Immunology Year One Study Notes

Comprehensive flashcard collection defining core immunological concepts, cellular mediators, lymphoid tissues, immunoglobulins, and serological assays from the Year One Immunology lecture notes.

Immunology

A branch of biology involved in the study of the body's defence mechanisms against microbes, parasites and foreign substances, or how the body maintains the consistency of its internal milieu against invasion, mutations, and unwanted cellular products.

Immunity

The resistance of a host organism to an invasive pathogen (or antigen) and their toxic products.

Susceptibility

The lack of immunity to an invasive pathogen or antigen.

Innate Immunity

Natural, non-specific immunity present at birth and present for life that acts as the first line of defence against invading micro-organisms, lacking immunological memory and remaining constant in intensity upon repeated exposure.

Phagocytosis

The ability of certain immune cells (phagocytes) to ingest and kill micro-organisms by extending pseudopodia, enclosing the particle in a phagosome, and fusing it with a lysosome to form a phagolysosome.

Reverse Phagocytosis

The discharge of neutrophil granule digestive and toxic contents into the exterior milieu when a target organism or particle is too large to be engulfed, resulting in tissue damage.

Inflammation

The non-specific response of tissue to injury or pathogens, characterised by vasodilation, increased vascular permeability, cellular infiltration, heat, redness, swelling, and pain.

Granuloma

A tiny nodule formed when damaged cells clump together over a persistent causative agent to contain it and prevent its spread.

Interferons

A family of glycoproteins produced by nucleated human cells that inhibit viral replication and tumour cell proliferation by inducing an antiviral state, while also modulating NK cells and macrophages.

Lysozyme

Enzymes secreted by macrophages and present in saliva, tears, and human milk that protect against Gram-negative bacteria by hydrolysing the peptidoglycan of the bacterial cell wall.

Defensins

Cationic peptides that form pores in bacterial membranes to kill bacteria, found as $\alpha$-defensins in the lower respiratory tract and $\beta$-defensins in the gastrointestinal tract.

Acute Phase Proteins

Serum proteins produced in high concentrations by the liver upon stimulation by cytokines such as IL-1 during infection, an example being C-reactive protein.

Complement System

A group of approximately $20$ distinct inactive protein factors present in normal serum that activate in an enzymatic cascade to potentiate phagocytosis, induce cell lysis, and stimulate inflammatory reactions.

Adaptive Immunity

An antigen-specific, acquired defence mechanism gained during body development that discriminates between antigenic variants, increases in intensity upon repeated exposure, and generates immunological memory.

Natural Passive Immunity

The transfer of pre-formed antibodies from a mother to an infant across the placenta (IgG) or through colostrum and breast milk.

Artificial Passive Immunity

The therapeutic administration of antitoxins, antibodies (gamma globulins), or sensitised white blood cells from an immune individual to a non-immune person.

Natural Active Immunity

Immunity acquired following natural daily exposure to an antigen, triggering the host immune system to produce antibodies and sensitised lymphocytes.

Artificial Active Immunity

Immunity induced by introducing prepared antigens into a susceptible individual via vaccination to stimulate antibody production and sensitised lymphocytes.

Lag Phase

The initial phase following primary antigen exposure during which B-cells proliferate and differentiate into effector plasma cells and antibodies are not yet detected in serum.

Antigen

A macromolecule (such as a protein, polysaccharide, or lipid) that induces a specific immune response in the body.

Immunogen

A substance that directly elicits an immune response; all immunogens are antigens, but not all antigens are immunogenic.

Hapten

A small molecule that is non-immunogenic on its own but can induce an immune response when coupled to a larger carrier molecule.

Epitope (Antigenic Determinant)

The specific portion of an antigen molecule that combines with the variable region of an antibody or with sensitised T-cell receptors.

Memory Cells

Experienced, quiescent lymphocytes generated during a primary immune response that rapidly respond upon re-exposure to the same antigen to drive secondary immunity.

Effector Cells

Differentiated lymphocytes actively mediating antigen clearance, such as antibody-secreting plasma cells, $\text{CD8}^+$ cytotoxic T-cells, and $\text{CD4}^+$ helper cells.

T-independent Antigens

Antigens that can directly stimulate B-cells to produce antibodies without requiring assistance from T-helper cells.

T-dependent Antigens

Antigens (typically proteins) that require helper T-cell cooperation to stimulate B-cells to produce antibodies.

Alloimmunity (Isoimmunity)

An immune response mounted against non-self antigens (alloantigens or isoantigens) derived from members of the same species.

Superantigens

Antigens, such as Staphylococcus exotoxins, capable of polyclonally activating a large proportion of T-cells (up to $25\%$).

Cytokines

Low-molecular-weight soluble signalling factors released by cells to communicate with and influence other cells through specific surface receptors.

Chemotaxis

The directional migration of a cell or organism in response to a chemical concentration gradient.

