Innate Immunity, Microbiome & Normal Flora – Exam 4 Review

A comprehensive set of question-and-answer flashcards covering innate immunity, the human microbiome, commensals, and related exam topics.

What are the two major components that maintain a healthy human system?

1) Normal microbiota living in and on us; 2) Our disease-resistance mechanisms (intrinsic/innate and adaptive).

Define pathogen.

A disease-producing microorganism that possesses virulence factors.

What does ID50 measure?

The infectious dose required to infect 50 % of a host population; a quantitative indicator of virulence.

Differentiate colonization from infection.

Colonization is the presence and growth of a microbe without tissue invasion or damage, whereas infection involves invasion and leads to disease.

What is meant by a “healthy carrier”?

An individual colonized by a pathogen without signs of infection but capable of transmitting the pathogen to others.

Loose association vs. tight cell-to-cell interaction – which one is more likely to lead to invasion?

Tight, specific cell-to-cell interaction (firm attachment) is more likely to result in microbial invasion of host tissue.

Define commensalism.

A relationship in which one partner benefits and the other is neither helped nor harmed.

List three beneficial roles of commensals.

1) Train and shape the immune system; 2) Prevent pathogen attachment/colonization; 3) Produce molecules (e.g., bacteriocins) that inhibit pathogens.

What is an opportunistic pathogen?

A normally harmless microbe that can cause disease when given the opportunity, such as overgrowth or entry into a normally sterile site.

Which skin characteristics help limit microbial growth?

Cooler, drier environment and continuous shedding of epithelial cells.

Which two Gram-positive skin commensals colonize exposed skin, sweat glands and dry areas?

Staphylococcus epidermidis and Corynebacteria.

Which commensal bacterium is associated with acne vulgaris?

Propionibacterium (now Cutibacterium) acnes.

What type of microbes commonly inhabit moist areas of the skin?

Gram-negative bacteria (transients).

Name two bacterial species commonly found in the nostrils.

Staphylococcus epidermidis and Staphylococcus aureus.

Can β-hemolytic Streptococcus (cause of strep throat) be carried by healthy individuals?

Yes, healthy individuals can asymptomatically carry β-hemolytic Streptococcus in the oropharynx.

How are microbes removed from the lower respiratory tract?

By the mucociliary escalator, coughing reflex, and resident phagocytic cells.

Which enzyme allows Helicobacter pylori to survive stomach acidity?

Urease, which converts urea to ammonia and CO₂, locally neutralizing acid.

Which organ harbors the largest microbial population in the body?

The colon (large intestine).

What antimicrobial compounds do many gut commensals secrete to inhibit pathogens?

Bacteriocins (small antimicrobial peptides).

Which bacteria produce bacteriocins that can kill Clostridia in the bowel?

Escherichia coli (commensal strains).

What organisms predominate in the female genital tract and help protect against pathogens?

Acid-tolerant Lactobacilli that produce bacteriocins.

Define antigen.

Any substance that can elicit an immune response.

Distinguish innate from adaptive immunity in terms of specificity and memory.

Innate immunity is nonspecific and lacks memory; adaptive immunity is specific to particular antigens and develops memory that improves with repeated exposure.

What are defensins and lysozyme?

Constitutively expressed antimicrobial proteins; defensins are peptides that disrupt membranes, while lysozyme degrades Gram-positive cell walls.

What dual role does mucus provide on mucous membranes?

It forms both a physical barrier and contains chemical antimicrobial factors.

What do Pattern Recognition Receptors (PRRs) detect?

Pathogen-Associated Molecular Patterns (PAMPs) such as LPS, peptidoglycan, flagellin, and microbial nucleic acids.

Name the main family of PRRs discussed in innate immunity.

Toll-like receptors (TLRs).

Activation of PRRs leads to production of what signaling molecules?

Pro-inflammatory cytokines and chemokines.

Which innate leukocyte bridges innate and adaptive immunity by presenting antigen?

Dendritic cells.

What cytotoxic molecules are released by Natural Killer (NK) cells?

Perforin and granzymes.

Describe the main function of macrophages.

Phagocytosis of pathogens and debris, and secretion of cytokines.

