Unit 3: Epidemiology, Innate Immunity, and Adaptive Host Defenses

Comprehensive vocabulary review flashcards covering Chapter 16 (Epidemiology and Disease Transmission), Chapter 17 (Innate Nonspecific Host Defenses), and Chapter 18 (Adaptive Specific Host Defenses).

Incidence

The number of individuals with new infections of a particular disease in a given period of time, calculated as \text{Incidence} = \frac{\text{# new cases of a disease}}{\text{total population}}.

Prevalence

The total number or proportion of individuals in a population ill with a specific disease at a given time, calculated as \text{Prevalence} = \frac{\text{total # of cases of a disease}}{\text{total population}}.

Sporadic Disease

A disease that is seen only occasionally and usually without geographic concentration (e.g., tetanus, rabies, plague).

Endemic Disease

A disease constantly present (often at a low level) within a particular geographic region (e.g., malaria in Brazil).

Epidemic Disease

A disease for which a larger than expected number of cases occurs in a short time within a geographic region (e.g., influenza in some years).

Pandemic Disease

An epidemic disease that spreads on a worldwide scale (e.g., HIV/AIDS).

Disease

Any condition in which the normal structure and function of the body is damaged or impaired.

Infection

The successful colonization of a microorganism within a host.

Etiology

The science of the causes of disease.

Etiologic Agent

The specific pathogen or causative agent responsible for causing a particular disease.

Morbidity

The state of being diseased or ill within a population.

Mortality

Death resulting from a disease within a population.

Notifiable / Reportable Diseases

Diseases of public health importance on a national scale for which all diagnosed cases must legally be reported to regional, state, and/or federal agencies such as the Centers for Disease Control and Prevention (CDC).

Emerging Infectious Disease

A disease that is new to the human population or has increased in prevalence over the previous 20 years.

Reemerging Infectious Disease

A disease that was once under control or largely eradicated but has begun causing new outbreaks due to changes in susceptible populations, the environment, or the pathogen itself.

Reservoir

A living host or nonliving site (such as soil, water, or the environment) in which a pathogenic organism can survive or multiply.

Passive Carrier

An individual capable of transmitting a pathogen to another individual without becoming infected themselves.

Active Carrier

An infected individual who can transmit the pathogen to others regardless of whether symptoms are currently present.

Asymptomatic Carrier

An infected individual who exhibits no signs or symptoms of disease yet remains capable of transmitting the pathogen to others.

Fomite

An inanimate item (such as a doorknob, towel, or needle) that may harbor microbes and facilitate indirect contact disease transmission.

Direct Contact Transmission

The spread of a pathogen between hosts via physical touch, such as kissing, holding hands, or sexual intercourse.

Droplet Transmission

Contact transmission of a pathogen via respiratory droplets that travel over distances of one meter or less when an infected person coughs or sneezes.

Vehicle Transmission

The transmission of pathogens through contaminated media such as food, water, or air.

Mechanical Vector

An animal (such as a fly) that physically carries a pathogen on its body from fecal matter or an environmental source to food or a host without becoming infected.

Biological Vector

An organism (most commonly an arthropod, such as a mosquito) that transmits a pathogen between hosts after the pathogen multiplies or develops inside its body.

Nosocomial Infections (HAIs)

Healthcare-Associated Infections acquired during a hospital or healthcare facility stay, developing at the intersection of microorganisms in the hospital environment, immunocompromised patients, and pathogen transmission among staff and patients.

Innate Immunity

Nonspecific defenses present in an individual at birth prior to pathogen exposure, consisting of physical barriers, phagocytes, and circulating complement proteins that provide an immediate initial response.

Adaptive Immunity

Specific, acquired immunity that develops following pathogen exposure; composed of specialized, systemic cells and processes (B cells, T cells, and antibodies) exhibiting targeted specificity and immunological memory.

Blood-Brain Barrier

Tight cell junctions of the endothelial cells lining the blood vessels that serve the central nervous system, preventing the passage of microbes from the bloodstream into the brain and cerebrospinal fluid.

Mucous Membranes

Moist layers of epithelial cells and interspersed goblet cells lining inner body cavities open to the outside (respiratory, gastrointestinal, and genitourinary tracts), bathed in antimicrobial secretions.

