Microbiology Study Guide: Infection, Immunity, and Adaptive Responses

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

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 the 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 mild malaise (prodromal), then fever and cough (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.

Additional info: Not all microbes are pathogens; many are harmless or beneficial.

Contamination, Infection, and Disease

Contamination: Presence of microbes on or in the body.
Infection: Successful invasion and colonization of the host by a microbe.
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 collection of all microorganisms living in association with the human body. These microbes play roles in digestion, immunity, and disease prevention.

Modes of Transmission

Contact Transmission: Direct (person-to-person), indirect (fomites), or droplet.
Vehicle Transmission: Spread via air, water, or food.
Vector Transmission: Transmission by living organisms (e.g., mosquitoes, ticks).

Epidemiology and Etiology

Epidemiology: Study of the distribution and determinants of health and disease 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 or region.
Epidemic: Sudden increase in cases above normal expectations.
Pandemic: Epidemic that spreads across countries or continents.

Koch's Postulates

Koch's postulates are criteria to establish a causative relationship between a microbe and a disease:

The suspected pathogen must be present in all cases of the disease and absent from healthy individuals.
The pathogen must be isolated and grown in pure culture.
The cultured pathogen must cause disease when introduced into a healthy host.
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.

Innate Immunity

Definition and Overview

Innate immunity is the body's first line of defense against pathogens, providing immediate, non-specific 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 resources, producing substances that inhibit pathogen growth.

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)

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)

Chemotaxis: Phagocyte moves toward microbe.
Adherence: Phagocyte attaches to microbe.
Ingestion: Microbe is engulfed into a phagosome.
Digestion: Fusion with lysosome and destruction of microbe.
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 (e.g., neurotoxins, enterotoxins)

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 the 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
IgM	First antibody produced; effective in agglutination	Pentamer structure
IgA	Secretory antibody (mucosal immunity)	Found in saliva, tears, 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 abnormal 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): Marking cells for destruction by NK cells.

Cytokines

Cytokines are signaling proteins released by immune cells to regulate immunity, inflammation, and hematopoiesis.

Clonal Expansion, Deletion, and Selection

Clonal Expansion: Proliferation of lymphocytes after activation by antigen.
Clonal Deletion: Elimination of self-reactive lymphocytes to prevent autoimmunity.
Clonal Selection: Activation of specific lymphocytes by antigen recognition.

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 responses to maintain tolerance.
Natural Killer (NK) Cells: Kill virus-infected and tumor cells without prior sensitization.