Exhaustive Study Notes on Cells, Tissues, and Innate Immunity

Principles and Overview of the Immune System

• Primary Challenges of Protective Immunity:

  • The immune system must rapidly respond to small numbers of diverse microbes introduced at any anatomic site.
  • In adaptive immunity, very few naive lymphocytes specifically recognize any single antigen.
  • Effector mechanisms (antibodies and effector T cells) must locate and eliminate microbes at tissue sites distant from where the immune response was originally induced.
  • These challenges are met through rapid cell activation, specialized structural organization in lymphoid tissues, and continuous cell recirculation.

• Surface Markers and Cluster of Differentiation (CD) Nomenclature:

  • Membrane proteins serve as phenotypic markers to identify and distinguish distinct cell populations (e.g., CD4 on helper T cells, CD8 on cytotoxic T lymphocytes).
  • Specific markers are defined using labeled monoclonal antibodies, which bind selectively to target surface proteins or carbohydrates.
  • The Cluster of Differentiation (CD) system provides uniform naming for cell surface molecules defined by groups (clusters) of monoclonal antibodies.
  • CD markers function as active participants in cell signaling, adhesion, and activation, and serve as targets for therapeutic antibodies in cancer and inflammatory diseases.

• Normal Blood Cell Counts in Adult Humans:

  • White blood cells (leukocytes): Mean number of $7400/\nu\text{L}$ (Normal range: $4500\text{--}11,000/\nu\text{L}$).
  • Neutrophils: Mean number of $4400/\nu\text{L}$ ($40\text{--}60\text{\%}$ of total leukocytes).
  • Eosinophils: Mean number of $200/\nu\text{L}$ ($1\text{--}4\text{\%}$ of total leukocytes).
  • Basophils: Mean number of $40/\nu\text{L}$ ($<1\text{\%}$ of total leukocytes).
  • Lymphocytes: Mean number of $2500/\nu\text{L}$ ($20\text{--}40\text{\%}$ of total leukocytes).
  • Monocytes: Mean number of $300/\nu\text{L}$ ($2\text{--}8\text{\%}$ of total leukocytes).

Phagocytes and Other Myeloid Effector Cells

• Neutrophils (Polymorphonuclear Leukocytes, PMNs):
  • Most abundant circulating white blood cells; principal cells mediating acute inflammatory reactions.
  • Morphologically spherical cells ($12\text{--}15\nu\text{m}$ in diameter) with numerous membranous projections and a nucleus divided into $3\text{--}5$ connected lobules.
  • Granule Types:
  • Specific granules: Majority of granules; contain enzymes such as lysozyme, collagenase, and elastase; do not stain strongly with acidic or basic dyes.
  • Azurophilic granules: Stained by azure A; contain myeloperoxidase, defensins, and cathelicidins.
  • Derived from bone marrow myeloid precursors driven by granulocyte colony-stimulating factor (G-CSF) and granulocyte-macrophage colony-stimulating factor (GM-CSF).
  • Adult human production exceeds $1 \times 10^{11}$ neutrophils per day.
  • Circulate in blood for a few hours up to $5\text{ days}$; function for only $1\text{--}2\text{ days}$ after entering inflamed tissues before dying.
  • Major functions: Rapid phagocytosis and destruction of opsonized microbes and necrotic cell debris in phagolysosomes; secretion of granule contents; extrusion of nuclear contents to form neutrophil extracellular traps (NETs).

• Mononuclear Phagocyte System:
  • Monocytes:
  • Circulating phagocytes ($10\text{--}15\nu\text{m}$ in diameter) with bean-shaped nuclei and finely granular cytoplasm containing lysosomes, phagocytic vacuoles, and cytoskeletal filaments.
  • Mature in bone marrow under the influence of monocyte colony-stimulating factor (M-CSF) and circulate for $1\text{--}7\text{ days}$.
  • Express MHC class II, CD11b, and CD86 in humans; CD115, CD11b, and CD64 in mice.
  • Classical (Inflammatory) Monocytes: Comprise $90\text{\%}\text{--}95\text{\%}$ of blood monocytes in humans; produce inflammatory mediators, highly phagocytic, rapidly recruited to sites of infection or tissue damage.
  • Nonclassical Monocytes: Comprise $5\text{\%}\text{--}10\text{\%}$ of blood monocytes; crawl along endothelial surfaces (patrolling), scavenge luminal microparticles, eliminate circulating microbes, and assist in endothelial repair.
  • Macrophages:
  • Monocyte-derived macrophages arise during inflammatory responses and survive for days or weeks.
  • Tissue-resident macrophages develop during fetal life from primitive progenitors in the yolk sac or fetal liver and persist for years via self-renewal.
  • Specialized tissue-resident populations: Kupffer cells (liver sinusoids), alveolar macrophages (lung alveoli), microglial cells (central nervous system), sinusoidal macrophages (spleen), myocardial macrophages (heart), lamina propria macrophages (gut), dermal macrophages (skin).

• Comparative Properties of Neutrophils vs. Macrophages:

  • Origin: Neutrophils arise from bone marrow hematopoietic stem cells (HSCs); Macrophages arise from bone marrow HSCs (inflammatory) or fetal yolk sac/fetal liver (tissue-resident).
  • Surface Markers (Human): Neutrophils express CD11b, CD18, CD15, CD66b, CD10; Macrophages express CD11b, CD14, CD68, CD16, CD64, CCR5.
  • Life Span: Neutrophils live $1\text{--}2\text{ days}$ in tissues; Inflammatory macrophages live days to weeks; Tissue-resident macrophages live years.
  • Enzymatic & Functional Responses: Neutrophils use rapid, short-lived activation of preformed enzymes via cytoskeletal rearrangements; Macrophages rely on slower, prolonged responses driven by gene transcription and new protein expression.
  • Reactive Species: Neutrophils rapidly induce reactive oxygen species (ROS) via phagocyte oxidase assembly (respiratory burst); Macrophages prominently induce nitric oxide (NO) via inducible nitric oxide synthase (iNOS) transcription.
  • Degranulation: Major response in neutrophils; not prominent in macrophages.
  • Cytokine Production: Low levels per cell in neutrophils; major function producing large amounts per cell in macrophages.
  • NET Formation: Rapidly induced in neutrophils; absent in macrophages.
  • Pyroptosis: Absent in neutrophils; prominent in macrophages via caspase-1 activation.

