| Acanthosis | Thickening of the epidermis (outer skin layer) due to increased keratinocyte proliferation |
| Adaptive immunity | The branch of the immune system that mounts specific, targeted responses and has memory; carried out by T cells and B cells |
| Aetiology | The cause or set of causes of a disease |
| AMP (antimicrobial peptide) | Small protein fragments produced by skin cells that kill microbes and also act as immune alarm signals; LL-37 is key in psoriasis |
| Antigen | Any substance that the immune system can recognise and respond to, such as a piece of a virus, bacterium, or (in autoimmunity) the body’s own tissue |
| Auspitz sign | Pinpoint bleeding when psoriatic scale is gently removed, caused by exposed dilated capillaries |
| BADBIR | British Association of Dermatologists Biologics and Immunomodulators Register, the UK/Ireland psoriasis safety registry |
| Biologic | A drug made from living cells (rather than chemically synthesised) that targets a specific molecule in the immune system |
| Biosimilar | A biologic drug that is highly similar to an existing approved biologic (the “reference” product) with no clinically meaningful differences |
| BSA | Body Surface Area, the percentage of total skin surface affected by psoriasis |
| Calcineurin inhibitor | A class of topical drugs (tacrolimus, pimecrolimus) that suppress local immune activity; useful for face, flexures, and genitals |
| CAR-T (Chimeric Antigen Receptor T cell) | An engineered immune cell therapy where T cells are modified to recognise and eliminate cells expressing a specific antigen; being explored for severe autoimmune diseases |
| CASPAR | Classification Criteria for Psoriatic Arthritis, the most widely used and validated classification tool for PsA, requiring inflammatory articular disease plus ≥3 points from clinical features |
| CGRP (calcitonin gene-related peptide) | A neuropeptide released by sensory nerves that promotes neurogenic inflammation and vasodilation in the skin |
| CCL20 | A chemokine (“come here” signal) that attracts Th17 cells to the skin |
| Certolizumab pegol | A PEGylated anti-TNF biologic that lacks the Fc region and therefore does not cross the placenta; preferred in pregnancy |
| Chemokine | A subset of cytokines whose job is to direct immune cells to specific locations |
| Comorbidity | A disease or condition that occurs alongside the primary condition (e.g., cardiovascular disease in a patient with psoriasis) |
| Cytokine | A small protein released by cells that acts as a chemical messenger regulating immune responses |
| Dactylitis | Diffuse swelling of an entire digit (“sausage finger” or “sausage toe”), caused by a combination of joint inflammation, tenosynovitis, and soft tissue oedema; highly suggestive of psoriatic arthritis |
| Dendritic cell | An antigen-presenting immune cell that acts as a messenger between innate and adaptive immunity |
| Differentiation | The process by which a general-purpose cell becomes a specialist (e.g., a naïve T cell becoming a Th17 cell) |
| DLQI | Dermatology Life Quality Index, a questionnaire measuring how skin disease affects daily life (score 0–30; ≥10 = significant impact) |
| DNA methylation | A chemical modification where methyl groups are added to DNA, altering gene expression without changing the DNA sequence; a key epigenetic mechanism studied in psoriasis disease memory |
| Drug survival | The length of time a patient remains on a given therapy; used in registries as a real-world proxy for treatment effectiveness and tolerability |
| Emollient | A moisturising preparation that softens skin and maintains the skin barrier |
| Enthesitis | Inflammation at the points where tendons and ligaments insert into bone (entheses); a hallmark feature of psoriatic arthritis that distinguishes it from rheumatoid arthritis |
| Epigenetics | The study of heritable changes in gene expression that don’t involve changes to the DNA sequence itself; mechanisms include DNA methylation and histone modification |
| Epidermis | The outermost layer of the skin |
| Erythema | Redness of the skin caused by increased blood flow; appears differently across skin tones |
| Erythrodermic psoriasis | A rare, severe form involving widespread inflammation covering most of the body; medical emergency |
| Fc region | The “tail” part of an antibody molecule that interacts with immune cells and is actively transported across the placenta |
| GPP (generalised pustular psoriasis) | A severe form featuring sterile pustules across the body, driven by the IL-36 pathway |
| GWAS (Genome-Wide Association Study) | A research method that analyses genetic variation across the entire genome to identify DNA variants associated with disease susceptibility |