Immune Adherence

The binding of complement-opsonised particles, bacteria, or complexes to erythrocytes via erythrocyte complement receptors, facilitating enhanced phagocytosis.

Adjuvants

Substances, such as aluminium potassium sulphate (alum), that enhance the immunogenicity of an antigen when injected together with it.

Haematopoiesis

The production, development, and maturation of all cellular components of blood and blood plasma from pluripotent haematopoietic stem cells.

Neutrophils

Short-lived polymorphonuclear phagocytes with a lifespan of $6\text{ to }12\text{ hours}$ that represent $60\%\text{ to }70\%$ of circulating leucocytes and whose accumulation after death forms pus.

Eosinophils

Granulocytes with a $2\text{ to }3$-lobed nucleus comprising $1\%\text{ to }4\%$ of total WBC that release toxic granule contents to kill extracellular helminth larvae and participate in asthmatic allergies.

Basophils

Circulating granulocytes measuring $10\text{ to }15\,\mu\text{m}$ in diameter comprising $0.5\%\text{ to }1\%$ of WBC that regulate vascular tone and effect IgE-mediated type 1 hypersensitivity.

Mast Cells

Tissue-resident granulocytes (mastocytes/labrocytes) with granules rich in histamine and heparin that drive IgE-mediated type 1 hypersensitivity.

Monocytes

Large mononuclear leucocytes ($12\text{ to }15\,\mu\text{m}$) with kidney-shaped nuclei comprising $5\%\text{ to }10\%$ of WBC that circulate for about $24\text{ hours}$ before entering tissues to mature into macrophages and dendritic cells.

Macrophages

Terminally differentiated, highly phagocytic mononuclear cells that act as professional antigen-presenting cells expressing MHC class II and produce monokines like IL-1.

Dendritic Cells

Motile, bone-marrow-derived cells with branched extensions that act as the most effective professional antigen-presenting cells in the body, migrating to secondary lymphoid organs to activate T-cells.

Plasma Cells

Terminally differentiated, tissue-fixed B-lymphocytes specialised for the secretion of large amounts of specific antibodies.

T-helper Cells (TH / CD4+)

T-lymphocytes bearing CD4 that assist B-lymphocytes and other immune cells; subdivided into Th1 (producing lymphokines) and Th2 (stimulating antibody production).

Cytotoxic T-lymphocytes (Tc / CD8+)

Thymus-derived lymphocytes expressing CD8 that destroy virus-infected cells, malignant cells, and allografts through MHC class I-restricted cytotoxicity.

Natural Killer (NK) Cells

Large granular null lymphocytes expressing CD16 that destroy virus-infected and cancer cells independently of antibodies or complement.

Killer (K) Cells

Cytotoxic null lymphocytes bearing Fc receptors that bind to IgG-coated target cells to execute antibody-dependent cell-mediated cytotoxicity.

Erythropoiesis

The formation of erythrocytes regulated by erythropoietin, progressing from a committed CFU-E through pro-erythroblast, erythroblast stages, normoblast, and reticulocyte to a mature red blood cell.

Megakaryocyte

A large, granule-saturated bone marrow precursor cell whose plasma membrane fragments to generate blood platelets.

Bone Marrow

A primary lymphoid organ located within bone cavities that serves as the major haematopoietic site, generating all blood cell lineages and supporting B-cell maturation.

Thymus

A primary lymphoid organ derived from the 3rd and 4th pharyngeal pouches where immature thymocytes undergo positive and negative selection, differentiation, and maturation into mature T-lymphocytes.

Red Pulp

The splenic tissue compartment containing venous sinuses and cords dominated by erythrocytes and macrophages, responsible for filtering out and destroying senescent red blood cells.

White Pulp

The lymphoid compartment of the spleen surrounding branches of the splenic artery, consisting of the T-cell periarteriolar lymphoid sheath (PALS) and B-cell follicles.

High Endothelial Venules (HEV)

Specialised post-capillary venules in lymph nodes lined by distinct plump endothelial cells through which circulating lymphocytes extravasate into lymphoid tissue.

Mucosa-Associated Lymphoid Tissue (MALT)

Non-encapsulated aggregates of lymphoid tissue distributed beneath mucosal membranes of the digestive, respiratory, and urogenital tracts to provide mucosal barrier defence.

Antibody-Dependent Cell-Mediated Cytotoxicity (ADCC)

A cytotoxic immune mechanism whereby effector cells (such as NK cells or eosinophils) bind to antibody-coated target cells via Fc receptors and discharge granules to kill the target extracellularly.

Fab (Fragment Antigen-Binding)

The arm fragment of an immunoglobulin molecule composed of one constant and one variable domain of both heavy and light chains, responsible for specific epitope recognition.