• Functional Roles of Macrophages:

  • Phagocytosis and Killing: Ingestion of microbes into phagosomes, fusion with lysosomes, generation of toxic ROS and reactive nitrogen species, enzymatic protein digestion.
  • Sentinel & Cytokine Secretion: Secretion of proinflammatory cytokines (TNF, IL-1, IL-6, IL-12) to recruit leukocytes and initiate adaptive immunity.
  • Clearance of Apoptotic Cells (Efferocytosis): Recognition and removal of necrotic tissue debris and apoptotic host cells (e.g., via TIM4, CD300b, Mer) without triggering inflammation.
  • Inhibition of Self-Phagocytosis: Healthy cells express CD47 ("don't eat me" signal), which engages the inhibitory receptor SIRP$\nu\text{a}$ on macrophages to prevent phagocytosis.
  • Antigen Presentation: Display of protein fragments to T cells in cell-mediated immunity; transfer of antigens to B cells and FDCs by subcapsular sinus and marginal zone macrophages.
  • Tissue Repair and Fibrosis: Production of vascular endothelial growth factor (angiogenesis) and TGF-$\beta$ (collagen synthesis and fibrosis).
  • Phenotypic Subsets: Classically activated (M1) macrophages mediate microbicidal activity and inflammation; alternatively activated (M2) macrophages promote tissue remodeling, repair, and anti-inflammatory functions.

• Mast Cells, Basophils, and Eosinophils:

  • Mast Cells:
  • Bone marrow-derived cells resident in tissues (skin and mucosal epithelia) adjacent to blood vessels and nerves; dependent on stem cell factor (SCF / c-KIT ligand).
  • Cytoplasm packed with membrane-bound granules containing histamine and acidic proteoglycans (staining dark blue/purple).
  • Express high-affinity IgE receptors (Fc$\nu\text{RI}$); activated by IgE-antigen crosslinking or direct TLR engagement to release histamine, lipid mediators (leukotrienes, prostaglandins), and cytokines.
  • Basophils:
  • Blood granulocytes comprising $<1\text{\%}$ of blood leukocytes; mature in bone marrow from distinct progenitors.
  • Synthesize structural and functional mediators similar to mast cells; express IgE receptors and circulate in blood.
  • Eosinophils:
  • Granulocytes with segmented nuclei and cytoplasmic granules containing basic proteins (major basic protein, eosinophil cationic protein) that stain red with acidic dyes (eosin).
  • Maturation driven by GM-CSF, IL-3, and IL-5; present in mucosal linings of respiratory, gastrointestinal, and genitourinary tracts.
  • Express Fc receptors for IgA and IgG, TLRs, and IL-5 receptors; mediate toxicity against helminthic parasites and participate in allergic inflammation.

Dendritic Cell Lineages and Sentinel Functions

• General Properties and Functions:
  • Bone marrow-derived cells characterized by long, thin membranous projections.
  • Widely distributed in epithelia, mucosal surfaces, and organ parenchyma as tissue sentinels.
  • Dependent on the cytokine FLT3L for development; express CD11c and MHC class II molecules.
  • Capture microbial antigens, migrate via afferent lymphatics to secondary lymphoid organs, and present peptide-MHC complexes to activate naive T cells or induce tolerance.

• Dendritic Cell Subsets:
  • Conventional DCs (cDCs / Classical DCs):
  • Major DC population in epithelia and lymphoid organs; derived from common DC precursors (pre-cDCs) in bone marrow.
  • cDC2 (Major Subset): Expresses transcription factors IRF4 and ZEB2; surface markers BDCA-1 (CD1c) and CD11c; highly efficient at capturing exogenous antigens and presenting to CD4+ T cells; produces IL-6 and IL-23.
  • cDC1 (Cross-Presenting Subset): Expresses transcription factors IRF8 and BATF3; surface markers BDCA-3 (CD141), CLEC9A, XCR1, and CD11c; specialized for antigen cross-presentation to naive CD8+ T cells and induction of Th1 responses; produces IL-12.
  • Plasmacytoid DCs (pDCs):
  • Morphologically resemble plasma cells; express transcription factors IRF4, IRF8, and E2-2; surface markers BDCA-2 (CD303), BDCA-4 (CD304), and CD123.
  • Express high levels of endosomal TLR7 and TLR9; serve as the primary source of type I interferons (IFN-$\nu\text{a}$, IFN-$\nu\text{b}$) in response to viral infections.
  • Monocyte-Derived DCs:
  • Arise from blood monocytes recruited into tissue inflammatory sites; express CD11c, CD11b, CCR2, CD14, and transcription factors MAFB and KLF4.
  • Langerhans Cells:
  • Resident DCs of the epidermal layer of skin; derived developmentally from embryonic yolk sac and fetal liver precursors.
  • Express transcription factors ID2 and RUNX3; surface markers Langerin (CD207), EpCAM, CD1a, and CD11b.
  • Contain unique tennis racket-shaped cytoplasmic organelles visible by electron microscopy called Birbeck granules.
  • Follicular Dendritic Cells (FDCs):
  • Non-hematopoietic stromal cells located in B-cell follicles of secondary lymphoid organs; do not express MHC class II or present antigens to T cells.
  • Express Fc receptors and complement receptors to display intact immune complexes to B cells during germinal center reactions.