| Guttate psoriasis | A form characterised by small, droplet-shaped lesions; often triggered by streptococcal infection |
| HDAC (Histone Deacetylase) | An enzyme that removes acetyl groups from histones; HDAC inhibitors block this activity to modulate gene expression in inflammatory diseases |
| Heritability | The proportion of variation in disease risk attributable to genetic factors (psoriasis heritability is ~66%) |
| Histopathology | Examination of tissue under a microscope to aid diagnosis |
| HLA (Human Leukocyte Antigen) | A group of genes on chromosome 6 encoding surface proteins that present antigens to T cells; HLA-C*06:02 is the strongest psoriasis risk allele |
| IGA (Investigator’s Global Assessment) | A clinical scoring system measuring overall psoriasis severity on a scale from 0 (clear) to 4 or 5 (severe); IGA 0 indicates completely clear skin |
| IFN-γ (Interferon gamma) | A cytokine produced by Th1 cells that activates macrophages |
| IFN-α (Interferon alpha) | A cytokine produced by plasmacytoid dendritic cells; important in psoriasis initiation |
| IL-17 (Interleukin-17) | The key effector cytokine in psoriasis, produced by Th17 cells; tells keratinocytes to multiply rapidly |
| IL-23 (Interleukin-23) | The “upstream master switch” cytokine that creates and sustains Th17 cells |
| Immunosuppressant | A drug that suppresses the immune system (e.g., methotrexate, ciclosporin) |
| Innate immunity | The body’s rapid, non-specific first line of defence against threats |
| JAK inhibitor | A class of oral drugs that block Janus kinase enzymes involved in cytokine signalling |
| Keratinocyte | The predominant cell type in the epidermis; in psoriasis, keratinocytes proliferate abnormally |
| Köbner phenomenon | The appearance of new psoriatic lesions at sites of skin trauma |
| LL-37 (cathelicidin) | An antimicrobial peptide that triggers the psoriasis immune cascade by binding to self-DNA |
| Methotrexate | A folic acid antagonist widely used as first-line systemic treatment for psoriasis; teratogenic |
| MHC (Major Histocompatibility Complex) | Surface proteins encoded by HLA genes that present antigen fragments to T cells |
| Munro microabscesses | Collections of neutrophils in the stratum corneum (uppermost epidermis); a histological hallmark of psoriasis |
| NB-UVB | Narrowband ultraviolet B (311 nm), the most commonly used form of phototherapy for psoriasis |
| Neuropeptide | A small protein released by nerve endings that regulates inflammation, pain, and itch signalling in the skin; examples include substance P and CGRP |
| Network meta-analysis (NMA) | A statistical method that compares multiple treatments simultaneously by combining direct and indirect evidence from clinical trials; used to rank biologic efficacy |
| NGF (nerve growth factor) | A protein that promotes the growth and survival of sensory nerve fibres; elevated in psoriatic skin, contributing to increased nerve density and itch |
| NK1R (neurokinin-1 receptor) | The receptor for substance P on immune cells, keratinocytes, and endothelial cells; mediates neurogenic inflammation in psoriasis |
| Nociceptor | A sensory nerve ending that detects painful or itch-inducing stimuli; nociceptors in psoriatic skin release neuropeptides that amplify local inflammation |
| NICE | National Institute for Health and Care Excellence, the UK body that issues treatment guidance and approves NHS funding |
| Onycholysis | Separation of the nail from the nail bed; common in nail psoriasis |
| PASI (Psoriasis Area and Severity Index) | The standard clinical scoring system for psoriasis severity (0–72) |
| PASI 75 / 90 / 100 | The percentage of improvement from baseline; PASI 100 = complete clearance |
| Pathogenesis | The biological process by which a disease develops and progresses |
| Pharmacogenomics | The study of how an individual’s genetic makeup affects their response to drugs; in psoriasis, HLA-C*06:02 status may predict biologic response |
| Plaque psoriasis | The most common form (~80–90%), featuring raised, red, scaly patches on extensor surfaces |
| Post-inflammatory hyperpigmentation (PIH) | Persistent dark discolouration at sites of healed inflammation; particularly significant in skin of colour |
| PUVA | Psoralen + UVA light therapy |
| Rete ridges | Finger-like projections of the epidermis into the dermis; elongated in psoriasis |
| Substance P | A neuropeptide released by sensory nerve endings that promotes inflammation, vasodilation, and immune cell activation via the NK1 receptor; elevated in psoriatic skin |
| Super-responder | A patient who achieves complete skin clearance that remains stable even when treatment is reduced or withdrawn; concept introduced by the GUIDE trial |
