Pattern Recognition, Phagocytosis, Toll-Like Receptors, and Interferons

Comprehensive vocabulary flashcards covering pattern recognition receptors, leukocyte extravasation, phagocytosis mechanisms, Toll-like receptor signaling cascades, inflammasomes, and interferon antiviral pathways.

Pathogen-Associated Molecular Patterns (PAMPs)

Broad, conserved structural motifs of microbial species (such as sugars, proteins, lipids, and nucleic acids) that are absent in the host, essential for microbial survival, and recognized by the innate immune system.

Pattern Recognition Receptors (PRRs)

Host receptors expressed on immune and epithelial cells that detect PAMPs, allowing the innate immune system to discriminate precisely between self and non-self molecules.

Extravasation (Diapedesis)

The migration process in which circulating leukocytes exit blood vessels by squeezing through the capillary walls to reach sites of injury and infection in response to chemotactic cytokines.

Opsonin

A host molecule, such as an antibody or complement component, that binds to microbial surfaces and to specific phagocyte receptors to enhance and activate phagocytosis.

Fc Receptors

Surface receptors on phagocytes that recognize the exposed Fc region of IgG antibodies coating bacteria, facilitating target binding and promoting phagocytosis.

Scavenger Receptors

Receptors expressed on macrophages (such as SR-A I/II and MARCO) that bind a broad range of polyanions and sialic acid on bacterial and yeast surfaces.

Mannose Receptor

A pattern recognition receptor found on macrophages and neutrophils that specifically recognizes mannose-containing carbohydrates (polysaccharides) on many bacteria.

CD14

A phagocytic pattern recognition receptor present on macrophages and neutrophils that binds the lipopolysaccharide (LPS) component of Gram-negative bacterial cell walls.

Phases of Phagocytosis

The seven-step process consisting of: 1. Microbe adherence, 2. Pseudopod engulfment, 3. Phagosome formation, 4. Fusion into a phagolysosome, 5. Digestion by proteolytic enzymes, 6. Residual body formation, and 7. Exocytosis of indigestible material.

Respiratory Burst (Oxygen-Dependent Killing)

A rapid release of reactive oxygen species (ROS) produced by phagocytes upon activation to destroy internalized microorganisms.

NADPH Oxidase

The enzyme complex in phagocytes that catalyzes the initial step of the respiratory burst: $2\text{O}_2 + \text{NADPH} \rightarrow 2\text{O}_2^- + \text{NADP}^+ + \text{H}^+$.

Superoxide Dismutase

An enzyme that converts toxic superoxide radicals generated during the respiratory burst into hydrogen peroxide and oxygen: $2\text{O}_2^- + 2\text{H}^+ \rightarrow \text{H}_2\text{O}_2 + \text{O}_2$.

Myeloperoxidase

A neutrophil enzyme that converts hydrogen peroxide and chloride ions into hypochlorite: $\text{H}_2\text{O}_2 + \text{Cl}^- \rightarrow \text{OCl}^- + \text{H}_2\text{O}$.

Lactoferrin

An oxygen-independent antimicrobial protein located in neutrophil granules that sequesters and removes essential iron ($\text{Fe}^{2+}$) from bacteria.

Lysozyme

An oxygen-independent antibacterial enzyme secreted by phagocytes that hydrolyzes and breaks down peptidoglycan within the bacterial cell wall.

Toll-like Receptor (TLR) Architecture

A transmembrane receptor organization composed of an extracellular or lumenal leucine-rich repeat (LRR) domain that recognizes PAMPs and an intracellular TIR domain that recruits adaptors.

TIR (Toll/Interleukin-1 Receptor) Domain

The conserved cytoplasmic signaling domain of Toll-like receptors containing Boxes 1, 2, and 3 that interacts with downstream signaling adaptor proteins.

TLR1:TLR2 Heterodimer

A plasma membrane receptor complex that recognizes triacyl lipopeptides, bacterial lipoproteins, and mycobacterial lipomannans.