Lymphocytes: Subsets, Development, and Life History

• General Repertoire and Distribution:

  • Unique cells of adaptive immunity; express clonally distributed antigen receptors generated by somatic recombination of gene segments.
  • Total body count in a healthy adult is approximately $5 \times 10^{11}$ lymphocytes.
  • Tissue distribution: $2\text{\%}$ in blood, $4\text{\%}$ in skin, $10\text{\%}$ in bone marrow, $15\text{\%}$ in mucosal lymphoid tissues, $65\text{\%}$ in secondary lymphoid organs (spleen and lymph nodes).

• Classes and Subsets of Lymphocytes:

  • B Lymphocytes:
  • Produce membrane-bound immunoglobulins (Ig) as antigen receptors; differentiate into antibody-secreting plasma cells.
  • Follicular B Cells: Most abundant ($5\text{--}20\text{\%}$ of blood, $20\text{--}25\text{\%}$ of lymph node, $40\text{--}45\text{\%}$ of spleen lymphocytes); express $\text{CD19}^+$, $\text{IgD}^{\text{high}}$, $\text{IgM}^+$, $\text{CD23}^+$; recognize diverse molecules and yield high-affinity antibodies and memory B cells.
  • Marginal Zone B Cells: Noncirculating cells in the spleen ($2\text{--}3\text{\%}$ of blood, $7\text{--}10\text{\%}$ of spleen); express $\text{CD19}^+$, $\text{IgM}^{\text{high}}$, $\text{CD23}^-$, $\text{CD27}^{\text{high}}$, $\text{CD21}^{\text{high}}$; possess limited receptor diversity targeting polysaccharide and lipid antigens.
  • B-1 Cells: Resident in mucosal tissues and peritoneal/pleural cavities ($1\text{--}3\text{\%}$ of blood); express $\text{IgM}^{\text{high}}$, $\text{CD27}^+$, $\text{CD43}^+$; produce low-affinity natural antibodies with restricted specificity.
  • T Lymphocytes:
  • Mediators of cellular immunity; express $\nu\text{a}\nu\text{b}$ or $\nu\text{g}\nu\text{d}$ T-cell receptors (TCRs) associated with CD3.
  • CD4+ Helper T Lymphocytes: Express $\text{CD3}^+$, $\text{CD4}^+$, $\text{CD8}^-$ ($35\text{--}60\text{\%}$ of blood lymphocytes); recognize peptide-MHC class II complexes; secrete cytokines to activate B cells, macrophages, and inflammation.
  • CD8+ Cytotoxic T Lymphocytes (CTLs): Express $\text{CD3}^+$, $\text{CD4}^-$, $\text{CD8}^+$ ($15\text{--}40\text{\%}$ of blood lymphocytes); recognize peptide-MHC class I complexes; kill virus-infected and tumor cells.
  • Regulatory T Cells: Express $\text{CD3}^+$, $\text{CD4}^+$, $\text{CD25}^+$, $\text{FOXP3}^+$ ($1\text{--}2\text{\%}$ of blood, $10\text{\%}$ of node/spleen lymphocytes); suppress immune responses and maintain self-tolerance.
  • Natural Killer T (NKT) Cells: Express $\nu\text{a}\nu\text{b}$ TCRs with limited diversity recognizing glycolipids presented by CD1d; express CD56, CD16, CD3.
  • Mucosa-Associated Invariant T (MAIT) Cells: Express $\nu\text{a}\nu\text{b}$ TCRs with limited specificity for bacterial riboflavin metabolites presented by MR1; comprise $5\text{\%}$ of blood T cells and up to $50\text{\%}$ of liver lymphocytes.
  • $\nu\text{g}\nu\text{d}$ T Lymphocytes: Express $\nu\text{g}\nu\text{d}$ TCRs with limited protein and nonprotein specificity; localized in epithelia.

• Stages in Lymphocyte Life History:
  • Naive Lymphocytes:
  • Immunologically inexperienced resting cells in the $G_0$ cell cycle stage ($8\text{--}10\nu\text{m}$ in diameter) with dense heterochromatin and scant cytoplasm.
  • Maintain basal energy via oxidative phosphorylation and fatty acid oxidation.
  • Lifespan is $1\text{--}3\text{ months}$; survival requires tonic antigen receptor signals plus specific cytokines: IL-7 for naive T cells and B cell-activating factor (BAFF) for naive B cells.
  • Total count is maintained via homeostatic proliferation driven by self-antigen recognition and IL-7.
  • Effector Lymphocytes:
  • Antigen-activated large lymphocytes or lymphoblasts ($10\text{--}12\nu\text{m}$ in diameter) utilizing aerobic glycolysis.
  • Effector T cells express activation markers (CD25 / IL-2R) and altered adhesion molecules/chemokine receptors.
  • Plasma Cells: Terminal B-lineage effector cells with eccentric nuclei, cartwheel chromatin, abundant rough endoplasmic reticulum, and prominent perinuclear Golgi apparatus; secrete thousands of antibody molecules per second; lack surface antigen receptors; resident in bone marrow and mucosal tissues.
  • Plasmablasts: Circulating, proliferating antibody-secreting cells expressing CD19 and low CD20.
  • Memory Lymphocytes:
  • Quiescent or slowly cycling long-lived cells generated during infection; express high IL-7R (CD127).
  • Human naive T cells express CD45RA, whereas memory and activated T cells express CD45RO.
  • Proportion of memory T cells shifts with age: $<5\text{\%}$ of blood T cells in newborns to $\nu\text{g}50\text{\%}$ in adults.

