Comprehensive Study Notes – Antigen–Antibody Reactions & Serological Techniques

Introduction and Biological Significance

Antigen–antibody reactions (AARs) are highly specific, non-covalent interactions between an antigen (Ag) and the antibody (Ab) that was elicited against it. These reactions:

  • Provide natural protection in vivo (neutralisation of toxins/viruses, opsonisation, complement activation).
  • Underlie diagnostic assays that detect either Ag or Ab in vitro.
  • May trigger tissue injury in hypersensitivity and autoimmune diseases.

Physicochemical Basis of Binding

  1. Bonds involved (all reversible): electrostatic, hydrogen, van der Waals and hydrophobic interactions.
  2. A “good fit” (steric complementarity between the Ag determinant and the Ab combining site) maximises multiple simultaneous weak bonds, stabilising the complex.
  3. No irreversible chemical modification of Ag or Ab occurs; complexes can dissociate at extreme pH or high ionic strength.

Quantitative Descriptors

  • Affinity (K_a) – strength of binding between a single epitope and one Fab; high-affinity Abs stay bound longer.
  • Avidity – overall strength when a multivalent Ag meets a multivalent Ab; depends on valencies and usually \text{avidity} > \sum \text{individual affinities}.
  • Specificity – capacity of a combining site or Ab population to react only with its homologous Ag.
  • Cross-reactivity – binding to structurally similar, “heterologous” epitopes; clinically causes heterophile tests (e.g., Weil–Felix) and some autoimmune phenomena.

Stages of the Antigen–Antibody Reaction

StageNatureBonds DominatingVisibility
PrimaryRapid, reversibleWeak non-covalentNone
SecondaryIrreversible lattice/networksCovalent stabilisationVisible events (precipitation, agglutination, complement fixation, etc.)

Titer is the highest reciprocal serum dilution still giving a positive secondary-stage read-out: e.g. \frac{1}{128} > \frac{1}{8} indicates more Ab.

Diagnostic Families of AARs

  1. Precipitation
  2. Agglutination
  3. Complement-dependent tests
  4. Neutralisation
  5. Opsonisation
  6. Immunofluorescence
  7. Enzyme immunoassay (ELISA)
  8. Radioimmunoassay (RIA)
  9. Western blotting
  10. Chemiluminescence assays
  11. Immunoelectron microscopy

Each exploits identical molecular interactions but translates them into different laboratory read-outs.

Precipitation Reactions

Occurs with soluble Ag + Ab → lattice that becomes insoluble.

Fundamental Principles

  • Requires bivalent Ab and at least bivalent (preferably polyvalent) Ag.
  • Detectable only in the zone of equivalence where Ag and Ab are roughly equal.
    • Prozone (Ab excess) and postzone (Ag excess) give false negatives; correct by diluting the excessive component.

Precipitation in Solution

  • Ring test – layering Ag over antiserum; precipitin ring (e.g., C-reactive protein, Lancefield grouping).
  • Flocculation – precipitate remains suspended; VDRL slide test, Kahn tube test.

Immunodiffusion (Precipitation in Agar)

Agar ≈ 0.3–1.5%0.3–1.5\% permits diffusion; agarose preferred (minimal charge).

  1. Single diffusion, 1-D (Oudin): Ag diffuses into Ab-containing gel → vertical bands.
  2. Single diffusion, 2-D (Radial immunodiffusion): diameter of ring ∝ [Ag]; used for quantitating IgG, IgM, IgA, complement, viral Abs.
  3. Double diffusion, 1-D (Oakley–Fulthrope): Ag and Ab diffuse towards each other through plain agar spacer.
  4. Double diffusion, 2-D (Ouchterlony): central Ab well + peripheral Ag wells produce arcs (identity), crossed lines (non-identity) or spur (partial identity). Useful in mycology, smallpox serology, ENA autoantibodies, Elek test for diphtheria toxin.