Fc (Fragment Crystallisable) Region

The tail portion of an immunoglobulin formed by constant domains of the heavy chains that interacts with cell-surface Fc receptors and complement proteins.

Hinge Region

A flexible, proline-rich segment between the Fab arms and Fc trunk of an antibody that permits variable angles for optimal dual-epitope binding.

Immunoglobulin G (IgG)

The predominant monomeric immunoglobulin in serum ($\sim 75\%$) with a molecular weight of $150{,}000\text{ Da}$ that fixes complement, neutralises toxins, opsonises pathogens, and uniquely crosses the placenta.

Immunoglobulin M (IgM)

A pentameric immunoglobulin of $900{,}000\text{ Da}$ joined by a J chain that serves as the primary antibody produced upon initial antigen exposure, excelling at agglutination and complement fixation.

J Chain

A specialised joining polypeptide that covalently links immunoglobulin subunits to form stable polymeric IgA dimers and IgM pentamers, facilitating their external secretion.

Immunoglobulin A (IgA)

The predominant antibody class in external secretions (saliva, tears, colostrum, mucus) consisting of dimers joined by a J chain and protected by a secretory component against proteolytic digestion.

Secretory Component

A polypeptide attached to dimeric IgA that mediates its transcytosis across mucosal epithelial cells and shields it from enzymatic degradation in secretions.

Immunoglobulin D (IgD)

A monomeric antibody ($180{,}000\text{ Da}$) making up less than $1\%$ of serum immunoglobulins that functions as an antigen receptor on naive B-cells and helps prevent tolerance induction.

Immunoglobulin E (IgE)

A trace monomeric immunoglobulin ($0.002\%$ of serum) that binds with high affinity to Fc receptors on mast cells and basophils, mediating type 1 hypersensitivity and anti-parasitic ADCC.

Antibody Specificity

The property of an individual antibody paratope to bind exclusively to a single complementary antigenic determinant based on spatial fit and non-covalent forces.

Cross-Reactivity

The ability of an antibody to bind to an unrelated antigen possessing identical or structurally similar epitopes, typically with lower binding avidity.

Affinity

The intrinsic non-covalent binding strength between a single antigen-binding site (paratope) of an antibody and a single monovalent epitope.

Avidity

The overall functional binding strength between multivalent antigens and multivalent antibodies, boosted by the synergistic bonus effect of multiple simultaneous bonds.

Serology

The in vitro scientific study and clinical evaluation of antigen-antibody reactions in blood serum or bodily fluids.

Agglutination

The visible clumping of insoluble particulate antigens (such as bacteria, fungal cells, or erythrocytes) when cross-linked by specific antibodies termed agglutinins.

Titer

The highest dilution factor of an antiserum that produces visible clumping in a quantitative agglutination assay.

Direct Agglutination

An agglutination assay wherein soluble antibodies react directly with natural surface cellular antigens on erythrocytes, bacteria, or fungi.

Passive Agglutination

An agglutination technique in which soluble antigens are adsorbed or chemically coupled onto insoluble carrier particles (such as latex beads or red blood cells) to detect antibodies.

Reverse Passive Agglutination

An agglutination assay in which specific antibodies are coated onto inert carrier particles to identify and capture soluble antigens present in a test sample.

Direct Coombs Test

An indirect agglutination method using anti-human globulin to detect non-agglutinating (incomplete) antibodies bound directly to the surface of a patient's erythrocytes.

Indirect Coombs Test

An agglutination test where a patient's serum is incubated with Rh-positive red cells and exposed to anti-human globulin to detect circulating incomplete antibodies.

Precipitation

The reaction between soluble antibodies and soluble antigens that combine at optimal proportions to form an insoluble, visible macromolecular aggregate.

Flocculation

A variant of precipitation in which the resulting antigen-antibody complex remains suspended as visible flakes or clumps rather than sedimenting to the bottom.

Prozone Effect

The phenomenon occurring in an antibody-excess zone where an overabundance of antibody relative to antigen inhibits cross-linking and prevents lattice precipitation.

Equivalence Zone

The optimal ratio of antigen to antibody concentrations at which extensive lattice networks form, resulting in maximum visible precipitation.

Post-zone Effect

The phenomenon occurring in an antigen-excess zone where excessive antigen saturates antibody binding sites, forming small soluble complexes that fail to precipitate.

Ouchterlony Assay

A double immunodiffusion assay wherein soluble antigen and antibody diffuse toward each other through agar gel, yielding distinct precipitin lines indicating identity, non-identity, or partial identity.

Single Radial Immunodiffusion

A quantitative immunodiffusion method where antigen placed in a well diffuses radially into an antibody-containing agar gel, forming a precipitin ring whose diameter is proportional to antigen concentration.

Neutralising Antibodies

Antibodies that bind directly to surface components of viruses, bacteria, or toxins, blocking target-cell adhesion, entry, or biological toxicity.