Architecture and Microenvironments of Lymphoid Tissues

• Primary (Generative) Lymphoid Organs:
  • Bone Marrow:
  • Site of hematopoiesis and B-cell maturation.
  • Hematopoiesis shifts during gestation: yolk sac/para-aortic mesenchyme $\nu\text{r}$ liver (months $3\text{--}4$) $\nu\text{r}$ bone marrow (birth onwards). Restricted to flat bones (sternum, vertebrae, iliac bones, ribs) by puberty.
  • Multipotent, self-renewing Hematopoietic Stem Cells (HSCs) express $\text{CD34}^+$ and $\text{c-KIT}^+$ ($\text{LIN}^-$).
  • Hematopoietic Cytokines:
    • Stem Cell Factor (c-KIT ligand) ($24\text{ kD}$): Acts on HSCs to drive all lineages.
    • IL-7 ($25\text{ kD}$): Bone marrow stromal cells; drives T cell and lymphoid progenitor development.
    • IL-3 ($20\text{--}26\text{ kD}$): T cells; acts on immature progenitors.
    • GM-CSF ($18\text{--}22\text{ kD}$): Granulocyte and monocyte progenitor maturation.
    • M-CSF ($70\text{--}90\text{ kD}$ dimer): Monocyte progenitor maturation.
    • G-CSF ($19\text{ kD}$): Granulocyte progenitor maturation.
    • FLT-3 Ligand ($30\text{ kD}$): Drives classical and plasmacytoid DC and B-cell development.
  • Thymus:
  • Bilobed organ in anterior mediastinum; site of T-cell maturation; involutes after puberty.
  • Outer cortex packed with immature thymocytes; inner medulla contains mature thymocytes, macrophages, DCs, and Hassall's corpuscles (whorls of keratinized epithelial cells).
  • Cortical epithelial cells produce IL-7; Medullary Thymic Epithelial Cells (MTECs) include mimetic cells expressing tissue-restricted self-antigens for central tolerance.
  • DiGeorge Syndrome: Genetic deletion preventing branchial pouch development, resulting in thymic aplasia and T-cell deficiency.
  • Nude Mouse Model: Mutation in Tbx1 transcription factor causing failure of thymic epithelium and hair follicle development.

• Secondary (Peripheral) Lymphoid Organs & Lymphatic System:
  • Lymphatic System:
  • Blind-ended lymphatic capillaries lined by overlapping endothelial cells absorb interstitial tissue fluid (lymph).
  • Afferent lymphatics enter lymph nodes; efferent lymphatics exit and merge into the thoracic duct ($2\text{ L/day}$ fluid returned to superior vena cava) or right lymphatic duct.

• Lymph Nodes:
  • Encapsulated secondary organs ($

u ext{g}500$ in humans) filtering lymph-borne antigens. * Subcapsular sinus floor lined by marginal reticular cells; contains subcapsular sinus macrophages that capture particulate antigens. * Outer Cortex: B-cell rich zone composed of primary follicles (naive B cells) and secondary follicles (germinal centers with dark zone centroblasts and light zone centrocytes). * Paracortex (T-Cell Zone): T-cell rich region containing high endothelial venules (HEVs) and mature DCs. * Organogenesis: Dependent on fetal Lymphoid Tissue-Inducer (LTi) cells expressing membrane lymphotoxin $\nu\text{a}_1\nu\text{b}_2$ trimers acting on stromal cells.

• Chemokine-Driven Compartmentalization in Nodes:
  • T-cell zone Fibroblastic Reticular Cells (FRCs) express Podoplanin and secrete CCL19 and CCL21. Naive T cells and DCs express CCR7, driving their co-localization in the paracortex.
  • B-cell zone FRCs and FDCs secrete CXCL13. Naive B cells express CXCR5, driving their migration into follicles.
  • FRC Conduits: Tubular structures ($0.2\text{--}3\nu\text{m}$ in diameter) composed of collagen bundles surrounded by fibrillin microfibers, laminin basement membrane, and an FRC sheath. Transport low-molecular-weight soluble antigens from subcapsular sinus directly to paracortical DCs.

• Spleen:
  • Highly vascularized organ ($150\text{ g}$ in adult) filtering blood-borne antigens and damaged erythrocytes.
  • Red Pulp: Blood-filled vascular sinusoids lined by macrophages that phagocytose senescent RBCs, immune complexes, and opsonized encapsulated bacteria (e.g., Pneumococcus, Meningococcus).
  • White Pulp: Lymphocyte-rich regions surrounding central arterioles. Includes Periarteriolar Lymphoid Sheaths (PALS, T-cell zone), B-cell follicles, marginal sinus, and Marginal Zone (populated by specialized B cells and macrophages).

• Mucosal and Cutaneous Immune Systems:
  • Unencapsulated structures (Mucosa-Associated Lymphoid Tissue, MALT) and diffuse barrier populations providing specialized defense while maintaining tolerance to commensal microbes.

Molecular Recognition in Innate Immunity

• PAMPs vs. DAMPs:

  • Pathogen-Associated Molecular Patterns (PAMPs): Shared microbial structures essential for survival/infectivity. Examples:
  • Nucleic Acids: ssRNA (viruses), dsRNA (viruses), unmethylated CpG motifs (bacteria, viruses).
  • Proteins: Pilin (bacteria), Flagellin (motile bacteria).
  • Lipids: Lipopolysaccharide (LPS, gram-negative bacteria), Lipoteichoic acid (gram-positive bacteria).
  • Carbohydrates: Mannan (fungi, bacteria), $\nu\text{b}$-glucans (fungi).
  • Damage-Associated Molecular Patterns (DAMPs): Endogenous host molecules released during non-apoptotic cell injury or stress. Examples:
  • Stress proteins: Heat shock proteins (HSPs).
  • Nuclear/Cytosolic proteins: High-mobility group box 1 (HMGB1), histones.
  • Crystals: Monosodium urate, calcium pyrophosphate.
  • Extracellular/Mitochondrial products: Extracellular ATP, formylated peptides, hyaluronan fragments, uric acid.

