The study of the distribution, determinants, and control of diseases in populations.
Etiology: The study of the cause or origin of disease.
Example: Epidemiologists track outbreaks; etiologists identify the causative agent.

Symbiotic Relationship Terminology

Mutualism: Both organisms benefit (e.g., gut bacteria and humans).
Commensalism: One benefits, the other is unaffected (e.g., skin microbes).
Parasitism: One benefits, the other is 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).

The Microbiome

Microbiome: The collection of all microbes living in and on the human body.
Role: Contributes to health, digestion, and immune function.

Contamination, Infection, and Disease

Contamination: Presence of microbes on surfaces or tissues.
Infection: Microbes invade and multiply in host tissues.
Disease: Infection leads to damage or dysfunction in host tissues.

Sporadic, Endemic, Epidemic, Pandemic

Sporadic: Occurs occasionally and irregularly.
Endemic: Constantly present in a population.
Epidemic: Sudden increase in cases above normal 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 the patient (e.g., pain, fatigue).

Virulence and Pathogenicity

Pathogenicity: The ability of a microbe to cause disease.
Virulence: The degree of pathogenicity; how severe the disease is.
Example: Highly virulent pathogens cause severe disease; low virulence may cause mild symptoms.

Chapter 15: Innate Immunity

Innate Immunity

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

Characteristics: Present from birth, responds quickly, does not require prior exposure.
Components: Physical barriers, chemical barriers, cellular defenses.

First and Second Lines of Defense

First Line: Skin, mucous membranes, secretions (e.g., lysozyme, acid).
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: Phagocytose bacteria and fungi.
Macrophages: Engulf pathogens and present antigens.
Natural Killer (NK) Cells: Destroy infected or abnormal cells.
Eosinophils: Target parasites.
Basophils: Release histamine in allergic responses.

Pathogen-Associated Molecular Patterns (PAMPs)

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

Fever and Inflammation

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

Steps of Phagocytosis (in order)

Chemotaxis: Phagocyte moves toward pathogen.
Adherence: Phagocyte binds to pathogen.
Ingestion: Pathogen is engulfed into a phagosome.
Digestion: Phagosome fuses with lysosome; enzymes degrade pathogen.
Exocytosis: Waste is expelled from the cell.

Endotoxins vs Exotoxins

Endotoxins: Lipopolysaccharides from Gram-negative bacteria; released upon cell death.
Exotoxins: Proteins secreted by bacteria; often highly toxic.

Endogenous vs Exogenous Antigens

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

Chapter 16: Adaptive Immunity

MHC I and II

Major Histocompatibility Complex (MHC) molecules are essential for antigen presentation and immune recognition.

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 by producing antibodies.

Antigen Presenting Cells (APCs)

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

Antibody Classes and Functions

Class	Main Function	Characteristics
IgG	Major serum antibody; neutralization, opsonization	Crosses placenta; long-term immunity
IgM	First antibody produced; agglutination	Pentamer; activates complement
IgA	Protects mucosal surfaces	Found in secretions (saliva, tears, milk)
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 in Immunity: Removes infected or self-reactive cells.

Primary vs Secondary Immune Responses

Primary Response: First exposure to antigen; slower, lower antibody levels.
Secondary Response: Subsequent exposures; 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 cells for destruction by NK 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	Main 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	Suppress immune responses
Natural Killer Cells	Kill infected or abnormal cells without MHC restriction