| Systemic disease | A disease that affects the whole body, not just one organ |
| T cell (T lymphocyte) | An adaptive immune cell that matures in the thymus; Th17 cells are key in psoriasis |
| Teratogen | A substance that causes birth defects (e.g., methotrexate, acitretin) |
| Th17 cell | A T helper cell subtype that produces IL-17; the central effector cell in psoriasis |
| TNF-α (Tumour Necrosis Factor alpha) | A major pro-inflammatory cytokine targeted by the first generation of psoriasis biologics |
| TRM cells (tissue-resident memory T cells) | Long-lived T cells that persist in previously affected skin; explain why psoriasis recurs at the same sites |
| Treg (regulatory T cell) | A T cell subtype that suppresses excessive immune responses; dysfunctional in psoriasis |
| TRPV1 | Transient receptor potential vanilloid 1, an ion channel on sensory nerve endings that detects heat, chemicals, and inflammatory mediators; upregulated in psoriatic skin and involved in itch and pain signalling |
| TRPA1 | Transient receptor potential ankyrin 1, an ion channel on sensory neurons that detects chemical irritants and cold; works alongside TRPV1 in psoriatic itch and pain |
| TYK2 | Tyrosine kinase 2, an intracellular enzyme in the IL-23, IL-12, and type I interferon signalling pathways; target of deucravacitinib |
| Allodynia | Pain caused by stimuli that aren’t normally painful (e.g., light touch on inflamed skin); results from peripheral and central sensitisation |
| Autophagy | A cellular self-cleaning process by which cells digest and recycle damaged components; impaired in psoriasis, contributing to NF-κB activation and IL-36 overexpression |
| Central sensitisation | Amplified pain processing in the spinal cord and brain caused by prolonged peripheral inflammation; present in ~43% of PsA patients |
| CRISPR-Cas9 | A gene-editing technology that allows precise modification of DNA sequences; being explored experimentally for targeting psoriasis-related genes |
| Excimer laser | A 308 nm UV light device used for localised psoriasis treatment; delivers targeted UV-B to individual plaques without exposing surrounding healthy skin |
| Extracellular vesicle (EV) | A tiny membrane-enclosed parcel (30–1000 nm) released by cells carrying proteins, lipids, and nucleic acids; serve as long-range molecular messengers and potential biomarkers |
| Exosome | A subtype of extracellular vesicle (30–150 nm) released by cells; psoriatic serum-derived exosomes modify keratinocyte behaviour |
| Ferroptosis | A form of regulated cell death driven by iron-dependent lipid peroxidation; recently identified in psoriatic keratinocytes as a potential pathogenic mechanism |
| IL-31 | A cytokine produced by Th2 cells that acts on sensory neurons via IL-31RA to generate itch signals; a key mediator of psoriatic pruritus |
| IL-36 | A family of pro-inflammatory cytokines (IL-36α, IL-36β, IL-36γ) that drive generalised pustular psoriasis via a distinct autoinflammatory pathway separate from IL-23/IL-17 |
| Metabolomics | The large-scale study of small molecules (metabolites) in biological samples; being used to discover psoriasis biomarkers and monitor treatment response |
| mNAPSI | Modified Nail Psoriasis Severity Index, a scoring system specifically for nail psoriasis, assessing 8 features across 10 fingernails on a 0–130 scale |
| Nanomedicine | The application of nanotechnology to drug delivery; includes liposomes, microneedles, and nanoparticles designed to enhance topical drug penetration |
| Obstructive sleep apnoea (OSA) | A sleep disorder where the airway repeatedly collapses during sleep; screening studies suggest significantly elevated risk in psoriasis patients |
| Palmoplantar psoriasis | Psoriasis specifically affecting the palms and soles; a distinct clinical entity with unique genetic and immunological features |
| Phage therapy | The use of bacteriophages (viruses that specifically kill bacteria) to treat microbial infections; being explored for skin microbiome-based psoriasis treatments |
| PGA × BSA | Physician’s Global Assessment multiplied by Body Surface Area, a composite severity score that is more sensitive than PASI for mild-to-moderate disease |
| Proteomics | The large-scale study of all proteins in a biological sample; being used to develop blood-based psoriasis biomarkers |
| Trained immunity | A form of innate immune memory in which prior inflammatory exposure causes long-lasting epigenetic changes in innate immune cells and tissue stem cells, enabling faster and stronger responses to subsequent stimuli |
| Transcriptomics | The study of all RNA molecules (gene expression) in a cell or tissue; used to identify psoriasis molecular subtypes and treatment response signatures |