Immunology Laboratory Manual: Experimental Protocols & Diagnostic Immunoassays
ABO and Rh (D) forward/reverse hemagglutination typing, quantitative radial immunodiffusion (Mancini), Ouchterlony double immunodiffusion (ODD) precipitin analysis, indirect and sandwich ELISA microplate assays, Widal tube titration for enteric fever, Ficoll-Paque density gradient isolation of peripheral blood mononuclear cells (PBMCs), and direct/indirect immunofluorescence microscopy aligned with B.Sc. (Hons), M.Sc., CSIR-NET Life Sciences (Unit 4), and GATE practical syllabi.
๐ก๏ธ Clinical Immunology & Biosafety Level 2 (BSL-2) Standard Operating Procedures (SOP)
Immunological diagnostics handle human blood, serum, viral/bacterial antigens, and cytotoxic dyes. Strict adherence to Universal Precautions (CDC/WHO) is mandatory:
Treat all human blood products as potentially infectious for bloodborne viruses (HBV, HCV, HIV). Wear double nitrile gloves, protective lab coats, and safety eyewear. Never mouth-pipette. Use sterile filter-barrier tips exclusively.
Sterile single-use blood lancets and venipuncture needles must be discarded immediately after use directly into puncture-resistant, rigid yellow biohazard sharps containers without recapping.
Tetramethylbenzidine (TMB), ortho-phenylenediamine (OPD), and sulfuric acid ($2,text{N H}_2text{SO}_4$) are toxic and irritating. FITC fluorochromes are light-sensitive. Prepare solutions in a chemical fume hood and store in amber foil-wrapped containers at $4^circtext{C}$.
Decontaminate all ELISA wash wastes and culture supernatants by adding $10%$ sodium hypochlorite (household bleach) for at least 30 minutes before sink disposal. Autoclave all plasticware at $121^circtext{C}$ for 30 minutes.
ABO & Rhesus (Rh D) Blood Group Typing by Forward & Reverse Hemagglutination
๐ฌ Principle & Theoretical Background
The human ABO blood group system was discovered by Karl Landsteiner in 1900, establishing the foundation of safe blood transfusion medicine. ABO antigens (agglutinogens) are carbohydrate oligosaccharide chains covalently linked to glycoproteins and glycosphingolipids on the erythrocyte plasma membrane:
- H-Antigen Precursor: The basic oligosaccharide precursor terminates in L-fucose attached by an $alpha(1to 2)$ fucosyltransferase (encoded by the *FUT1* gene).
- A-Antigen: The *A* allele encodes an $alpha(1to 3)$-N-acetylgalactosaminyltransferase that attaches N-acetylgalactosamine (GalNAc) to the terminal galactose of the H antigen.
- B-Antigen: The *B* allele encodes an $alpha(1to 3)$-galactosyltransferase that attaches D-galactose to the H antigen.
- O-Phenotype: The *O* allele contains a single base deletion ($Delta 261text{G}$) causing a frameshift and producing an inactive enzyme; the unextended H antigen remains unaltered.
Landsteiner’s Law:
1. If an agglutinogen (antigen) is present on an individual’s red blood cells, the corresponding isoantibody (agglutinin) must be absent from plasma.
2. If an agglutinogen is absent from red blood cells, the corresponding isoantibody must be present in plasma (naturally occurring IgM antibodies produced in response to gut bacterial microbiota).
Rhesus (Rh) System: The Rh blood group system depends on transmembrane protein antigens encoded by the *RHD* and *RHCE* genes on Chromosome 1. The D antigen is the most clinically immunogenic. Individuals possessing the D antigen are classified as Rh-positive ($text{Rh}^+$); individuals lacking the D antigen (often due to homozygous deletion of the *RHD* gene) are Rh-negative ($text{Rh}^-$). Unlike ABO antibodies, anti-D antibodies are not naturally occurring; they are acquired IgG antibodies generated following exposure through transfusion or feto-maternal hemorrhage (causing hemolytic disease of the fetus and newborn, HDFN / erythroblastosis fetalis).
