Applied Microbiology Lecture 10 - The Human Immune Response to Microbes

Topic = The Human Immune Response to Microbes

Overview of Host Resistance and Immunity

• Pathogenic Requirements: To successfully cause infection, most pathogens must exhibit the ability to:    

• Overcome physical and chemical surface barriers.     

• Reach underlying tissues.     

• Evade or overcome the mechanisms of the host immune system.

• Host Defences: These are categorized into two major branches:     

• Innate (Nonspecific) Defences: Provide broad, immediate protection.     

• Adaptive (Specific) Immune Responses: Provide targeted, long-term protection.

• The Immune System: Defined as a complex network of cells, tissues, and organs distributed throughout the body. Its primary functions are to:     * Detect foreign substances and microorganisms.     * Work to neutralize, eliminate, and prevent infection.

• Key Definitions:     * Immunity: The inherent ability of the host to resist infection or disease.     * Immunology: The dedicated study of the immune system and its various responses to challenges.

Characteristics of Innate vs. Adaptive Immunity

• Innate (Nonspecific) Immunity:     

• Terminology: Also known as natural or nonspecific immunity.     

• Timing: Acts as the first line of defence and provides a rapid response.     

• Specificity: Responds broadly to microbes and foreign material rather than specific strains.     

• Origin: Present from birth.    

• Memory: Does not generate immunological memory.

• Adaptive (Specific) Immunity:     

• Terminology: Also known as acquired or specific immunity.     

• Timing: Exhibits a slower initial response compared to innate immunity.     

• Specificity: Specifically targets unique pathogens or antigens.     

• Origin: Develops following exposure to a pathogen or via vaccination.     

• Memory: Generates immunological memory, causing the response to become faster and stronger upon repeated exposure.

Innate Immunity: Barriers, Processes, and Cells

• Barriers: Act rapidly (immediately to within hours) to prevent pathogen entry and colonization.  

• Normal Microbiota: Commensal organisms that compete with pathogens.     

• Physical Barriers: Skin, epithelium, and mucous membranes.     

• Chemical Barriers: Antimicrobial peptides, enzymes, and cytokines.

• Innate Processes: These coordinate and amplify responses over hours or days.     

• Inflammation: Immediate response to injury or infection.     

• Opsonisation: Coating microbes to enhance phagocytosis.     

• Cell Communication: Facilitated via cytokines and chemokines to recruit immune cells and link innate and adaptive responses.

• Innate Cells: Rapidly respond (within $6\text{--}12$ hours) to destroy microbes and infected cells.     

• Cell Types: Macrophages, neutrophils, dendritic cells, and natural killer ($NK$) cells.     

• Functions: Phagocytosis, cytotoxic killing, and cell signalling.

Physical Barriers and Mechanical Flushing

• First Line of Defence: These barriers inhibit microbes before infection occurs and work alongside mechanical flushing mechanisms:     

• Tears, saliva, mucus, urination, and defecation.

• Primary Examples of Barriers:     

• Intact skin and mucous membranes.     

• Cilia located in the respiratory tract.     

• Stomach acid and digestive enzymes.     

• Antimicrobial substances found in tears, saliva, sweat, and mucus.

• Influencing Factors: The effectiveness of these barriers is affected by age, genetics, nutrition, overall health, fever, physiology, personal hygiene, and living conditions.

Detailed Physical Barriers: Skin and Mucous Membranes

• The Skin:     

• Mechanical Protection: Consists of multiple tightly packed cell layers containing keratin produced by keratinocytes.     

• Hostile Environment: Conditions that limit microbial growth include:         

• Constant shedding (desquamation) which removes attached organisms.         

• Slightly acidic surface $pH$.         

• High salt conditions created by sweat.         

• Surface dryness.     

• Microbial Competition: Normal microbiota compete with potential pathogens for space and nutrients.     

• Immune Surveillance: Specialized immune cells within the skin detect invading microbes.

• Mucous Membranes:     

• Function: Line body openings and cavities, trapping microbes in mucus to prevent tissue invasion.    

• Antimicrobial Secretions:         

• Lysozyme: An enzyme that breaks down bacterial cell walls by targeting peptidoglycan.         

• Lactoferrin: A protein that binds iron, making it unavailable for bacterial growth.         

• Lactoperoxidase: An enzyme that produces antimicrobial reactive molecules.     

• Specific Defences:         

• Tears flush the eyes.         

• Saliva cleanses the oral cavity.         

• Mucus and cilia move microbes out of the respiratory tract.         

• Secretory $IgA$ ($sIgA$) prevents microbial attachment.

Barrier Defences of the Respiratory and Gastrointestinal Systems

• Respiratory System:     

• Mucociliary System: Inhaled microbes are trapped in mucus and moved away from the lungs by cilia, a process known as the "mucociliary escalator."     

