Antigen-Antibody Reactions and Hypersensitivity Reactions

Antigen-Antibody Reaction

A specific interaction between an antigenic determinant (epitope) of an antigen and the antigen-binding site (paratope) of its corresponding antibody, based on lock-and-key specificity via non-covalent binding.

Epitope

An antigenic determinant located on an antigen.

Paratope

An antigen-binding site located on an antibody.

Specificity

The property where an antibody reacts specifically with its corresponding antigen (e.g., Anti-HBs reacting with HBsAg).

Cross-reactivity

The phenomenon where an antibody produced against one antigen reacts with a structurally similar antigen, which can contribute to molecular mimicry.

Affinity

The intrinsic binding strength between a single antigenic epitope and a single antibody-binding site.

Avidity

The overall combined strength of binding between a multivalent antigen and a multivalent antibody.

Prozone

A zone of antibody excess in an antigen-antibody interaction where antibody molecules saturate antigenic sites, preventing proper lattice formation and causing false negative or weak reactions.

Zone of Equivalence

The optimal antigen-to-antibody ratio that allows maximum lattice formation and produces the maximum visible reaction.

Postzone

A zone of antigen excess where antigen molecules saturate antibody-binding sites without forming a large lattice, resulting in a weak or negative reaction.

Primary Reactions

The initial, non-visible actual antigen-antibody binding interactions, such as ELISA, Radioimmunoassay, and Immunofluorescence.

Secondary Reactions

Antigen-antibody interactions that produce demonstrable visible effects, including precipitation, agglutination, complement fixation, and neutralization.

Tertiary Reactions

The biological consequences resulting from antigen-antibody interactions, such as immunity, hypersensitivity, and tissue injury.

Precipitation Reaction

A reaction occurring between a soluble antigen and its corresponding soluble antibody in optimal proportions, forming an insoluble precipitate lattice.

Ring Precipitation

A precipitation technique where antigen and antibody solutions are layered over each other, forming a visible white precipitate ring at the junction.

Gel Precipitation

A reaction in which antigen and antibody diffuse through a gel medium and create a visible line of precipitation where they meet in optimal proportions.

Ouchterlony Double Diffusion

A double gel diffusion method where both antigen and antibody diffuse to compare antigenic relationships, displaying patterns of identity, non-identity, or partial identity.

Immunoelectrophoresis

A diagnostic technique combining electrophoresis and immunodiffusion to separate proteins electrophoretically before reacting them with specific antibodies.

Agglutination Reaction

Visible clumping formed by the cross-linking and lattice formation of a particulate antigen with its specific antibody.

Direct Agglutination

Agglutination where naturally occurring particulate antigens are directly clumped by specific antibodies, such as in the Widal test, Weil-Felix test, or blood grouping.

Passive Agglutination

Agglutination in which soluble antigens are artificially coated onto carrier particles (such as latex or RBCs) to detect corresponding antibodies.

Reverse Passive Agglutination

An agglutination test where specific antibodies are attached to carrier particles to detect soluble antigen present in a patient specimen.

Co-agglutination

An agglutination technique using antibody attached to protein A-containing Staphylococcus aureus cells, where protein A binds the Fc region of IgG while leaving Fab free to bind antigen.

Agglutination Inhibition

A test principle where the presence of antigen in a patient sample neutralizes added antibodies, preventing agglutination and indicating a positive result.

Complement Fixation Test

A serological test where a patient's antibody-antigen complex consumes complement, leaving no complement available to lyse an added indicator system of sheep RBCs and anti-sheep RBC antibodies (absence of haemolysis indicates a positive test).

Direct Immunofluorescence

A diagnostic procedure using a known fluorescent-labelled primary antibody to directly detect antigen in a patient specimen under a fluorescence microscope.

Indirect Immunofluorescence

A two-antibody diagnostic system where patient antibodies bind to a known antigen, followed by signal amplification using fluorescent-labelled anti-human globulin.

ELISA

Enzyme Linked Immunosorbent Assay; a assay using enzyme-labelled antigens or antibodies to produce a visible substrate-driven color reaction.

Hypersensitivity

An exaggerated or inappropriate immune response occurring in a previously sensitized individual upon subsequent exposure to a specific antigen, causing tissue injury or disease.

Allergen

An antigen that produces a hypersensitivity or allergic response.

Type I Hypersensitivity

An immediate, IgE-mediated anaphylactic reaction occurring within minutes after allergen cross-links IgE bound to mast cells, releasing preformed and newly synthesized mediators.

Anaphylaxis

A severe, rapidly developing systemic Type I hypersensitivity reaction characterized by urticaria, angioedema, bronchospasm, laryngeal edema, hypotension, and shock.

Type II Hypersensitivity

A cytotoxic or antibody-mediated hypersensitivity reaction driven by IgG or IgM binding to cell-surface or tissue antigens, causing cell lysis, phagocytosis, ADCC, or cell dysfunction.

ADCC

Antibody-Dependent Cellular Cytotoxicity; a Type II mechanism where natural killer (NK) cells recognize antibody-coated target cells via Fc receptors to destroy them.

Type III Hypersensitivity

An immune-complex mediated hypersensitivity caused by circulating soluble antigen-antibody (IgG/IgM) complexes depositing in tissues, activating complement, and recruiting neutrophils.

Serum Sickness

A systemic Type III immune-complex disease characterized by fever, rash, arthralgia, lymphadenopathy, and proteinuria.

Arthus Reaction

A localized Type III immune-complex reaction that develops at an injection site in a sensitized individual, leading to localized edema, erythema, and necrosis.

Type IV Hypersensitivity

A delayed, cell-mediated hypersensitivity response mediated by sensitized T lymphocytes and cytokines, typically requiring $48\text{--}72\text{ h}$ or longer to manifest.

Tuberculin-type Reaction

A delayed (Type IV) hypersensitivity response exemplified by the Mantoux test, where intradermal PPD triggers memory T-cell activation, cytokine release, and mononuclear cell induration read at $48\text{--}72\text{ h}$.

Granulomatous Hypersensitivity

A form of Type IV hypersensitivity caused by persistent intracellular organisms (such as Mycobacterium tuberculosis)