• Key Comparative Features of Innate vs. Adaptive Immunity:

  • Specificity: Innate targets shared structural patterns (PAMPs/DAMPs, $\nu\text{g}1000$ patterns); Adaptive targets specific antigenic determinants ($>10^7$ antigens).
  • Receptors: Innate uses germline-encoded, nonclonal invariant Pattern Recognition Receptors (PRRs, $<100$ types); Adaptive uses somatically rearranged, clonally distributed receptors (Ig and TCR, millions of variations).
  • Self/Nonself Discrimination: Innate based on lack of target ligands on healthy cells or active self-protective regulatory proteins; Adaptive based on central/peripheral selection eliminating self-reactive clones.
  • Kinetics: Innate is immediate/rapid; Adaptive requires days for expansion and differentiation.
  • Memory: Innate shows minimal/no specific memory (trained immunity observed in some NK/macrophages); Adaptive displays secondary recall responses.

• Evolutionary Origins:

  • Innate immunity evolved $\nu\text{g}750 \text{ million years ago}$ in early multicellular organisms (defensins, TLRs, NF-$\nu\text{b}$ signaling conserved in plants, insects, and vertebrates).
  • Adaptive immunity appeared $\nu\text{g}350\text{--}500 \text{ million years ago}$ in jawed vertebrates.
  • Human genome encodes $\nu\text{g}850$ innate immunity-related genes vs. $\nu\text{g}575$ adaptive immunity genes.

Cellular Pattern Recognition Receptors (PRRs)

• Toll-Like Receptors (TLRs):
  • Homodimeric or heterodimeric type I transmembrane glycoproteins with extracellular Leucine-Rich Repeat (LRR) modules ($16\text{--}28$ repeats) and cytoplasmic Toll/IL-1 Receptor (TIR) domains.
  • Human genome contains 10 functional TLRs (TLR1 to TLR10).

• Plasma Membrane TLRs:

  • TLR1:TLR2 Heterodimer: Recognizes triacylated bacterial lipopeptides.
  • TLR2 Homodimer: Recognizes bacterial peptidoglycans and fungal zymosan.
  • TLR2:TLR6 Heterodimer: Recognizes diacylated bacterial lipopeptides and lipoteichoic acid.
  • TLR4 Homodimer: Recognizes gram-negative LPS; requires extracellular LPS-Binding Protein (LBP), CD14, and MD2 (Myeloid Differentiation protein 2).
  • TLR5 Homodimer: Recognizes bacterial flagellin.

• Endosomal Membrane TLRs:

  • Transported from ER to endosomes via the chaperone UNC93B. Mutations in UNC93B or TLR3 cause herpes simplex virus encephalitis.
  • TLR3: Recognizes viral double-stranded RNA (dsRNA).
  • TLR7 & TLR8: Recognize viral single-stranded RNA (ssRNA) rich in guanosine/uridine.
  • TLR9: Recognizes unmethylated cytosine-guanine (CpG) DNA motifs in bacteria and viruses.

• TLR Signaling Pathways:

