MI2.1-8 | Immunology: Basic, Applied & Immunological Disorders — Glossary

Administration of antigen (vaccine) to stimulate the host's own immune system to generate antibody and memory cells; protection develops over weeks but persists for years.

The antigen-specific, memory-forming arm of immunity mediated by lymphocytes; develops over days to weeks.

A mechanism where IgE (or IgG) coats a target (e.g., helminth larva); effector cells (eosinophils, NK cells) bind via Fc receptors and degranulate, killing the target.

A substance that enhances immunogenicity of an antigen without being antigenic itself; examples include alum and MF59.

The progressive increase in antibody–antigen binding affinity during a germinal centre reaction, driven by somatic hypermutation and selection.

An immunological test in which particulate antigens or antibody-coated particles clump visibly upon reaction with specific antibody or antigen; examples include the Widal test and Brucella agglutination.

Autoantibodies directed against nuclear antigens; detected by indirect immunofluorescence on HEp-2 cells; highly sensitive for SLE (positive in ~95% of cases).

The rapid, high-titre, predominantly IgG antibody response on re-exposure to a previously encountered antigen; basis of booster vaccination.

Autoantibodies against double-stranded DNA; both sensitive and specific for SLE; elevated titres correlate with disease activity and lupus nephritis.

A phenomenon where pre-existing, non-neutralising antibodies facilitate viral entry into Fc receptor-bearing cells (macrophages), leading to increased viral replication and severity; classically described in secondary dengue.

Any substance specifically recognised by lymphocytes or antibodies, typically a protein or polysaccharide on a pathogen's surface.

A condition in which the immune system fails to maintain self-tolerance and attacks the body's own tissues, generating autoreactive T cells and/or autoantibodies.

T lymphocyte bearing the CD4 surface marker that recognises antigen on MHC class II; orchestrates both humoral and cell-mediated immune responses.

T lymphocyte bearing the CD8 surface marker that recognises antigen on MHC class I; kills virally infected cells and tumour cells via perforin/granzyme.

A monoclonal antibody that blocks inhibitory checkpoint molecules (anti-PD-1, anti-CTLA-4), releasing T-cell suppression and restoring anti-tumour immunity; used in melanoma, lung cancer, cervical cancer.

A primary phagocyte immunodeficiency caused by NADPH oxidase deficiency; phagocytes can ingest but cannot kill catalase-positive organisms; NBT/DHR test is diagnostic.

The process by which a B cell changes the isotype of antibody it produces (e.g., IgM to IgG) while maintaining the same antigen specificity; driven by T-cell cytokines.

The system of refrigerated storage and transport that maintains vaccine potency from manufacture to administration; critical for live attenuated vaccines.

A cascade of ~30 serum proteins that can be activated by antibody–antigen complexes, MBL, or microbial surfaces; functions include opsonisation, chemotaxis, and cell lysis via MAC.

A vaccine in which a polysaccharide antigen is chemically linked to a carrier protein, converting a T-independent antigen to a T-dependent antigen capable of generating memory in infants.

A primary T-cell immunodeficiency caused by thymic aplasia due to 22q11 deletion; characterised by absent T cells, absent thymic shadow on chest X-ray, hypocalcaemia, and recurrent viral/fungal infections.

An immunological technique in which a fluorescence-labelled antibody is applied directly to a specimen to detect the specific antigen; used in rabies diagnosis (brain DFA) and Chlamydia detection.

The most widely used diagnostic immunological test; detects antigen or antibody via enzyme-linked secondary antibody producing a colour change; used for HIV, HBsAg, dengue serology.

The specific molecular region (antigenic determinant) on an antigen that binds to an antibody or T-cell receptor.

A technique that counts and characterises cells in suspension using laser light and fluorescence-labelled antibodies against surface markers; used for CD4 T-cell counting in HIV monitoring.

A complication of bone marrow/HSCT where immunocompetent donor T cells attack recipient tissues; acute GvHD affects skin, gut, liver; chronic GvHD resembles autoimmune disease.

A collection of activated macrophages (epithelioid cells), giant cells, and lymphocytes that walls off intracellular pathogens such as Mycobacterium tuberculosis that resist killing.

A small molecule that is antigenic but not immunogenic alone; becomes immunogenic when coupled to a larger carrier protein.

Indirect protection of susceptible individuals when a sufficient proportion of the population is immune, breaking chains of transmission.

The minimum proportion of the population that must be immune to break transmission chains; calculated as 1 − 1/R₀.

The human MHC gene complex encoding polymorphic cell surface molecules that present peptide antigens to T lymphocytes; major determinant of transplant compatibility.

Transplant rejection occurring within minutes to hours due to pre-existing donor-specific antibodies (anti-ABO or anti-HLA); causes thrombotic graft destruction; prevented by crossmatch testing.

An exaggerated or inappropriate immune response to an antigen, causing tissue damage; classified into Types I–IV by the Gell and Coombs system.

Dimeric antibody found in mucosal secretions (saliva, breast milk, intestinal secretions); primary defence at mucosal surfaces.