Immunodiffusion with Electric Field

  • Immunoelectrophoresis (IEP) – electrophoretic separation followed by diffusion; profiles serum proteins, detects myeloma bands.
  • Counter-current immunoelectrophoresis (CIEP) – Ag moves to anode, Ab to cathode; line forms within 30–60 min30–60\,\text{min}. Rapid detection of HBsAg, cryptococcal Ag in CSF.
  • Rocket electrophoresis (Laurell) – Ag migrates into Ab-gel during electrophoresis; rocket height ∝ [Ag].
  • 2-D immunoelectrophoresis – orthogonal second run; qualitative + quantitative serum survey.

Light-Scattering Methods

  • Turbidimetry – measures loss of incident light.
  • Nephelometry – detects light scattered at an angle; linear relationship allows automated measurement of IgG, complement, RA factor, ASLO, etc.

Agglutination Reactions

Visible clumping when particulate Ag (cells, beads) binds Ab.

Contrasts with Precipitation

  • Surface phenomenon; Ag must be exposed.
  • Because cells present many determinants, prozone is rarer.
  • Blocking (incomplete) Abs may coat but not agglutinate (e.g., anti-Rh).

Direct Agglutination

  1. Slide test – rapid ID of Salmonella, Shigella, Vibrio; blood grouping.
  2. Tube test – quantitative (Widal for typhoid; Brucella SAT – beware prozone; use Coombs’).
  3. Heterophile tests – Weil–Felix (Proteus OX strains ↔ rickettsiae), Paul–Bunnell (sheep RBC ↔ EBV), Streptococcus MG ↔ M. pneumoniae.
  4. Coombs’ (antiglobulin) test – detects RBC-bound incomplete IgG.
    • Direct – Ab already on patient RBCs (HDN, transfusion reaction).
    • Indirect – patient serum tested against donor RBCs (prenatal screening).

Passive/Indirect Agglutination (Particulate Carrier Added)

  • Latex agglutination – polystyrene beads coated with Ag or Ab; detect CRP, ASLO, RA factor, B-streptococcal Ag, N. meningitidis polysaccharide.
  • Indirect hemagglutination (IHA) – RBCs coated with Ag. Used for amoebiasis, hydatid, toxoplasmosis. Reverse passive HA detects HBsAg.
  • Viral hemagglutination inhibition – serum Abs prevent virus-mediated RBC agglutination (influenza, mumps, measles).
  • Coagglutination – Staphylococcus aureus Cowan I protein A binds Fc of IgG, leaving Fab free. More stable than latex; detects cryptococcal, amoebic, hydatid Ags; groups streptococci/mycobacteria.

Complement-Dependent Tests

Complement = >20 serum proteins forming lytic cascade.

1. Complement Fixation Test (CFT)

  • Test system: patient serum (heat-inactivated) + specific Ag + standardized guinea-pig complement.
  • Indicator system: “sensitized” sheep RBC + anti-sheep Ab.
    • If patient Ab–Ag fixes complement → none left → no RBC lysis = POSITIVE.
    • If no patient Ab → free complement → hemolysis = NEGATIVE.
  • Indirect CFT and conglutinination variants accommodate sera incapable of fixing guinea-pig complement.
  • Historically for Wassermann (syphilis), Mycoplasma, fungal serology; now largely abandoned (technical complexity).

2. Immune Adherence

Complement + specific Ab causes bacteria to adhere to RBC/platelets, aiding phagocytosis (e.g., cholera V. cholerae, T. pallidum).

3. Immobilisation

Treponema pallidum immobilisation test: live spirochetes lose motility in presence of Ab + complement.

4. Cytolytic/Cytocidal Reactions

Complement-mediated killing of target cells/bacteria when coated with Ab.

Neutralisation Tests

  • Virus neutralisation – Ab prevents viral entry/replication in cell culture, embryonated eggs or animals (e.g., poliovirus, influenza HAI).
  • Toxin neutralisation
    • In vivo: Schick (diphtheria), C. welchii toxin test.
    • In vitro: ASO (antistreptolysin O), Nagler reaction (lecithinase of C. perfringens on egg-yolk agar neutralised by antitoxin).