  • MyD88-Dependent Pathway: Used by all TLRs except TLR3. MyD88 recruits IRAK4 and TRAF6, activating the IKK complex. IKK phosphorylates I$\nu\text{b}$ u ext{a}$, targeting it for degradation and releasing NF-$ u ext{b} to transcribe proinflammatory cytokines (TNF, IL-1, IL-6), chemokines, and costimulators (CD80, CD86).\n * *TRIF-Dependent Pathway*: Used by TLR3 and TLR4. TRIF recruits TRAF3 and activates TBK1 (TANK-Binding Kinase 1), phosphorylating IRF3 and IRF7 to induce Type I IFNs (IFN- u ext{a}$, IFN-$ u ext{b}).\n\n\n\n* **NOD-Like Receptors (NLRs) and NOD Signalosome**:\n * Cytosolic family (>20 proteins) containing C-terminal LRRs, central NOD/NACHT oligomerization domains, and variable N-terminal effector domains (BIR [NLRB], CARD [NLRC], Pyrin [NLRP]).\n * **NOD1**: Recognizes diaminopimelic acid (DAP) from gram-negative bacterial peptidoglycan.\n * **NOD2**: Highly expressed in Paneth cells; recognizes muramyl dipeptide (MDP) from gram-positive and gram-negative bacteria. Recruits RIP2 kinase to form the NOD signalosome, activating NF- u ext{b}.\n * *Pathology*: *NOD2* loss-of-function polymorphisms increase susceptibility to Crohn's disease; gain-of-function mutations cause Blau syndrome.\n\n* **Inflammasomes and Pyroptosis**:\n * Cytosolic multiprotein complexes that recruit procaspase-1, inducing auto-cleavage into active caspase-1.\n * **Inflammasome Sensors**:\n * *NLRP3*: Activated indirectly by ROS, lysosomal rupture (silica, asbestos, urate crystals), cytosolic K^+ efflux, extracellular ATP, or pore-forming toxins.\n * *NLRC4*: Activated by cytosolic flagellin and bacterial type III secretion system components.\n * *NLRP1*: Activated by anthrax lethal toxin.\n * *AIM2*: Activated by cytosolic dsDNA.\n * *Pyrin*: Encoded by *MEFV*; senses Rho GTPase inactivation by bacterial toxins.\n * **Assembly & Processing**: Sensor oligomerization recruits ASC (Apoptosis-associated Speck-like protein containing a CARD) filaments, clustering procaspase-1. Active caspase-1 cleaves pro-IL-1 u ext{b}$and pro-IL-18 into active IL-1$ u ext{b} and IL-18.\n * **Pyroptosis**: Caspase-1 or noncanonical caspases (caspase-11 in rodents; caspase-4/5 in humans binding intracellular LPS) cleave Gasdermin D. The N-terminal Gasdermin D fragment forms plasma membrane pores, causing cytoplasmic leakage, IL-1 u ext{b} secretion, osmotic lysis, and inflammatory cell death.\n * **Inflammasomopathies & Clinical Conditions**: Familial Mediterranean Fever (*MEFV* mutations), Cryopyrin-Associated Periodic Syndromes (CAPS; *NLRP3* mutations), Gout (monosodium urate crystals), Pseudogout, Atherosclerosis (cholesterol crystals), Type 2 Diabetes, Alzheimer's disease ( u ext{b}-amyloid).\n\n\n\n* **RIG-Like Receptors (RLRs)**:\n * Cytosolic sensors of viral RNA containing two N-terminal CARD domains, an RNA-helicase domain, and a C-terminal domain.\n * **RIG-I**: Recognizes short ssRNA/dsRNA with 5'-triphosphate moieties.\n * **MDA5**: Recognizes long dsRNA (1 ext{--}6 ext{ kb}) unmethylated at the 2'-O-ribose position.\n * **Signaling**: Ligand binding causes RLRs to interact with MAVS (Mitochondrial Antiviral-Signaling protein) on the outer mitochondrial membrane. MAVS polymerizes into filaments, recruiting TRAFs to activate TBK1/IKK, driving IRF3, IRF7, and NF- u ext{b}.\n * *Vaccine Innovation*: Incorporating modified nucleosides (e.g., pseudouridine) into mRNA vaccines prevents RLR/TLR detection and rapid degradation.\n\n\n\n* **Cytosolic DNA Sensors (CDSs) and STING Pathway**:\n * **cGAS-STING Pathway**: Cytosolic dsDNA activates cGAS (cyclic GMP-AMP synthase) to synthesize 2'3'-cGAMP from ATP and GTP. cGAMP binds STING (Stimulator of IFN Genes) on the ER membrane, causing STING translocation to the Golgi, recruitment of TBK1, and activation of IRF3/7 and type I IFNs.\n * STING also senses bacterial cyclic dinucleotides (c-di-GMP, c-di-AMP) and induces autophagy.\n * *Interferonopathies*: Gain-of-function STING mutations cause SAVI (STING-Associated Vasculopathy with onset in Infancy); loss of nucleic acid degrading enzymes (e.g., TREX1, SAMHD1, RNASEH2) causes Aicardi-Goutières Syndrome (AGS).\n\n\n\n* **Other Cell-Associated PRRs**:\n * **C-Type Lectin Receptors (CLRs)**: Integral membrane lectins binding carbohydrates in a ext{Ca}^{2+}-dependent manner.\n * *Mannose Receptor (CD206)*: Recognizes terminal D-mannose, L-fucose, N-acetyl-D-glucosamine; mediates phagocytosis.\n * *Dectin-1 (CD369)*: Binds fungal u ext{b}$-glucan; contains cytoplasmic ITAM; signals via Syk and CARD9 to activate NF-$ u ext{b} and drive Th17 differentiation.\n * *Dectin-2 & Mincle*: Recognize high-mannose structures; utilize associated FcR u ext{g} ITAM subunit.\n * *DC-SIGN (CD209)*: Binds terminal mannose/fucose; target for HIV-1 and Hepatitis C entry.\n * *Langerin (CD207)*: Expressed on Langerhans cells; involved in antigen presentation.\n * **Scavenger Receptors**: SR-A and CD36; mediate phagocytosis of oxidized lipoproteins, LPS, lipoteichoic acid, and micro-organisms.\n * **Formyl-Peptide Receptors**: FPR1 is a G-protein coupled receptor (GPCR) binding bacterial N-formylmethionyl peptides, inducing high-affinity leukocyte chemotaxis.\n\n# Cellular Components of Innate Immunity\n\n\n\n* **Epithelial Barriers and Host Defense Peptides**:\n * Continuous tight junctions, keratinization (skin), mucus coat, and ciliary action.\n * **Defensins**: Cationic peptides (29 ext{--}34 ext{ amino acids}) containing 3 intrachain disulfide bonds; insert into and disrupt microbial membranes.\n * * u ext{a}-defensins*: Produced by neutrophils, NK cells, CTLs, and intestinal Paneth cells (crypticidins).\n * * u ext{b}-defensins*: Produced constitutively or inducibly by mucosal epithelial cells.\n * **Cathelicidins**: Synthesized as an 18 ext{ kD} precursor protein; cleaved to yield active peptides including LL-37 (neutralizes LPS, activates pDCs).\n * **Intraepithelial Lymphocytes**: Express limited-diversity u ext{g} u ext{d}$or$ u ext{a} u ext{b} TCRs; provide rapid local antimicrobial defense.\n\n* **Cytokine-Producing Innate Lymphoid Cells (ILCs)**:\n * Non-antigen-specific bone marrow-derived cells lacking TCRs; dependent on the Id2 transcription factor.\n * **ILC1**: Expresses T-BET; secretes IFN- u ext{g} in response to IL-12 and IL-15; mediates immunity against intracellular pathogens.\n * **ILC2**: Expresses GATA3; secretes IL-5 and IL-13 in response to IL-25, IL-33, and TSLP; mediates defense against helminths and drives allergic inflammation.\n * **ILC3**: Expresses ROR u ext{g}$t; secretes IL-17 and IL-22 in response to IL-1$ u ext{b} and IL-23; maintains mucosal barrier integrity and antifungal defense. Includes Lymphoid Tissue-Inducer (LTi) cells.\n\n\n\n* **Natural Killer (NK) Cells**:\n * Cytotoxic lymphocytes (5 ext{--}20 ext{\%}$of circulating lymphocytes); expressed as$ ext{CD56}^+$,$ ext{CD3}^-$,$ ext{CD16}^+.\n * **Effector Functions**: Direct exocytosis of perforin and granzymes to induce target cell apoptosis; secretion of IFN- u ext{g} to activate macrophage microbicidal activity.\n\n\n\n * **Receptor Integration & "Missing Self" Hypothesis**:\n * NK cell function is regulated by the balance between activating and inhibitory signals.\n * Healthy host cells express self MHC class I molecules, which engage inhibitory receptors.\n * Virally infected or transformed cells downregulate MHC class I ("missing self") and upregulate stress ligands, shifting the balance to NK activation.