What specialized structure can neutrophils release to trap microbes?

Neutrophil extracellular traps (NETs).

What is the primary function of mast cells in innate immunity?

Release of pharmacologically active molecules (e.g., histamine) and antimicrobial compounds during inflammation.

State two purposes of the lymphatic system relevant to immunity.

1) Drains interstitial fluid and filters out debris/pathogens; 2) Provides sites (lymph nodes) where innate and adaptive cells interact.

Approximately how many lymph nodes does an average young adult have?

About 400–450 lymph nodes.

What is complement?

A system of nine soluble plasma proteins that, when activated, leads to pathogen lysis.

Which complement component can spontaneously bind bacterial cell walls to initiate the cascade?

C3.

What terminal structure formed by complement causes bacterial death?

The Membrane Attack Complex (MAC), which punches a pore in the pathogen membrane.

List three health issues prevented by a healthy gut–lung microbiome connection.

Chronic kidney disease, cardiovascular complications, and abnormal immunity.

Why are most transient microbes killed in the stomach?

Because of its very low pH, unless they pass quickly, are in fatty food, or are ingested in large numbers.

Give one reason why microbiome diversity can vary among individuals.

Differences in ethnicity, diet, culture, environment, age, or antibiotic use.

What feature of the skin removes roughly 600,000 cells per day?

Continuous shedding of the outer epithelial layer (stratum corneum).

What are the two major components that maintain a healthy human system?

A healthy human system is maintained by two major components:1. Normal microbiota residing in and on the body, which are beneficial microorganisms forming a protective barrier and aiding various bodily functions.2. Our disease-resistance mechanisms, encompassing both intrinsic/innate (non-specific, immediate) and adaptive (specific, memory-based) immunity, which protect against harmful invaders.

Define pathogen.

A pathogen is a disease-producing microorganism, such as a bacterium, virus, fungus, or parasite, that possesses specific virulence factors (e.g., toxins, enzymes, adhesion molecules) enabling it to cause damage and illness in a host.

What does ID_{50} measure?

ID{50} (Infectious Dose 50%) measures the number of microbial cells or virions required to infect 50% of a given host population. It serves as a quantitative indicator of a pathogen's virulence; a lower ID{50} generally indicates higher virulence.

Differentiate colonization from infection.

Colonization describes the presence and multiplication of a microorganism on or within a host without causing any tissue invasion or observable host damage. In contrast, infection specifically involves the invasion of host tissues by microorganisms, leading to cellular damage, inflammation, and the onset of disease symptoms.

What is meant by a “healthy carrier”?

A healthy carrier is an individual who is colonized by a pathogen — meaning the pathogen is present and multiplying within them — but shows no signs or symptoms of infection or disease themselves. Despite their asymptomatic state, they are capable of transmitting the pathogen to susceptible individuals.

Loose association vs. tight cell-to-cell interaction – which one is more likely to lead to invasion?

Tight, specific cell-to-cell interaction (firm attachment) is significantly more likely to result in microbial invasion of host tissue. This firm adhesion allows pathogens to overcome host defenses, establish a more stable foothold, and initiate mechanisms for entering host cells or deeper tissues, unlike loose associations which are easily dislodged.

Define commensalism.

Commensalism is a symbiotic relationship between two different species where one organism, the commensal, benefits from the association, while the other organism (the host) is neither significantly helped nor harmed. For example, some skin bacteria feed on desquamated cells without affecting the host.

List three beneficial roles of commensals.

Commensal microorganisms play several beneficial roles:1. They train and shape the host immune system, helping it distinguish between harmful pathogens and harmless microbes.2. They prevent pathogen attachment and colonization by occupying ecological niches and competing for nutrients, a phenomenon known as "colonization resistance."3. They produce beneficial molecules, such as bacteriocins (small antimicrobial peptides) or vitamins (e.g., Vitamin K, B vitamins), that can directly inhibit the growth of pathogens or contribute to host health.

What is an opportunistic pathogen?