๐ Step-by-Step Slide Grouping Protocol
- Tile Preparation: Clean a dry white glazed porcelain tile or three-well glass slide. Label the three reaction zones as “Anti-A”, “Anti-B”, and “Anti-D”.
- Antisera Dispensing: Place one drop ($~50,mutext{L}$) of monoclonal Anti-A reagent (blue dye) in the first well, one drop of Anti-B reagent (yellow dye) in the second well, and one drop of Anti-D reagent (clear) in the third well.
- Blood Sample Addition: Clean the subject’s fingertip with an alcohol swab, allow to air-dry, and puncture with a sterile single-use disposable lancet. Wipe away the first drop of blood. Add one drop of whole blood (or a $5%$ RBC suspension in saline) adjacent to each antiserum drop.
- Mixing & Rocking: Using separate clean applicator sticks for each circle (never reuse sticks to prevent cross-contamination), mix the blood and reagent thoroughly across a $20,text{mm}$ area.
- Reading Agglutination: Rock the tile gently back and forth for exactly 2 minutes. Observe under strong incident light for visible brick-red clumping/lattice formation (hemagglutination) versus smooth uniform suspension.
๐ Blood Group Phenotype Interpretation Matrix
| Anti-A (Blue) | Anti-B (Yellow) | Anti-D (Clear) | RBC Antigen(s) | Serum Antibody | Resulting Blood Group |
|---|---|---|---|---|---|
| Agglutination (+) | No Clumping (-) | Agglutination (+) | A and D | Anti-B | A Positive (Aโบ) |
| Agglutination (+) | No Clumping (-) | No Clumping (-) | A only | Anti-B | A Negative (Aโป) |
| No Clumping (-) | Agglutination (+) | Agglutination (+) | B and D | Anti-A | B Positive (Bโบ) |
| No Clumping (-) | Agglutination (+) | No Clumping (-) | B only | Anti-A | B Negative (Bโป) |
| Agglutination (+) | Agglutination (+) | Agglutination (+) | A, B, and D | None | AB Positive (ABโบ) |
| Agglutination (+) | Agglutination (+) | No Clumping (-) | A and B | None | AB Negative (ABโป) |
| No Clumping (-) | No Clumping (-) | Agglutination (+) | D only | Anti-A and Anti-B | O Positive (Oโบ) |
| No Clumping (-) | No Clumping (-) | No Clumping (-) | None | Anti-A and Anti-B | O Negative (Oโป) |
Quantitative Radial Immunodiffusion (RID – Mancini Technique) for Serum Immunoglobulin Assay
๐ฌ Principle & Mathematical Derivation
Radial Immunodiffusion (RID), developed by Giuliano Mancini, J. Carbonara, and J.F. Heremans in 1965, is a single diffusion technique in which a monospecific polyclonal antibody is incorporated at uniform concentration into molten agarose.
When antigen is pipetted into cylindrical wells cut into the gel, it diffuses radially outward in all directions. Initially, near the well margin, antigen is present in large excess ($Ag > Ab$), forming soluble antigen-antibody complexes. As the antigen diffuses outward, its concentration drops until it reaches the Zone of Equivalence, where the ratio of multivalent antigen to bivalent antibody forms an insoluble, high-molecular-weight crosslinked three-dimensional lattice. This produces an opaque, circular precipitin ring.
At diffusion equilibrium (the Mancini endpoint method, typically 48 to 72 hours), all available antigen has migrated out of the well and reacted with antibody. Mathematically, the area of the precipitin ring ($pi r^2$) is directly proportional to the total amount of antigen applied:
Where $d$ is the outer diameter of the precipitin ring in millimeters, $m$ is the slope of the calibration line, $[text{Ag}]$ is the antigen concentration, and $c$ is the intercept on the y-axis (the square of the well diameter, $d_0^2$).
๐ Step-by-Step Laboratory Protocol
- Gel Preparation: Prepare a $1.2%$ solution of agarose in PBS (pH 7.4, with $0.02%$ sodium azide preservative). Boil until clear, then cool in a water bath to exactly $50^circtext{C}$ (higher temperatures denature antibody; lower temperatures cause premature gelling).