\n\n\n\n * **Activating Receptors**:\n * *NKG2D*: Lectin-like receptor; binds MIC-A, MIC-B, and ULBP stress proteins; signals via DAP10.\n * *CD16 (Fc u ext{g}$RIIIA)*: Low-affinity IgG receptor; binds IgG1 and IgG3 coated on target cells to trigger Antibody-Dependent Cell-Mediated Cytotoxicity (ADCC); signals via Fc$ u ext{e}$RI$ u ext{g}$or$ u ext{z} ITAM-containing chains.\n * *Activating KIRs*: KIR2DS and KIR3DS; associate with DAP12 signaling subunit containing ITAMs.\n * **Inhibitory Receptors**:\n * *Inhibitory KIRs*: KIR2DL and KIR3DL; bind classical HLA-A, HLA-B, HLA-C molecules.\n * *CD94/NKG2A*: Lectin-like heterodimer; binds non-classical HLA-E displaying signal peptides of classical MHC class I molecules.\n * *Signaling Mechanism*: Inhibitory receptors contain Immunoreceptor Tyrosine-based Inhibition Motifs (ITIMs). Ligand binding phosphorylates ITIM tyrosines, recruiting SHP-1 tyrosine phosphatase to dephosphorylate activating pathway intermediates.\n\n\n\n# Soluble Effector Molecules of Innate Immunity\n\n* **The Complement System**:\n * Enzymatic cascade of plasma proteins functioning in opsonization, leukocyte recruitment, and cell lysis.\n * **Initiation Pathways**:\n * *Classical Pathway*: C1q detects surface-bound IgM or IgG antibody-antigen complexes or pentraxins (CRP, SAP); activates C1r and C1s proteases.\n * *Alternative Pathway*: Spontaneous low-level hydrolysis of C3; C3b covalently binds microbial surfaces lacking regulatory proteins.\n * *Lectin Pathway*: Mannose-Binding Lectin (MBL) or Ficolins bind terminal mannose or N-acetylglucosamine residues; activates MASP1 and MASP2 proteases.\n * **Cascade Convergence and Effector Functions**:\n * All pathways generate C3 convertase, which cleaves C3 into C3a (anaphylatoxin) and C3b (opsonin covalently attached to microbes).\n * C3b forms C5 convertase, cleaving C5 into C5a (potent chemoattractant and anaphylatoxin) and C5b.\n * C5b recruits C6, C7, C8, and C9 to assemble the Membrane Attack Complex (MAC, C5b-9), forming a membrane pore causing osmotic cell lysis.\n * *Clinical Susceptibility*: Deficiencies in C3 lead to severe recurrent bacterial infections; MAC deficiencies selectively increase susceptibility to *Neisseria* infections.\n\n\n\n* **Pentraxins**:\n * Pentameric plasma proteins binding microbial structures and activating classical complement via C1q.\n * *Short Pentraxins*: C-Reactive Protein (CRP) binds phosphorylcholine; Serum Amyloid P (SAP) binds phosphatidylethanolamine. Synthesized as acute-phase reactants by hepatocytes driven by IL-6, IL-1, and TNF.\n * *Long Pentraxins*: PTX3 produced by DCs, macrophages, and endothelial cells in response to TLR ligands and TNF.\n\n* **Collectins and Ficolins**:\n * **Collectins**: Hexameric structures containing a collagen-like stalk and a C-type lectin head. Includes MBL (opsonin and complement activator) and Surfactant Proteins SP-A and SP-D (bind lipids/carbohydrates in pulmonary alveoli to opsonize microbes).\n * **Ficolins**: Structurally homologous to collectins but possess a fibrinogen-type carbohydrate recognition domain binding N-acetylglucosamine and lipoteichoic acid.\n\n\n\n# The Inflammatory Response and Cytokine Networks\n\n\n\n* **Major Proinflammatory Cytokines**:\n * **Tumor Necrosis Factor (TNF)**:\n * Produced as a 17 ext{ kD}$monomer; forms a$51 ext{ kD} homotrimer cleaved by TACE metalloproteinase.\n * Binds TNFRI (K_d u ext{g} 1 imes 10^{-9} ext{ M}$) or TNFRII ($K_d u ext{g} 5 imes 10^{-10} ext{ M}$), recruiting TRAF adaptors to activate NF-$ u ext{b} or caspases.\n * Local effects: Endothelial activation (E-selectin, ICAM-1, VCAM-1 expression), chemokine production, neutrophil activation.\n * Systemic effects: Hypothalamic fever, liver acute-phase protein production, bone marrow leukocyte release.\n * Pathologic high-dose effects: Decreased cardiac output, endothelial thrombosis, microvascular permeability/leak leading to septic shock, and muscle/fat cachexia.\n\n\n\n * **Interleukin-1 (IL-1)**:\n * Produced as a 33 ext{ kD}$precursor (pro-IL-1$ u ext{b}$); cleaved by inflammasome caspase-1 into mature$17 ext{ kD}$IL-1$ u ext{b}.\n * Signals through Type I IL-1 Receptor containing a TIR domain; inhibited by decoy Type II IL-1 Receptor and competitive IL-1 Receptor Antagonist (IL-1RA).\n * **Interleukin-6 (IL-6)**:\n * Synthesized by phagocytes, endothelial cells, and fibroblasts; signals through IL-6R and gp130 via STAT3.\n * Drives liver acute-phase protein synthesis and Th17 differentiation. Soluble IL-6R mediates trans-signaling.\n\n* **Other Innate Cytokines**:\n * *IL-12*: Heterodimer (35 ext{ kD}$p35 +$40 ext{ kD}$p40); drives IFN-$ u ext{g} production by NK/T cells and Th1 differentiation via STAT4.\n * *IL-23*: Heterodimer (19 ext{ kD}$p19 +$40 ext{ kD} p40); maintains Th17 cells.\n * *IL-15*: Presented bound to IL-15R u ext{a} on DCs to nearby NK and memory CD8+ T cells to promote survival.\n * *IL-18*: Inflammasome-dependent; enhances NK cytotoxicity and IFN- u ext{g} synthesis.\n * *IL-25, TSLP, IL-33*: Epithelial-derived alarmins inducing type 2 responses.\n\n* **Sequence of Events in Inflammation**:\n 1. Tissue sentinels (macrophages, DCs, mast cells) sense PAMPs/DAMPs.\n 2. Release of histamine, prostaglandins, TNF, IL-1, IL-6, and chemokines.\n 3. Arteriolar vasodilation and increased venular permeability.\n 4. Endothelial expression of E-selectin, P-selectin, ICAM-1, and VCAM-1.\n 5. Leukocyte rolling, high-affinity integrin binding, transvascular migration, and CXCL8/CCL2 chemokine gradient chemotaxis.\n 6. Neutrophil infiltration within minutes to hours, followed by monocyte influx.\n\n* **Ingestion and Intracellular Killing**:\n * Phagocytosis mediated by PRRs, Fc u ext{g}RI, or C3b receptors.\n * **Phagocyte Oxidase (NADPH Oxidase)**: Multisubunit enzyme assembled in phagolysosome membrane during respiratory burst. Converts O_2$into superoxide radicals ($O_2^{ullet-}$), hydrogen peroxide ($H_2O_2$), and myeloperoxidase-catalyzed hypohalous acids ($HOCl). Deficiency causes Chronic Granulomatous Disease (CGD).\n * **Inducible Nitric Oxide Synthase (iNOS)**: Catalyzes arginine conversion to Nitric Oxide (NO$); reacts with superoxide to form peroxynitrite radicals ($ONOO^-).\n * **Proteolytic Enzymes**: Lysosomal elastase and cathepsin G.\n * **Neutrophil Extracellular Traps (NETs)**: Extruded chromatin threads decorated with elastase, lysozyme, and defensins. Requires histone citrullination by Peptidylarginine Deiminase 4 (PAD4); causes cell death via NETosis.\n\n# Antiviral Responses and Adaptive Immune Induction\n\n* **Antiviral Defense & Type I/III Interferons**:\n * Type I IFNs (IFN- u ext{a}$multi-protein family, IFN-$ u ext{b}$) and Type III IFNs (IFN-$ u ext{l}) induced via endosomal TLR3/7/8/9, RLRs, or cGAS-STING activating IRF3/7.\n * Bind IFNAR1/IFNAR2 receptors on uninfected cells, activating JAK-STAT signaling (STAT1, STAT2, IRF9).\n * **Induction of the Antiviral State**:\n * *PKR*: dsRNA-activated protein kinase; phosphorylates eIF2 u ext{a}$$, inhibiting viral protein translation.
  • 2',5'-Oligoadenylate Synthetase & RNase L: Synthesize oligo A to activate RNase L, degrading viral RNA.
  • IFITs: Bind eIF3 and 5'-triphosphate motifs to block viral translation.
  • IFITMs: Block entry of enveloped viruses across cellular membranes.
  • Mx GTPases: Polymerize to inhibit viral nucleocapsid assembly and gene expression.

