Microbiology Study Guide: Infection, Immunity, and Adaptive Responses

Infection, Infectious Diseases, and Epidemiology

Stages of Infection and Disease

The progression of infectious disease occurs in a predictable sequence of stages:

• Incubation Period: Time between pathogen entry and appearance of symptoms.

• Prodromal Period: Short period with mild, nonspecific symptoms.

• Illness: Most severe stage; characteristic signs and symptoms appear.

• Decline: Symptoms subside as immune response or treatment reduces pathogen numbers.

• Convalescence: Recovery and return to normal function.

Example: Influenza typically has a 1-4 day incubation period before symptoms begin.

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; some strains are pathogens.

Epidemiology and Etiology

• Epidemiology: The study of the distribution and determinants of health-related states in populations.

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

Example: Epidemiologists track the spread of COVID-19; etiology identifies SARS-CoV-2 as the cause.

Symbiotic Relationships

• Mutualism: Both organisms benefit (e.g., gut bacteria synthesizing vitamins).

• Commensalism: One benefits, the other is unaffected (e.g., skin flora).

• Parasitism: One benefits at the expense of the other (e.g., pathogenic bacteria).

Modes of Transmission

• Contact Transmission: Direct (person-to-person), indirect (fomites), or droplet.

• Vehicle Transmission: Through air, water, or food.

• Vector Transmission: Via insects or animals (mechanical or biological vectors).

The Microbiome

• Microbiome: The collection of all microorganisms living in association with the human body.

• Plays roles in digestion, immunity, and disease prevention.

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 the host.

Sporadic, Endemic, Epidemic, and Pandemic

• Sporadic: Occurs occasionally and irregularly.

• Endemic: Constantly present in a population.

• Epidemic: Sudden increase in cases above normal expectancy.

• Pandemic: Epidemic that spreads across countries or continents.

Koch's Postulates

• The microorganism must be found in all cases of the disease.

• It must be isolated and grown in pure culture.

• The cultured microbe must cause disease when introduced into a healthy host.

• It 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.

Innate Immunity

Definition and Overview

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

First and Second Lines of Defense

• First Line: Physical and chemical barriers (skin, mucous membranes, secretions).

• Second Line: Cellular defenses (phagocytes, natural killer cells), inflammation, fever, antimicrobial proteins.

Microbial Antagonism

• Normal microbiota compete with pathogens for nutrients and space, inhibiting pathogen colonization.

Leukocytes in Innate Immunity

• Neutrophils: Phagocytose and destroy microbes.

• Macrophages: Engulf pathogens and present antigens.

• Natural Killer (NK) Cells: Destroy infected or abnormal cells.

• Eosinophils and Basophils: Involved in defense against parasites and allergic responses.

Pathogen-Associated Molecular Patterns (PAMPs)

• Molecular structures found on pathogens (e.g., LPS, flagellin) recognized by host pattern recognition receptors (PRRs).

Fever and Inflammation

• Fever: Elevated body temperature that enhances immune response and inhibits pathogen growth.

• Inflammation: Localized response to injury or infection, characterized by redness, heat, swelling, and pain.

Steps of Phagocytosis (in order)

1. Chemotaxis: Phagocyte moves toward microbe.

2. Adherence: Phagocyte attaches to microbe.

3. Ingestion: Microbe is engulfed into a phagosome.

4. Digestion: Fusion with lysosome and destruction of microbe.

5. Exocytosis: Expulsion of debris.

Endotoxins vs. Exotoxins

Feature	Endotoxins	Exotoxins
Source	Gram-negative bacteria (outer membrane)	Gram-positive and Gram-negative bacteria (secreted)
Chemical Nature	Lipopolysaccharide (LPS)	Proteins
Heat Stability	Stable	Unstable
Toxicity	Low	High
Effect	General (fever, shock)	Specific (neurotoxin, enterotoxin, etc.)

Endogenous vs. Exogenous Antigens

• Endogenous Antigens: Generated within cells (e.g., viral proteins in infected cells).

• Exogenous Antigens: Enter the body from outside (e.g., bacteria, toxins).

Adaptive Immunity

MHC I and II

• MHC I: Found on all nucleated cells; presents endogenous antigens to cytotoxic T cells (CD8+).

• MHC II: Found on antigen-presenting cells; presents exogenous antigens to helper T cells (CD4+).

T Cells and B Cells

• T Cells: Develop in thymus; mediate cellular immunity (cytotoxic, helper, regulatory functions).

• B Cells: Develop in bone marrow; produce antibodies (humoral immunity).

Antigen Presenting Cells (APCs)

• Cells that process and present antigens via MHC II to helper T cells.

• Examples: Dendritic cells, macrophages, B cells.

Antibody Classes and Functions

Class	Main Function	Characteristics
IgG	Major serum antibody; crosses placenta	Long-term immunity
IgM	First antibody produced; agglutination	Pentamer; activates complement
IgA	Mucosal immunity	Found in secretions (tears, saliva)
IgE	Allergic responses; defense against parasites	Binds to mast cells and basophils
IgD	B cell receptor	Low concentration in serum

Apoptosis

• Programmed cell death; eliminates infected or abnormal cells without causing inflammation.

Primary vs. Secondary Immune Responses

• Primary Response: First exposure to antigen; slower, lower antibody production.

• Secondary Response: Subsequent exposure; faster, stronger due to memory cells.

Antibody Functions

• Opsonization: Coating pathogens to enhance phagocytosis.

• Neutralization: Blocking pathogen binding sites or toxins.

• Complement Activation: Triggering complement cascade to lyse pathogens.

• Inflammation: Promoting inflammatory response.

• Agglutination: Clumping pathogens for easier clearance.

• Antibody-Dependent Cellular Cytotoxicity (ADCC): Recruiting NK cells to destroy antibody-coated cells.

Cytokines

• Small proteins released by cells to communicate and regulate immune responses.

• Examples: Interleukins, interferons, tumor necrosis factors.

Clonal Expansion, Deletion, and Selection

• Clonal Selection: Activation of lymphocytes specific to an antigen.

• Clonal Expansion: Proliferation of activated lymphocytes.

• Clonal Deletion: Removal of self-reactive lymphocytes to prevent autoimmunity.

Lymphocyte Functions

• Cytotoxic T Cells (CD8+): Destroy infected or cancerous cells.

• B Cells: Produce antibodies.

• Helper T Cells (CD4+): Activate B cells and cytotoxic T cells; coordinate immune response.

• Regulatory T Cells: Suppress immune response to prevent autoimmunity.

• Natural Killer (NK) Cells: Kill virus-infected and tumor cells without prior sensitization.