TLR2:TLR6 Heterodimer

A cell-surface receptor complex that detects diacyl lipopeptides, Gram-positive lipoteichoic acid, cell-wall $\beta$-glucans, and yeast zymosan.

TLR3

An endosomal Toll-like receptor that specifically detects viral double-stranded RNA (dsRNA).

TLR4

A cell-surface Toll-like receptor that partners with MD-2 and CD14 to recognize lipopolysaccharide (LPS) from Gram-negative bacteria.

TLR5

A plasma membrane Toll-like receptor expressed on intestinal epithelium and immune cells that recognizes bacterial flagellin.

TLR7 and TLR8

Endosomal Toll-like receptors specialized in recognizing viral single-stranded RNA (ssRNA).

TLR9

An endosomal Toll-like receptor that detects unmethylated CpG motifs in bacterial and viral (herpesvirus) DNA.

TLR11

A mouse-specific Toll-like receptor expressed on macrophages, dendritic cells, and epithelial tissues that recognizes profilin and profilin-like proteins from Toxoplasma gondii and uropathogenic bacteria.

MyD88 (Myeloid Differentiation Primary Response Gene 88)

A primary cytoplasmic adaptor protein recruited by most TLRs that triggers downstream activation of the $\text{NF-}\kappa\text{B}$, MAP kinase, and IRF pathways.

TRIF (TIR-Domain-Containing Adaptor-Inducing IFN-$\beta$)

An adaptor protein utilized by TLR3 and TLR4 that mediates IRF-dependent signaling leading to the transcription of type I interferons.

TIRAP (Mal)

A sorting adaptor protein that bridges MyD88 recruitment to plasma membrane TLRs, specifically TLR1/TLR2, TLR2/TLR6, and TLR4.

TRAM (TRIF-Related Adaptor Molecule)

A bridging adaptor protein recruited exclusively to TLR4 that facilitates downstream binding of TRIF.

$\text{NF-}\kappa\text{B}$ Pathway

A major signaling pathway downstream of TLRs that upregulates gene expression for pro-inflammatory cytokines such as IL-1, IL-6, TNF-$\alpha$, and IL-12.

MAP Kinase Pathway

A signaling cascade involving ERK, p38, and JNK activated by TLR engagement that regulates transcription factors controlling cell proliferation, differentiation, and cytokine production.

IRF Pathway

A signaling cascade involving transcription factors IRF3 and IRF7 activated downstream of TLR3, TLR4, TLR7, TLR8, and TLR9 to induce the expression of type I interferons.

NOD-Like Receptors (NLRs)

Cytoplasmic pattern recognition receptors featuring a tripartite-domain organization with a conserved nucleotide-binding oligomerization domain (NOD) and leucine-rich repeats (LRRs).

Inflammasome

A multiprotein cytoplasmic complex assembled by NLRs that recruits and activates caspase-1 to cleave immature pro-IL-$1\beta$ into biologically active IL-$1\beta$.

Type I Interferons (IFN-$\alpha$ and IFN-$\beta$)

Secretory cytokines produced by virus-infected leukocytes and other cells that bind interferon receptors to induce antiviral effector proteins as part of innate immunity.

$2',5'$-Oligo(A) Synthetase

An interferon-induced enzyme that is activated by double-stranded RNA to synthesize $2',5'$-oligoadenylate chains from ATP, subsequently activating Ribonuclease L.

Ribonuclease L (RNase L)

An endoribonuclease that transitions from an inactive to an active state upon binding $2',5'$-oligo(A), which then degrades cellular and viral mRNA to halt viral replication.

Protein Kinase R (PKR / P1 Kinase)

An interferon-induced enzyme activated by double-stranded RNA that phosphorylates $\text{eIF}_2$ to inactivate it, shutting down cellular and viral translation and inducing apoptosis.