• Upregulate MHC class I expression on infected cells, enhancing CD8+ CTL recognition.

• Induce CD69 expression on lymphocytes, internalizing S1PR1 to retain lymphocytes in lymph nodes for antigen encounter.

  • Stimulation of Adaptive Immunity (Two-Signal Hypothesis):

• Signal 1: Specific antigen recognition by TCRs or BCRs.

• Signal 2: Second signals induced by innate responses to microbes (costimulators CD80/CD86, cytokines IL-12/IL-1/IL-6, or complement fragment C3d).

• Adjuvants: Substances (e.g., Alum, killed mycobacteria, CpG) co-administered with protein antigens to trigger innate PRR signaling, upregulate MHC/costimulators, and recruit DCs.

Mechanisms Limiting Innate Immune Responses

• Inhibitory Cytokines:
  • IL-10: Secreted by macrophages, DCs, and regulatory T cells; acts via negative feedback to suppress macrophage activation and inhibit IL-1, TNF, and IL-12 production.
• Competitive Inhibitors & Decoy Receptors:
  • IL-1 Receptor Antagonist (IL-1RA): Binds IL-1R without signaling, blocking IL-1 binding. Deficiencies cause severe bone and skin inflammation.
  • Type II IL-1 Receptor: Functions as a decoy receptor binding IL-1 without transducing intracellular signals.
• Intracellular Signaling Inhibitors:
  • SOCS (Suppressors of Cytokine Signaling): Induced by TLRs; inhibit JAK-STAT signaling pathways.
  • SHP-1: Intracellular protein tyrosine phosphatase that dephosphorylates activating intermediate kinases.