Mucociliary Escalator

A respiratory defense mechanism in which ciliated epithelial cells beat synchronously to push mucus and trapped debris upward and out of the respiratory tract toward the mouth to be swallowed or coughed out.

Peristalsis

Coordinated muscular contractions of the gastrointestinal tract that propel ingested material, mucus, and resident or pathogenic microbes through the digestive tract and out of the body.

Microbial Antagonism

The competitive interaction in which normal resident microbiota compete for available nutrients and cellular binding territory, thereby suppressing the growth and colonization of opportunistic pathogens.

Plasma

The fluid portion of whole blood containing all clotting factors (such as fibrinogen), complement proteins, cytokines, and inflammation-eliciting mediators.

Complement System

A group of over 20 circulating plasma proteins that function in an enzymatic cascade triggered by classical, alternative, or lectin pathways, resulting in pathogen lysis, opsonization, and inflammation.

Alternative Pathway

A complement activation pathway triggered directly by the spontaneous binding of the complement fragment C3b to the surface of microbial invaders.

Lectin Pathway

A complement activation pathway triggered when host mannose-binding lectin (MBL) binds to specific carbohydrates on microbial surfaces.

Classical Pathway

A complement activation pathway triggered by the binding of specific antibodies to antigens on the surface of microbial invaders.

Membrane Attack Complex (MAC)

A ring structure assembled from complement proteins C5b, C6, C7, C8, and C9 that penetrates the outer membrane of targeted pathogens (effective against Gram-negative bacteria), causing cell lysis.

Opsonization

The process of coating a pathogen with molecules such as complement fragment C3b or antibodies, tagging it for enhanced recognition and engulfment by phagocytic cells.

Cytokine

A soluble protein molecule produced by cells in response to stimulation that functions as a chemical communication signal between immune cells and stimulates nonspecific defenses.

Interferon

A cytokine secreted by virus-infected cells that instructs neighboring uninfected cells to degrade RNA and reduce protein synthesis, signals infected neighbors to undergo apoptosis, and activates immune cells.

Phagocytosis

A form of endocytosis by which specialized cells engulf large particles or microorganisms through membrane invagination, enclosing them in a pocket that pinches off as an intracellular vacuole.

Pathogen-Associated Molecular Patterns (PAMPs)

Conserved, nonself molecular motifs found on pathogens (such as peptidoglycan, flagellin, lipopeptides, and viral nucleic acids) that activate innate immune responses upon detection.

Pattern Recognition Receptors (PRRs)

Receptors located on the surface or within the endosomes/interior compartments of phagocytes that bind directly to pathogen-associated molecular patterns (PAMPs).

Toll-Like Receptors (TLRs)

A category of pattern recognition receptors (PRRs) situated either on the exterior plasma membrane of phagocytes or facing inward in intracellular compartments to recognize microbial components.

Phagosome

A membrane-bound intracellular compartment in the cytoplasm of a phagocyte that contains the engulfed pathogen enclosed by a portion of the cell membrane.

Phagolysosome

A digestive intracellular compartment formed by the fusion of a phagosome with a lysosome, leading to pathogen destruction through exposure to digestive enzymes and reactive chemicals.

Extravasation / Diapedesis

The process by which circulating leukocytes squeeze between the endothelial cells of capillary walls along a chemical gradient to migrate out of the blood and into damaged or infected tissues.

Pyrogens

Chemical substances that act on the hypothalamus to raise the body's internal thermostat, initiating a systemic fever that slows pathogen replication and optimizes immune cell function.

Inflammation

A localized innate defense response to injury or infection characterized by vasoconstriction, vasodilation, increased vascular permeability, leukocyte influx, and signs of erythema, edema, heat, pain, and altered function.

Cellular Immunity

The branch of adaptive immunity driven by T cells that directs the destruction of infected host cells and intracellular pathogens.

Humoral Immunity

The branch of adaptive immunity mediated by B cells and circulating antibodies targeting extracellular pathogens and toxins.

Antigen / Immunogen

A pathogen-specific molecular structure that triggers an adaptive immune response and stimulates the generation of antibodies or specific T-cell activation.