Opsonisation and the Opsonic Index

Opsonins (complement C3b, IgG Fc) coat particles → enhanced phagocytosis. Opsonic index = phagocytes with ingested bacteria in patient bloodsame in normal control\frac{\text{phagocytes with ingested bacteria in patient blood}}{\text{same in normal control}}; once used to monitor therapy.

Immunofluorescence (IF)

Fluorochromes: fluorescein isothiocyanate (green), lissamine rhodamine (orange-red).

Direct IF (DFA)

Labeled Ab attaches directly to Ag in tissue smear (e.g., rabies virus in nape-of-neck skin, N. gonorrhoeae in urethral exudate).

Indirect IF (IFA)

Unlabeled patient Ab binds fixed Ag → secondary fluorescent anti-Ig detects. Multipurpose, more sensitive, allows isotype typing, lymphocyte phenotyping, ANA screening.

Limitations: cost of fluorescent microscope/reagents, need for expertise, subjective reading.

Enzyme Immunoassays (ELISA family)

Enzymes: alkaline phosphatase, horseradish peroxidase, β-galactosidase.

Indirect ELISA

Plate coated with Ag; patient Ab → enzyme-anti-isotype conjugate → substrate → color ∝ Ab concentration. Mainstay for HIV, JE, dengue serology.

Sandwich ELISA

Capture Ab on plate → patient Ag → enzyme-linked second Ab → substrate. Detects rotavirus, E. coli enterotoxin, many hormones/cytokines.

Competitive ELISA

Patient Ab competes with enzyme-labelled Ab for plate-bound Ag. Color development inversely proportional to patient Ab amount. Widely used for HIV screening.

ELISPOT

Antibody or cytokine secreted by individual cells captured on membrane; spots counted to enumerate secreting cells.

Radioimmunoassay (RIA)

Competitive binding between radio-labelled (^125I, ^131I) and cold Ag for limited Ab. Sensitivity down to picogram/nanogram. Quantifies hormones, drugs, HBsAg. Drawbacks: cost, short half-life isotopes, radioactive waste.

Western Blot (Immunoblot)

  1. SDS-PAGE separates proteins by size.
  2. Transfer (electroblot) to nitrocellulose/ PVDF membrane.
  3. Probe with patient serum (primary Ab).
  4. Enzyme- or radio-labelled anti-human Ig (secondary). Development with chromogen or chemiluminescence produces bands corresponding to viral or other proteins.

Applications: confirmatory HIV diagnosis (bands to gp120, gp41, p24), detect neurocysticercosis Ab, TB meningitis Ab, size quantification.

Chemiluminescence Immunoassay

Antigen-Ab reaction triggers light-emitting chemical; automated, highly sensitive. Employed in drug susceptibility testing for Mycobacterium tuberculosis and many endocrine assays.

Immunoelectron Microscopy

  • Immunoelectronmicroscopy – virions + Ab cluster; rotavirus, HAV detection in stool.
  • Immunoenzyme EM – peroxidase-labelled Ab reveals tissue Ag ultrastructurally.
  • Immunoferritin – ferritin-conjugated Ab gives electron-dense signal on Ag.

Practical & Ethical Considerations

  • Select the assay balancing sensitivity, specificity, speed, cost, hazard (radioactivity) and equipment.
  • Understand prozone/postzone to avoid false negatives.
  • Cross-reactions mandate confirmatory testing (e.g., Western blot after ELISA for HIV).
  • Laboratory professionals must ensure proper disposal of biohazard and radioactive materials.

Connections to Prior Knowledge and Clinical Relevance

  • Builds on basic protein–ligand chemistry, diffusion laws (Graham’s law), electrochemistry (electrophoresis).
  • Integral to blood banking, vaccine efficacy monitoring, transplantation typing and autoimmunity work-ups.
  • Provides real-world examples of how molecular recognition translates into bedside diagnostics, reinforcing immunological principles of specificity and memory.