Key Molecules of the Immune System – Dr Liliana Shalamanova

Vocabulary flashcards covering innate and adaptive immunity characteristics, key receptors, cytokines, chemokines, MHC molecules, antigen-presentation pathways, complement, antibodies, and related immunological terms.

Innate immunity

The first line of immune defence that acts within minutes to hours, uses fixed recognition elements, has limited specificity, and lacks memory.

Adaptive immunity

The second line of defence that develops over days to weeks, employs variable recognition elements, is highly specific, and generates immunological memory.

Pattern-recognition receptor (PRR)

A germline-encoded receptor of the innate immune system that recognises conserved microbial patterns.

B-cell receptor (BCR)

Membrane-bound immunoglobulin on B cells that binds specific antigens and initiates B-cell activation.

T-cell receptor (TCR)

Heterodimeric receptor on T lymphocytes that recognises antigenic peptides presented by MHC molecules.

Cytokine

A low-molecular-weight protein secreted by cells that binds specific receptors to regulate immunity, inflammation, and hematopoiesis in autocrine, paracrine, or endocrine fashion.

Chemokine

A cytokine subset that directs cell migration (chemotaxis) and adhesion of leukocytes.

Hematopoietin family

A cytokine family that includes interleukins produced by leukocytes to act on other leukocytes.

Interferon family

Cytokines (e.g., IFN-α, IFN-β, IFN-γ) with prominent antiviral and immunomodulatory functions.

Tumour-necrosis factor (TNF) family

Cytokines that regulate inflammation and can induce cell death, e.g., TNF-α.

Autocrine signalling

Cytokine action on the same cell that secreted it.

Paracrine signalling

Cytokine action on neighbouring cells.

Endocrine signalling

Cytokine action on distant cells via the circulation.

Major Histocompatibility Complex (MHC)

A cluster of >200 genes on human chromosome 6 encoding antigen-presenting glycoproteins and other immune regulators.

MHC class I molecule

Membrane glycoprotein expressed on all nucleated cells that presents endogenous peptides to CD8⁺ T cells.

MHC class II molecule

Membrane glycoprotein expressed only on professional antigen-presenting cells that presents exogenous peptides to CD4⁺ T cells.

Professional antigen-presenting cell (APC)

Cell type (dendritic cell, macrophage, B cell) capable of processing antigen and expressing MHC II with co-stimulatory molecules.

Endogenous antigen pathway

Cytosolic proteins are ubiquitylated, degraded by the proteasome, transported by TAP into the ER, and loaded onto MHC I.

Exogenous antigen pathway

Extracellular proteins are endocytosed, processed in acidic vesicles, and loaded onto MHC II in MIIC compartments.

Complement system

30 soluble and membrane proteins that recognise extracellular pathogens, mediate lysis, opsonisation, inflammation, and immune-complex clearance.

Opsonisation

Tagging of pathogens with complement fragments or antibodies to enhance phagocytosis.

Antibody (immunoglobulin)

Soluble glycoprotein produced by B cells that neutralises pathogens, opsonises targets, activates complement, and mediates ADCC.

IgG

Monomeric antibody class; main serum antibody, crosses placenta, activates complement.

IgM

Pentameric antibody class; first produced in primary response, strong complement activator.

IgA

Antibody found as monomer in serum and dimer in secretions such as saliva and breast milk.

IgE

Monomeric antibody class that binds mast cells and basophils, mediating allergic reactions and defence against parasites.

IgD

Monomeric antibody primarily serving as B-cell receptor alongside IgM on naïve B cells.

Antibody-dependent cell-mediated cytotoxicity (ADCC)

Destruction of antibody-coated target cells by NK cells or other effector leukocytes via Fc receptor engagement.

Inflammasome

Cytoplasmic receptor complex that activates inflammatory cytokines such as IL-1β (mentioned indirectly under intracellular receptors).

Death receptor

Cell-surface receptor (e.g., Fas) that triggers apoptosis upon ligand binding.

Signal transduction

Intracellular cascade initiated by receptor-ligand binding leading to changes in gene expression or cell behaviour.

Chemotaxis

Directed movement of cells toward a chemokine gradient.

Endocytosis

Process by which cells internalise extracellular material via vesicle formation.

Phagocytosis

Engulfment and digestion of large particles or microbes by cells such as macrophages and neutrophils.

Immunological memory

Enhanced, faster response of adaptive immunity upon re-exposure to the same antigen.

Interleukin-1 (IL-1)

Pro-inflammatory cytokine produced by macrophages and others; induces fever and leukocyte activation.

Interleukin-6 (IL-6)

Cytokine that promotes B-cell differentiation, acute-phase response, and acts as both pro- and anti-inflammatory mediator.

Interferon-γ (IFN-γ)

Type II interferon produced mainly by TH1 cells and NK cells; activates macrophages and up-regulates MHC expression.

TAP (Transporter associated with Antigen Processing)

ER membrane protein complex that transports proteasome-generated peptides into the ER for MHC I loading.

Immunoproteasome

Cytokine-induced variant of the proteasome that generates peptides suited for MHC I presentation.