An opportunistic pathogen is a normally harmless microorganism that typically resides as a commensal or environmental microbe but can cause disease when given a specific "opportunity." This opportunity often arises from changes in the host's normal conditions, such as:

• Overgrowth due to a disrupted normal microbiota (e.g., after antibiotic use).
• Entry into a normally sterile body site (e.g., deep tissue wound, bloodstream).
• A weakened host immune system (e.g., in immunocompromised individuals).

Which skin characteristics help limit microbial growth?

The skin's unique characteristics effectively limit microbial growth:

• Its cooler and drier environment is less hospitable for many moisture-loving microorganisms.
• The continuous shedding of dead epithelial cells from the outer layer (stratum corneum) physically removes attached microbes, preventing their long-term colonization and buildup.

Which two Gram-positive skin commensals colonize exposed skin, sweat glands and dry areas?

The two Gram-positive skin commensals that commonly colonize exposed skin, sweat glands, and dry areas are Staphylococcus epidermidis and Corynebacteria species.

Which commensal bacterium is associated with acne vulgaris?

The commensal bacterium primarily associated with the development of acne vulgaris is Propionibacterium (now reclassified as Cutibacterium) acnes.

What type of microbes commonly inhabit moist areas of the skin?

Moist areas of the skin, such as the armpits, groin, and between toes, are more commonly inhabited by Gram-negative bacteria, often considered transients. These are microorganisms that are temporarily present on the skin but typically do not establish permanent residence or cause disease under normal circumstances unless the skin barrier is compromised.

Name two bacterial species commonly found in the nostrils?

Two bacterial species commonly found colonizing the nostrils are Staphylococcus epidermidis and Staphylococcus aureus.

Can β-hemolytic Streptococcus (cause of strep throat) be carried by healthy individuals?

Yes, healthy individuals can indeed asymptomatically carry β-hemolytic Streptococcus (e.g., Streptococcus pyogenes, the cause of strep throat) in their oropharynx. These carriers show no symptoms themselves but can act as a source of infection for others, transmitting the bacteria to susceptible individuals.

How are microbes removed from the lower respiratory tract?

Microbes are efficiently removed from the lower respiratory tract through several mechanisms:

• The mucociliary escalator, a coordinated action of mucus-producing cells and cilia that trap and propel foreign particles upwards.
• The coughing reflex, which forcefully expels mucus and trapped microbes from the airways.
• The presence of resident phagocytic cells, primarily alveolar macrophages, that engulf and destroy inhaled microbes and debris.

Which enzyme allows Helicobacter pylori to survive stomach acidity?

Helicobacter pylori utilizes the enzyme urease to survive the extreme acidity of the stomach. Urease converts urea (a common compound in the stomach) into ammonia (NH3) and carbon dioxide (CO2). The ammonia then acts as a local buffer, neutralizing the surrounding gastric acid and creating a more hospitable microenvironment for the bacterium to thrive.

Which organ harbors the largest microbial population in the body?

The colon (large intestine) harbors the largest and most diverse microbial population in the human body, comprising trillions of microorganisms (the gut microbiota).

What antimicrobial compounds do many gut commensals secrete to inhibit pathogens?

Many gut commensals secrete bacteriocins, which are small, ribosomally synthesized antimicrobial peptides. These compounds are produced by bacteria to inhibit the growth of or kill specific strains of other bacteria, often closely related species, thereby providing a competitive advantage against pathogens.

Which bacteria produce bacteriocins that can kill Clostridia in the bowel?

Commensal strains of Escherichia coli are known to produce various bacteriocins that can effectively inhibit the growth of and kill Clostridia species, including pathogenic ones like Clostridioides difficile, in the bowel.

What organisms predominate in the female genital tract and help protect against pathogens?

Acid-tolerant Lactobacilli (e.g., Lactobacillus crispatus, Lactobacillus jensenii) are the predominant organisms in the healthy female genital tract. They protect against pathogens by:

• Producing lactic acid, which lowers the vaginal pH, creating an acidic environment unfavorable for many pathogenic bacteria.
• Secreting bacteriocins and hydrogen peroxide, which directly inhibit the growth of competing microorganisms.

Define antigen.

An antigen is any substance, typically a protein or polysaccharide, that can be specifically recognized by the immune system and elicit an immune response (e.g., antibody production or cell-mediated immunity). Antigens can be components of pathogens (e.g., bacterial toxins, viral coats) or non-microbial substances (e.g., pollen, food allergens).