Antibody mediating Type I hypersensitivity (allergy and anaphylaxis); also important in anti-helminth immunity via eosinophil activation.

Most abundant serum antibody; mediates secondary immune responses; only isotype that crosses the placenta to protect the neonate.

Pentameric immunoglobulin; the first antibody produced in a primary immune response; best complement activator.

Inhibitory signalling pathways (e.g., PD-1/PD-L1, CTLA-4/B7) that normally prevent autoimmunity by restraining T-cell activation; exploited by tumours to evade immunity.

An antigen capable of inducing an immune response (antibody production or T-cell activation).

Glycoprotein produced by plasma cells; consists of two heavy and two light chains forming a Y-shape with antigen-binding Fab and effector Fc regions.

The non-specific, rapid (minutes to hours) first-line defence present from birth, including physical barriers, phagocytes, NK cells, and complement.

A vaccine containing dead or chemically inactivated pathogen; generates mainly antibody; safe in immunocompromised; usually requires booster doses.

A vaccine containing a living but weakened form of the pathogen; generates strong and durable CMI and antibody with memory; cannot be used in immunocompromised individuals.

The terminal complement complex (C5b–C9) that inserts into bacterial membranes, creating pores that lyse the organism.

Long-lived B lymphocytes generated during a germinal centre reaction that persist after antigen clearance and mount a rapid secondary response upon re-exposure.

Cell surface molecule expressed on all nucleated cells that presents endogenous (intracellular) antigens to CD8+ T cells.

Cell surface molecule expressed on professional antigen-presenting cells (dendritic cells, macrophages, B cells) that presents exogenous antigens to CD4+ T cells.

Innate lymphocytes that kill cells lacking MHC class I (e.g., tumour cells, virus-infected cells) without prior sensitisation.

Antibody-mediated blocking of pathogen attachment or entry into host cells, preventing infection.

Any molecule (IgG, C3b) that coats a pathogen and enhances its recognition and phagocytosis by macrophages and neutrophils.

Coating of a pathogen with antibody (IgG) or complement (C3b) to enhance phagocytosis by macrophages and neutrophils.

A measure of the proportion of a panel of donor lymphocytes that react with a transplant recipient's serum antibodies; high PRA indicates broad sensitisation and difficulty finding a compatible donor.

Administration of pre-formed antibodies (immunoglobulin preparations or antisera) for immediate but temporary protection; no memory is generated.

A terminally differentiated B lymphocyte that secretes large quantities of antigen-specific antibody.

The average number of secondary cases generated by one primary case in a completely susceptible population; determines transmissibility and HIT.

A systemic Type III hypersensitivity reaction occurring 7–14 days after administration of heterologous (animal) antisera; caused by immune complex deposition.

CD4+ T helper subset that produces IFN-γ and IL-2; drives macrophage activation and cytotoxic responses against intracellular pathogens like Mycobacterium tuberculosis.

A CD4+ T helper cell response characterised by IFN-γ and IL-2 production; essential for macrophage activation and control of intracellular pathogens.

CD4+ T helper subset that produces IL-4, IL-5, IL-13; drives IgE production, eosinophil activation, and mast cell responses; important in allergy and helminth immunity.

A CD4+ T helper cell response characterised by IL-4, IL-5, IL-13 production; drives IgE synthesis, eosinophil activation, and anti-helminth immunity.

Pattern recognition receptors on innate immune cells that recognise conserved pathogen-associated molecular patterns (PAMPs) such as LPS, flagellin, and viral RNA.

A vaccine made from chemically inactivated bacterial exotoxin; generates antibody against the toxin (e.g., diphtheria and tetanus toxoids).

An antigen present on tumour cells and also on some normal cells; often overexpressed in tumours; used as a tumour marker (e.g., CEA, AFP, PSA).

IgE-mediated immediate hypersensitivity; mast cell degranulation releases histamine and leukotrienes; clinical examples include anaphylaxis, atopic asthma, allergic rhinitis.

Cytokines produced rapidly by virus-infected cells that induce an antiviral state in neighbouring cells and activate NK cells.

Antibody (IgG/IgM)-mediated cytotoxic hypersensitivity targeting cell-surface antigens; examples include haemolytic transfusion reactions and Goodpasture syndrome.

Immune complex-mediated hypersensitivity; soluble antigen–antibody complexes deposit in vessel walls and tissue; activate complement; examples include serum sickness and SLE nephritis.

Delayed-type hypersensitivity (DTH) mediated by sensitised Th1 cells and macrophages, not antibody; onset 24–72 hours; examples include tuberculin test and contact dermatitis.

A flocculation screening test for syphilis detecting reagin (anti-cardiolipin antibodies); can yield biological false-positives in SLE, TB, malaria, and pregnancy.

A confirmatory immunological test in which proteins are separated by SDS-PAGE and transferred to a membrane; probed with patient serum to detect antibodies to specific antigens; used for HIV confirmation.

A primary humoral immunodeficiency caused by a BTK gene mutation; characterised by absent circulating B cells, absent immunoglobulins, and recurrent pyogenic infections beginning at 6 months.