The Three Tiers of Human Immune Defense and Infectious Disease

Comprehensive question-and-answer study flashcards covering the foundations of health and disease, pathogen classifications, and the first, second, and third lines of human immune defense.

How is a disease defined in the study of biology and health?

Any disorder of the body that is not the result of a physical injury, producing specific signs or symptoms.

What is the definition of a symptom?

A physical or mental problem experienced by a person that indicates a health condition.

What is an infection?

The invasion and multiplication of microorganisms within tissues or the body that are not normally present.

What are the key structural characteristics and treatment method for bacteria?

Microscopic single-celled living organisms without a nucleus ($1\text{--}5\,\mu\text{m}$) with cell walls that reproduce by binary fission, release tissue-damaging toxins, and are effectively treated with antibiotics.

What are the structural characteristics of viruses, and can they be treated with antibiotics?

Ultra-small non-living particles ($20\text{--}400\,\text{nm}$) containing DNA or RNA in a protein coat that require a host cell to replicate; antibiotics are completely ineffective against them.

What are fungi and what structures do they typically invade in the human body?

Plant-like eukaryotic organisms that grow in warm, damp environments and invade keratinized structures such as skin, hair, and nails.

What are protists (protozoa) and how are they typically transmitted?

Single-celled microscopic eukaryotic organisms with a defined nucleus that are transmitted through vectors or contaminated food and water.

How are parasites defined, and what are common examples?

Organisms living on or inside a host organism that derive nutrients at the direct expense and harm of the host, such as helminths (intestinal worms), lice, and ticks.

What are the four primary pathways of pathogen transmission?

Direct Contact (skin-to-skin, kissing, sexual contact), Droplet & Airborne (sneezing, coughing, speaking), Vehicles (contaminated water, food, blood, fomites), and Vectors (living organisms like mosquitoes, ticks, or fleas).

How do innate immunity and adaptive immunity differ in onset speed and specificity?

Innate immunity is present from birth, acts immediately (within minutes to hours), and provides non-specific defense; adaptive immunity is acquired over time, has a slower initial response (taking days), and custom-targets specific pathogens.

How do innate immunity and adaptive immunity differ regarding immunological memory?

Innate immunity has no immunological memory and reacts with identical speed and intensity each time, whereas adaptive immunity develops long-term memory via memory B and T cells.

What constitutes the first line of defense, and what is its primary function?

External physical and chemical barriers at entry portals that immediately prevent pathogen spread and entry by trapping, killing, washing away, or expelling foreign microbes.

What physical and chemical defenses are provided by the skin (epidermis)?

A tough, waterproof layer of dead keratinized cells that physically blocks entry, acidic sebum that lowers pH, and beneficial microbiota that outcompete pathogens.

Where are mucous membranes located and how do they defend against pathogens?

They line the respiratory, digestive, urinary, and reproductive tracts, as well as eyes and ears, producing a sticky substance called mucus that traps pathogens and debris.

What antibacterial enzyme is present in tears and saliva, and what is its action?

Lysozyme, an enzyme that breaks down bacterial cell walls to destroy microbes.

What chemical defense is secreted by the stomach to destroy ingested pathogens?

Hydrochloric acid ($\text{HCl}$).

What is the second line of defense?

General, non-specific internal responses to pathogens that breach the body's barriers, including inflammation, phagocytes, antimicrobial proteins, and fever.

What is the role of mast cells and histamine in the second line of defense?

Mast cells in connective tissue release histamine upon injury or infection, which dilates blood vessels and increases capillary permeability to recruit immune components.

What physiological changes occur during an inflammatory response?

Increased blood flow (vasodilation), increased capillary permeability causing swelling, recruitment of macrophages and neutrophils to the site of infection, and elevated mucus production.

What is phagocytosis and which immune cells perform it?

The cellular process of engulfing and digesting pathogens, dead cells, and debris using vesicles with lytic enzymes, carried out by phagocytes such as macrophages and neutrophils.

What are the two major types of antimicrobial proteins in innate immunity and their functions?

The complement system (plasma proteins that punch holes in bacterial membranes, causing cell bursting) and interferons (chemicals released by virus-infected cells to warn neighboring cells and block viral replication).

What triggers a fever response and how does it assist the immune system?

Triggered by signaling chemicals called pyrogens, fever creates an unfavorable thermal environment that slows pathogen replication while accelerating white blood cell activity and tissue repair.

What defines the third line of defense?

The adaptive immune system, which differentiates among specific pathogens, attacks and destroys them using specialized cells and antibodies, and retains long-term immunological memory.

What is an antigen?

A unique surface chemical marker or protein found on the surface of a pathogen that stimulates the production of antibodies.

What are antibodies and how do they function against pathogens?

Y-shaped protective proteins produced in blood and lymph in response to antigens; they bind matching antigens like a lock and key to neutralize pathogens, prevent host entry, and cause clumping.

What is the function of Plasma B cells during an immune response?

Specialized B cells that synthesize and secrete thousands of custom antibodies per second into circulation to neutralize specific pathogens.

What is the role of Memory B cells?

They persist long-term in lymph nodes and the bloodstream, retaining the antibody blueprint to launch a faster and stronger secondary response upon re-encountering the same pathogen.

Where do Helper T cells mature and what is their role?

They mature in the thymus and release signaling proteins called cytokines upon activation by antigen-presenting cells to coordinate B and T cell attacks.

What is the role of Cytotoxic (Killer) T cells in the adaptive response?

They bind directly to foreign antigens expressed on infected host cells and inject lethal chemicals to destroy infected or abnormal cells.

What is the role of Memory T cells?

They remain in circulation after an infection to coordinate rapid T-cell reactivation during subsequent exposures to the same antigen.

What are the five steps of the adaptive defense sequence?

1. Recognition (identifying foreign antigens); 2. Antibody Production (cloning plasma cells to make antibodies); 3. Clumping/Agglutination (antibodies bind pathogen antigens); 4. Digestion (phagocytes engulf clumped pathogens); 5. Long-Term Storage (antibodies and memory cells persist).

How does the secondary immune response compare to the primary immune response?

The primary response is slow and weak because the body requires time to recognize antigens and synthesize antibodies; the secondary response is faster and stronger because memory cells rapidly generate massive antibody concentrations before symptoms appear.

How do vaccines provide immunity against infectious diseases?

They introduce a weakened, dead, or partial form of an antigen to stimulate a harmless primary immune response, generating memory B and T cells that grant long-term protection without disease illness.

What is herd immunity?

Indirect community-level protection that occurs when a high proportion of a population is immune, reducing overall pathogen transmission and protecting vulnerable or unimmunized individuals.