Distinguish innate from adaptive immunity in terms of specificity and memory.

• Innate immunity is the body's first line of defense; it is nonspecific, meaning it responds in the same general way to a wide range of pathogens, and lacks memory, so it does not improve with repeated exposure to the same pathogen.
• Adaptive immunity is a more specialized defense system; it is highly specific to particular antigens, targeting very precise molecular structures. Crucially, it develops immunological memory, which allows for a faster, stronger, and more effective response upon subsequent encounters with the same antigen.

What are defensins and lysozyme?

Both defensins and lysozyme are constitutively expressed antimicrobial proteins, meaning they are continuously produced by the body as part of its innate immune defenses:

• Defensins are small, cationic peptides that primarily function by disrupting microbial membranes, leading to cell lysis.
• Lysozyme is an enzyme that degrades peptidoglycan, a key component of bacterial cell walls, making it particularly effective against Gram-positive bacteria.

What dual role does mucus provide on mucous membranes?

Mucus on mucous membranes serves a critical dual role:1. It forms a physical barrier, trapping inhaled particles, microbes, and debris due to its viscous, gel-like consistency, preventing them from adhering to and invading underlying epithelial cells.2. It contains various chemical antimicrobial factors, such as lysozyme, lactoferrin, and antibodies (e.g., IgA), which actively neutralize or destroy pathogens trapped within the mucus layer.

What do Pattern Recognition Receptors (PRRs) detect?

Pattern Recognition Receptors (PRRs) are germline-encoded host proteins, expressed by innate immune cells, that detect highly conserved molecular structures known as Pathogen-Associated Molecular Patterns (PAMPs). PAMPs are fundamental microbial components essential for pathogen survival (e.g., bacterial LPS in Gram-negative bacteria, peptidoglycan in bacterial cell walls, bacterial flagellin, and microbial nucleic acids like viral RNA or bacterial DNA). Recognition of PAMPs by PRRs triggers immediate innate immune responses.

Name the main family of PRRs discussed in innate immunity.

The main family of Pattern Recognition Receptors (PRRs) extensively discussed in the context of innate immunity are the Toll-like receptors (TLRs), which are transmembrane proteins found on cell surfaces and within endosomes, recognizing different PAMPs.

Activation of PRRs leads to production of what signaling molecules?

The activation of Pattern Recognition Receptors (PRRs) upon detecting PAMPs triggers intracellular signaling pathways that lead to the rapid production and secretion of pro-inflammatory cytokines (e.g., IL-1, TNF-\alpha, IL-6) and chemokines. Cytokines are signaling proteins that modulate immune responses, while chemokines are chemoattractant cytokines that guide immune cells to the site of infection or inflammation.

Which innate leukocyte bridges innate and adaptive immunity by presenting antigen?

Dendritic cells are the key innate leukocytes that bridge innate and adaptive immunity. Following the engulfment of pathogens in peripheral tissues, they migrate to lymph nodes where they process and present pathogen-derived antigens to naive T cells, thereby initiating specific adaptive immune responses.

What cytotoxic molecules are released by Natural Killer (NK) cells?

Natural Killer (NK) cells, a type of lymphocyte in innate immunity, release perforin and granzymes as their primary cytotoxic molecules. Perforin creates pores in the target cell's membrane, allowing granzymes (serine proteases) to enter and induce programmed cell death (apoptosis) in infected or cancerous cells.

Describe the main function of macrophages.

Macrophages are crucial phagocytic cells with several main functions:1. Phagocytosis: They efficiently engulf and digest pathogens, cellular debris, and dead cells through a process called phagocytosis, clearing the body of harmful substances.2. Antigen Presentation: As professional antigen-presenting cells (APCs), they can process antigens and present them to T lymphocytes, contributing to the initiation of adaptive immune responses.3. Cytokine Secretion: They secrete a variety of cytokines and other signaling molecules that modulate inflammation, recruit other immune cells, and influence tissue repair.

What specialized structure can neutrophils release to trap microbes?