• Removal Mechanism: Trapped microbes are expelled via coughing or sneezing, or swallowed into the stomach.     

• Alveolar Macrophages: Phagocytic cells in the alveoli that destroy microbes reaching the lower respiratory tract.    

• Airflow Dynamics: Turbulent airflow helps deposit particles onto mucosal surfaces.

• Gastrointestinal ($GI$) Tract:    

• Stomach: Gastric acid ($HCl$) destroys many ingested microbes.     

• Intestines: Survival is inhibited by digestive enzymes and bile.    

• Mechanical Protection: Peristalsis and the shedding of epithelial cells remove microorganisms.     

• Gut-Associated Lymphoid Tissue ($GALT$): Monitors for pathogens and activates responses.     

• Paneth Cells: Located in the intestinal epithelium; they produce lysozyme and defensins (also known as cryptdins or cryptins).

Chemical Mediators and Antimicrobial Peptides

• Chemical Barriers: Includes gastric acid, lysozyme, urea, complement proteins, and cytokines.

• Antimicrobial Peptides ($AMPs$): Cationic peptides found in blood, lymph, and mucosal secretions that damage microbial membranes.     

• First Class: Cathelicidins: Linear $\alpha$-helical peptides produced by neutrophils, respiratory/urogenital epithelial cells, and alveolar macrophages. They are produced from a single precursor and cleaved into active fragments.     

• Second Class: Defensins:         

• $\alpha$-defensins: Produced by neutrophils, Paneth cells, and intestinal/respiratory epithelial cells.        

• $\beta$-defensins: Produced mainly by epithelial cells.     

• Third Class: Large Amino-Acid Enriched Peptides: Contain repeating structural motifs.        

• Histatins: Present in saliva; possess significant antifungal activity.

• Bacteriocins: Antimicrobial peptides produced by the normal microbiota to inhibit closely related bacterial species.    

• Colicins: Produced by Escherichia coli.     

• Lantibiotics: Produced by Gram-positive bacteria.

The Complement System

• Composition: A cascade of over $30$ serum proteins that augment (complement) the antibacterial activity of antibodies.

• Activation Properties:     

• Produced in inactive forms and activated via enzymatic cleavage.     

• Must be activated in a specific, sequential order (cascade).     

• All three pathways converge at the activation of protein $C3$.

• The Three Pathways:     

• Classical Pathway: Activated by antigen-antibody complexes; links innate to adaptive immunity; typically slower than other pathways.     

• Lectin (Mannose-Binding Lectin/$MBL$) Pathway: Initiated when lectins bind to mannose-containing carbohydrates on microbial surfaces.     

• Alternative Pathway: Provides rapid, nonspecific defense against bacteria and fungi in the bloodstream; triggered by repetitive structures on microbial surfaces without requiring antibodies.

• Major Outcomes of Complement Activation:     

• Opsonisation: Microbes are coated with opsonins (e.g., $C3b$) to enhance recognition by phagocytes.     

• Chemotaxis and Inflammation: Proteins such as $C3a$ and $C5a$ recruit neutrophils and macrophages.     

• Cytolysis (Membrane Attack Complex): Direct killing of microbes.

• The Membrane Attack Complex ($MAC$):     

• Composition: Composed of $C5b$, $C6$, $C7$, $C8$, and $10\text{--}19$ molecules of polymerized $C9$

• Mechanism: Forms a transmembrane pore in the target cell membrane, causing uncontrollable movement of water and ions, leading to osmotic lysis.     

• Effectiveness: Highly effective against Gram-negative bacteria; less effective against Gram-positive bacteria and fungi due to cell wall differences.

Cytokines and Cell Communication

• Definition: Soluble proteins or glycoproteins released by immune cells that act as signaling molecules to coordinate communication.

• Mechanism: Bind to specific receptors to activate intracellular pathways that regulate gene transcription and protein production.

• Major Categories:     

• Monokines: Released by mononuclear phagocytes (macrophages).    

• Lymphokines: Released by $T$ lymphocytes.     

• Interleukins: Released by one leukocyte to act on other leukocytes.     

• Colony-Stimulating Factors ($CSFs$): Stimulate growth and differentiation of immature leukocytes in the bone marrow.

• Functional Groups:     

• Innate Response Regulators: Promote inflammation (e.g., $IL-1$, $IL-6$, $IL-8$, $IFN-\alpha$, $IFN-\beta$, $TNF-\alpha$).     

• Adaptive Immunity Regulators: Coordinate activation/differentiation (e.g., $IL-2$, $IL-4$, $IL-12$, $IFN-\gamma$).     

• Hematopoiesis Stimulators: Promote blood cell production (e.g., $IL-3$, $IL-7$, $CSF-1$, $CSF-2$, $CSF-3$).

