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Microbiology Study Guide: Infection, Immunity, and Immune Response

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Infection, Infectious Diseases, and Epidemiology

Stages of Infection and Disease

The progression of infectious disease occurs in distinct stages, each with characteristic features.

  • 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: After exposure to influenza virus, a person may experience a 1-2 day incubation, followed by fever and malaise (prodromal), then cough and high fever (illness), gradual improvement (decline), and full recovery (convalescence).

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 causing foodborne illness.

Contamination, Infection, and Disease

  • Contamination: Presence of microbes on surfaces or tissues.

  • Infection: Successful invasion and multiplication of microbes in the host.

  • Disease: Disruption of normal body function due to infection.

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).

Microbiome

The microbiome refers to the community of microorganisms living in and on the human body, contributing to health and disease resistance.

Modes of Transmission

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

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

  • Vector Transmission: Via animals (usually arthropods) that carry pathogens.

Epidemiology and Etiology

  • Epidemiology: Study of disease distribution and determinants in populations.

  • Etiology: Study of the cause of disease.

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

Criteria to establish a causative relationship between a microbe and a disease:

  1. Microbe must be found in all cases of the disease.

  2. Microbe must be isolated and grown in pure culture.

  3. Pure culture must cause disease when introduced into a healthy host.

  4. Microbe 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: Ability of a microbe to cause disease.

  • Virulence: Degree of pathogenicity; how severe the disease is.

Innate Immunity

Overview of Innate Immunity

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

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, preventing their colonization (competitive exclusion).

Leukocytes in Innate Immunity

  • Neutrophils: Phagocytose bacteria and fungi.

  • Macrophages: Engulf and digest pathogens; 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)

PAMPs are conserved microbial molecules (e.g., LPS, flagellin) recognized by host pattern recognition receptors (PRRs), triggering immune responses.

Fever and Inflammation

  • Fever: Elevated body temperature that enhances immune function and inhibits pathogens.

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

Phagocytosis Steps (in order)

  1. Chemotaxis: Phagocyte moves toward pathogen.

  2. Adherence: Phagocyte binds to pathogen.

  3. Ingestion: Pathogen is engulfed into a phagosome.

  4. Digestion: Fusion with lysosome forms phagolysosome; enzymes degrade pathogen.

  5. Exocytosis: Debris is expelled from the cell.

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

Fever, shock

Specific effects (e.g., neurotoxins)

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.

  • 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 (cytotoxic, helper, regulatory functions).

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

Antigen Presenting Cells (APCs)

  • Function: Process and present antigens to T cells via MHC molecules.

  • Types: Dendritic cells, macrophages, B cells.

Antibody Classes and Functions

Class

Main Function

Characteristics

IgG

Major serum antibody; crosses placenta

Long-term immunity, opsonization

IgM

First antibody produced

Pentamer, effective in agglutination

IgA

Secretory antibody

Found in mucosal areas, breast milk

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 unnecessary cells without causing inflammation.

Primary vs. Secondary Immune Responses

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

  • Secondary Response: Subsequent exposure; rapid, robust antibody production 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 responses.

  • Agglutination: Clumping pathogens for easier clearance.

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

Cytokines

Small proteins released by cells that regulate immune responses, inflammation, and cell communication.

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 abnormal cells.

  • Helper T Cells (CD4+): Activate B cells, cytotoxic T cells, and macrophages.

  • Regulatory T Cells: Suppress immune responses to maintain tolerance.

  • B Cells: Produce antibodies; differentiate into plasma and memory cells.

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

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