Neutrophils, a type of granulocyte, can release specialized web-like structures called Neutrophil Extracellular Traps (NETs). NETs are composed of decondensed chromatin (DNA), histones, and antimicrobial proteins released from the neutrophil's nucleus and granules, which effectively ensnare and kill bacteria, fungi, and other pathogens, preventing their spread.

What is the primary function of mast cells in innate immunity?

The primary function of mast cells in innate immunity is the rapid release of stored pharmacologically active molecules and antimicrobial compounds from their granules, typically in response to allergens or pathogen-associated molecules. A key molecule released is histamine, which induces vasodilation and increased vascular permeability, contributing to the characteristic redness, swelling, and itchiness (inflammation) associated with allergic reactions and immune responses.

State two purposes of the lymphatic system relevant to immunity.

The lymphatic system serves two crucial purposes relevant to immunity:1. It drains excess interstitial fluid (lymph) from tissues and filters out cellular debris, waste products, and pathogens, returning the fluid to the bloodstream and preventing tissue swelling.2. It provides specialized sites for immune cell interaction and activation, particularly in structures like lymph nodes, where naive lymphocytes encounter antigens presented by dendritic cells, leading to the initiation and proliferation of adaptive immune responses.

Approximately how many lymph nodes does an average young adult have?

An average young adult has approximately 400–450 lymph nodes distributed throughout the body, each acting as a filter for lymph and a key site for immune cell activation and interaction.

What is complement?

Complement is a complex system consisting of over 30 soluble plasma proteins, primarily produced by the liver, that circulate in an inactive form. When activated (often by pathogens or immune complexes), these proteins undergo a cascade of proteolytic cleavages, leading to a potent amplification loop that enhances phagocytosis (opsonization), directly lyses pathogens, and promotes inflammation, serving as a critical component of innate immunity.

Which complement component can spontaneously bind bacterial cell walls to initiate the cascade?

The complement component C3 is unique because a small proportion of it can spontaneously hydrolyze in plasma to C3(H_2O), which can then bind to bacterial cell walls. This spontaneous binding initiates the alternative pathway of complement activation, acting as a constant surveillance mechanism to detect and target pathogens even without prior antibody recognition.

What terminal structure formed by complement causes bacterial death?

The terminal structure formed by the activated complement cascade that directly causes bacterial death is the MembrAne Attack Complex (MAC). The MAC is a complex of complement proteins (C5b, C6, C7, C8, and multiple C9 molecules) that inserts into the pathogen's lipid membrane, creating a transmembrane pore. This pore disrupts the membrane integrity, leading to an uncontrolled influx of water and ions, ultimately causing osmotic lysis and death of the target cell.

List three health issues prevented by a healthy gut–lung microbiome connection.

A healthy and balanced gut–lung microbiome connection is increasingly recognized for its role in preventing various health issues, including:1. Chronic kidney disease progression.2. Various cardiovascular complications.3. The development of abnormal immunity or dysregulated immune responses, contributing to conditions like asthma, allergies, and autoimmune diseases.

Why are most transient microbes killed in the stomach?

Most transient microbes ingested orally are killed in the stomach primarily because of its very low pH (highly acidic environment, typically between 1.5 and 3.5). This extreme acidity denatures microbial proteins and disrupts cellular structures. However, some microbes may survive if they pass through quickly (e.g., with food), are protected within fatty foods, or are ingested in very large numbers.

Give one reason why microbiome diversity can vary among individuals.

Microbiome diversity can vary significantly among individuals due to a multitude of factors, including but not limited to:

• Ethnicity and genetics.
• Dietary habits.
• Cultural practices and lifestyle.
• Environmental exposures (e.g., urban vs. rural living, farm exposure).
• Age.
• Medical interventions, such as antibiotic use or mode of birth (vaginal vs. C-section).

What feature of the skin removes roughly 600,000 cells per day?

The continuous shedding of the outer epithelial layer, specifically the stratum corneum (the outermost layer of the epidermis), is a key feature of the skin that results in the loss of approximately 600,000 dead skin cells per day. This shedding serves as a physical defense mechanism, effectively removing adherent microbes and preventing the accumulation of pathogens on the skin surface.