Blood Cell Composition and Roles

• Normal Adult Blood Cell Counts (per $\text{mm}^3$ of blood):     

• Red Blood Cells ($RBCs$): Approximately $5,000,000$; responsible for oxygen transport.     

• Platelets: Approximately $250,000$; essential for clotting and wound repair.     

• White Blood Cells ($WBCs$): Total approximately $7,400$.         

• Neutrophils: $4,320$ ($60\%$); major phagocytes for acute bacterial infections.         

• Lymphocytes: $2,160$ ($30\%$); includes $B$ cells, $T$ cells, and $NK$ cells.         

• Monocytes: $430$ ($6\%$); differentiate into macrophages and dendritic cells.         

• Eosinophils: $215$ ($3\%$); defense against parasites and allergic responses.        

• Basophils: $70$ ($1\%$); release histamine for inflammation.

• Mast Cells:     

• Found in connective tissues and near body surfaces; mature within tissues.     

• Granules contain histamine, heparin, and inflammatory mediators.     

• Activation causes degranulation, increasing blood vessel permeability (redness, swelling, heat, itching).

Granulocytes, Monocytes, and Macrophages

• Granulocytes: Distinguished by irregular/multilobed nuclei and cytoplasmic granules.     

• Neutrophils (Polymorphonuclear Neutrophils/$PMNs$): Highly phagocytic; migrate from blood to infection sites to kill microbes with lytic enzymes and reactive oxygen metabolites.     

• Eosinophils: Defend against protozoan and helminth (worm) parasites.     

• Basophils: Nonphagocytic; release vasoactive mediators like histamine, prostaglandins, serotonin, and leukotrienes.

• Mononuclear Cells:     

• Monocytes: Circulate for roughly $8$ hours before migrating into tissues.     

• Macrophages: Large, tissue-resident phagocytes. They possess Pathogen Recognition Receptors ($PRRs$) to recognize Pathogen-Associated Molecular Patterns ($PAMPs$).         

• Tissue-Specific Names: Alveolar macrophages (lungs), Kupffer cells (liver), microglia (brain).

Dendritic Cells and Antigen Presentation

• Dendritic Cells: Use long, branching neuron-like appendages to sample the environment. Found in blood, skin, and mucosal membranes.

• Link Between Systems: They act as sentinel cells, capturing and processing antigens to present to $T$ lymphocytes, thereby bridging innate and adaptive immunity.

• Antigen-Presenting Cells ($APCs$): Macrophages and dendritic cells serve as $APCs$.     

• Process: Microbes are phagocytosed and processed into fragments; fragments are displayed on the surface bound to Major Histocompatibility Complex ($MHC$) molecules.

Antigens and the Major Histocompatibility Complex ($MHC$)

• Antigens: Large, complex foreign molecules that elicit an immune response.     

• Epitopes: Specific regions recognized by an antibody or receptor.     

• Valence: The number of epitopes on a single antigen.     

• Binding Strength: Affinity is the strength between one site and one epitope; Avidity is the overall strength across multiple sites.

• MHC (Human Leukocyte Antigen/$HLA$ Complex): Located on Human Chromosome $6$; essential for self vs. non-self recognition.     

• MHC Class I: Found on almost all nucleated cells; present endogenous (intracellular) antigens; monitored by cytotoxic $T$ cells and $NK$ cells.     

• MHC Class II: Found only on $APCs$ (macrophages, dendritic cells, $B$ cells); present exogenous (extrinsic) antigens to helper $T$ cells.     

• MHC Class III: Encode secreted immune proteins.

Natural Killer ($NK$) Cells

• Function: Large granular, non-phagocytic lymphocytes that kill virus-infected and malignant cells.

• Recognition Mechanisms:     

• Missing Self: Detect cells with reduced or missing $MHC\text{ I}$.     

• Antibody-Dependent Cell-Mediated Cytotoxicity ($ADCC$): Bind to antibodies already coating target cells.

• Killing Mechanism: Attachment forms a "lytic cleft." The $NK$ cell releases Perforin (forms pores) and Granzymes (trigger apoptosis).

Lymphoid Organs and Tissues

• Primary Lymphoid Organs: Sites of development and maturation.     

• Bone Marrow: Site of hematopoiesis and $B$ cell maturation.     

• Thymus: Site where immature $T$ cells proliferate and undergo Thymic Deletion (removal of self-reactive cells).

• Secondary Lymphoid Organs: Sites where mature lymphocytes encounter antigens.     

• Spleen: The most highly organized lymphoid organ; filters blood for microbes; site where antigens are presented to lymphocytes.     

• Lymph Nodes: Filter lymph draining from tissues; sites of $B$ cell differentiation into plasma and memory cells.     