Epitope

A specific, smaller exposed region on an antigen molecule that is directly recognized and bound by antibodies, B-cell receptors, or T-cell receptors.

Antibody (Immunoglobulin)

A Y-shaped glycoprotein made of two heavy and two light chains produced by plasma cells; it features an antigen-binding Fab region and a constant Fc region that interacts with immune effector mechanisms.

IgG

A monomeric immunoglobulin that constitutes approximately $80\%$ of total serum antibodies; it is the only antibody capable of crossing the placenta, fixes complement, binds to phagocytes, and mediates ADCC.

IgM

An immunoglobulin comprising $6\%$ of serum antibodies; it exists as a monomeric B-cell receptor on naive B cells and is secreted as a pentamer that excels at agglutination, complement fixation, and primary response.

Secretory IgA

A dimeric antibody containing a secretory component ($13\%$ of serum in monomeric form) found in body secretions (saliva, tears, breast milk, and mucus) that traps and neutralizes pathogens.

IgD

A monomeric immunoglobulin comprising $<1\%$ of serum antibodies that serves primarily as an antigen receptor on the plasma membrane of naive B cells.

IgE

A monomeric immunoglobulin present at $<1\%$ in serum whose Fc region binds tightly to receptors on mast cells and basophils, triggering responses against parasitic worms and mediating allergic reactions.

Neutralization

An antibody-mediated mechanism wherein antibodies coat viral surfaces or toxins (such as diphtheria toxin) to physically block their attachment to target host cell receptors.

Agglutination

The cross-linking and clumping of particulate antigens or whole bacterial cells by multivalent antibodies into large aggregations, facilitating easier mechanical filtration and phagocytosis.

Antibody-Dependent Cell-Mediated Cytotoxicity (ADCC)

A mechanism in which the Fab regions of IgG antibodies bind to a large pathogen and the Fc regions bind Fc receptors on Natural Killer (NK) cells, triggering the release of cytotoxic granules that destroy the pathogen.

Major Histocompatibility Complex (MHC)

A family of transmembrane glycoproteins involved in antigen presentation; MHC Class I is found on all nucleated cells (presenting to CD8+ CTLs), while MHC Class II is expressed on APCs (presenting to CD4+ T cells).

Helper T Cells

T cells carrying CD4 surface molecules that become activated by antigen-presenting cells displaying foreign peptides on MHC II; they orchestrate humoral and cellular immunity and activate macrophages and NK cells.

Cytotoxic T Cells (CTLs)

T cells carrying CD8 surface molecules that are activated by antigen presented on MHC I; they eliminate infected host cells by releasing perforins and granzymes to induce programmed apoptosis.

Regulatory T Cells

A population of CD4+ T cells activated by MHC II-peptide complexes that maintains peripheral tolerance and prevents destructive autoimmune responses.

Central Tolerance

The process of thymic selection that screens developing T cells in the thymus, eliminating self-reactive cells through negative selection to prevent autoimmunity.

T-Cell Receptor (TCR)

A membrane-bound protein complex composed of an alpha ($\alpha$) and beta ($\beta$) chain that specifically recognizes processed peptide antigens presented exclusively within the cleft of MHC I or MHC II molecules.

B-Cell Receptor (BCR)

A membrane-bound monomeric immunoglobulin molecule (membrane-bound IgM or IgD) anchored on the B-cell plasma membrane that directly binds free extracellular antigens.

Plasma Cell

An activated, terminally differentiated B lymphocyte that acts as an antibody factory, secreting large volumes of soluble antibodies identical in antigen specificity to its parent BCR.

Memory

The hallmark characteristic of the adaptive immune system that enables it to rapidly mount a heightened, prolonged secondary antibody and cellular response upon re-exposure to a previously encountered antigen.

Apoptosis

Programmed, non-lytic cellular death that systematically dismantles an infected or damaged host cell without triggering widespread inflammation or releasing intracellular pathogens into the surrounding tissue.

Superantigen

A class of bacterial exotoxin that non-specifically bridges MHC II molecules on APCs directly to T-cell receptors without typical antigen processing, causing an uncontrolled, systemic cytokine storm.

Herd Immunity

A reduction in disease prevalence and spread across a population that occurs when a high proportion of individuals are immune, providing indirect protection to susceptible members.