• Diffuse Lymphoid Tissues:         

• Skin-Associated Lymphoid Tissue ($SALT$): Features Langerhans cells and intraepidermal lymphocytes.         

• Mucosal-Associated Lymphoid Tissue ($MALT$): Protects mucosal surfaces; includes $GALT$ (gut), $BALT$ (bronchial), and urogenital $MALT$.

Phagocytosis and Pathogen Recognition

• Mechanism: Recognition occurs via two paths:     

• Opsonin-Independent (Non-opsonic): Direct recognition of $PAMPs$ using $PRRs$.     

• Opsonin-Dependent (Opsonic): Recognition of microbes coated in antibodies or complement.

• Pathogen-Associated Molecular Patterns ($PAMPs$): Conserved microbial structures (e.g., $LPS$ in Gram-negative bacteria, peptidoglycan in Gram-positive bacteria).

• Toll-Like Receptors ($TLRs$): A major class of $PRRs$ located on cell surfaces or intracellular membranes. Activation triggers cytokine production and immune cell recruitment.

Adaptive Immunity: T Cells and B Cells

• T Cells (Cell-Mediated Immunity):     

• Helper $T$ Cells ($T_H$; $CD4+$): Activated by $MHC\text{ II}$. Subsets include $T_H 1$ (macrophage activation), $T_H 2$ (B cell stimulation), $T_H 17$ (inflammation), $T_{FH}$ (antibody regulation), and $Treg$ (suppression/tolerance).    

• Cytotoxic $T$ Lymphocytes ($CTL$; $CD8+$): Activated by $MHC\text{ I}$; destroy infected host cells.     

• Activation: Requires Signal 1 (Antigen recognition by $TCR$) and Signal 2 (Co-stimulatory signal, e.g., $CD28-B7$ interaction).

• B Cells (Humoral Immunity):     

• Activation Pathways:         

• T-Dependent: Requires $T_H$ co-stimulation; produces high-affinity antibodies and memory cells.         

• T-Independent: Triggered by repetitive antigens (e.g., $LPS$); produces weaker responses with little memory.     

• Differentiation: B cells become Plasma Cells (secrete antibodies) or Memory B cells.

• Superantigens: Proteins (e.g., $TSST-1$) that bypass antigen specificity by directly linking $MHC\text{ II}$ and $TCR$, causing a "cytokine storm" and potentially circulatory shock.

Antibodies (Immunoglobulins)

• Structure: Flexible Y-shaped glycoprotein consisting of two identical heavy chains and two identical light chains connected by disulfide bonds.    

• Fab Region: Tips of the Y; contains variable ($V$) regions for specific antigen binding.     

• Fc Region: Stem of the Y; consists of constant ($C$) regions; binds to immune cells and activates complement.

• Antibody Classes:    

• $IgG$: $80\%$ of serum $Ig$; crosses the placenta; primary goal of vaccination.    

• $IgM$: First antibody in primary response; pentameric structure excellent for agglutination.    

• $IgA$: Found in mucosal secretions (tears, breast milk) as secretory $IgA$ ($sIgA$).    

• $IgD$: Part of the $B$ cell receptor complex; signals $B$ cells to start production.    

• $IgE$: Lowest concentration; triggers mast cell degranulation in allergies and parasitic infections.

• Antibody Response Dynamics:    

• Primary Response: Lag phase of several days; $IgM$ appears first.    

• Secondary Response: Driven by memory cells; faster, stronger, and dominated by high-affinity $IgG$.

• Diversity Generation: Achieved through combinatorial gene rearrangement, junctional diversity, and somatic hypermutation.

Consequences of Antigen-Antibody Binding

• Neutralisation: Antibodies block toxins or viruses from interacting with host cells.

• Opsonisation: Antibodies create a bridge between the pathogen and phagocyte via the $Fc$ receptor.

• Complement Activation: Triggering the classical pathway to cause lysis.

• Precipitation: Soluble antigens are cross-linked into complexes that settle out of solution.

• Agglutination: Cells or particles are cross-linked, making them easier for phagocytes to ingest.

The Inflammatory Response

• Five Cardinal Signs: Redness (rubor), warmth (calor), pain (dolor), swelling (tumor), and altered function (functio laesa).

• Acute Inflammation Phases:     

• Margination: Leukocytes move toward vessel walls.     

• Diapedesis (Extravasation): Leukocytes squeeze through endothelial cells.     

• Chemotaxis: Movement toward injury signals (histamine, bradykinin).

• Chemical Mediators:     

• Selectins: Adhesion molecules that slow down leukocytes.     

• Integrins: Receptors on leukocytes for firm attachment to vessel walls.

• Chronic Inflammation: Development of permanent tissue damage, fibrosis, and Granuloma formation (walled-off collections of epithelioid macrophages and giant cells).