- Antibody Incorporation: Add monospecific goat or rabbit anti-human IgG antiserum to the agarose at a pre-calibrated dilution (typically $1:20$ to $1:50$). Gently invert to mix without generating bubbles, and pour immediately onto a level glass slide or RID plastic plate ($1.5,text{mm}$ uniform gel thickness). Allow to set at room temperature for 30 minutes.
- Well Punching: Using a sharp $3.0,text{mm}$ gel punch and template, punch 4 to 6 clean cylindrical wells spaced at least $15,text{mm}$ apart. Remove the agar plugs by gentle vacuum suction.
- Sample Loading: Using a calibrated precision micro-pipette, deliver exactly $5.0,mutext{L}$ of standard IgG calibrators ($25,mutext{g/mL}$, $50,mutext{g/mL}$, $100,mutext{g/mL}$, $200,mutext{g/mL}$) and unknown patient serum into designated wells. Avoid touching the well rim.
- Incubation: Place the plate horizontally inside a moist, sealed humid chamber (petri dish with wet filter paper) and incubate at room temperature ($20-25^circtext{C}$) for 48 hours to reach complete equilibrium.
- Ring Measurement: Place the plate on an illuminated RID viewer with a graduated eyepiece micrometer or dark-field magnifying viewer. Measure the ring diameter ($d$) to the nearest $0.1,text{mm}$ in two perpendicular directions and record the mean diameter.
Ouchterlony Double Immunodiffusion (ODD): Qualitative Epitope Analysis & Antigenic Relationships
๐ฌ Principle & Theoretical Background
The Ouchterlony Double Immunodiffusion (ODD) assay was developed by the Swedish bacteriologist รrjan Ouchterlony in 1948. It is a passive double diffusion technique in which both soluble antigen and soluble antibody diffuse radially toward each other through a neutral, antibody-free agarose matrix.
As antigen and antibody migrate toward one another, opposing concentration gradients develop. At the region of optimal proportion (the equivalence zone), multivalent antigen and bivalent antibody crosslink into an insoluble precipitin line visible to the naked eye. The spatial morphology of the precipitin lines where adjacent diffusion patterns meet provides unambiguous qualitative diagnostic information about antigenic relatedness:
- 1. Reaction of Full Identity (Smooth Continuous Arc): Observed when two adjacent wells contain identical antigens sharing the same epitopes. The two advancing precipitin lines fuse seamlessly into a single, smooth curved arc with no spur or extension.
- 2. Reaction of Non-Identity (Crossed Lines / X-Pattern): Observed when adjacent wells contain completely different, unrelated antigens that react with two distinct antibody populations present in the central antiserum well. The two independent precipitin lines cross each other completely without any interaction or fusion.
- 3. Reaction of Partial Identity (Arc with a Spur): Observed when two antigens are phylogenetically related cross-reactive species (e.g., BSA and sheep serum albumin, or HSA). The antiserum contains antibodies directed against epitopes shared by both antigens ($A$), as well as antibodies directed against unique epitopes ($B$) present only on the homologous immunogen. The shared antibodies form a fused continuous arc, while the antibodies specific for the unique epitope diffuse past the arc to form an extended projection or “spur” that always points toward the simpler or cross-reactive antigen!
๐ Step-by-Step Experimental Protocol
- Gel Casting: Dissolve $1.0,text{g}$ of agarose in $100,text{mL}$ of PBS buffer (pH 7.2) with $0.01%$ thimerosal. Heat until molten, cool to $55^circtext{C}$, and cast onto glass slides ($3-4,text{mL}$ per slide) to achieve a uniform gel thickness of $2,text{mm}$.
- Pattern Punching: Using an Ouchterlony template punch, cut clusters of circular wells consisting of one central antibody well surrounded symmetrically by 4 to 6 peripheral antigen wells ($3.0,text{mm}$ well diameter, $5.0,text{mm}$ edge-to-edge distance). Carefully aspirate agar cores.
- Loading Reactants:
- Cluster 1 (Identity): Central well = Anti-BSA; Peripheral Well 1 = BSA ($1,text{mg/mL}$); Peripheral Well 2 = BSA ($1,text{mg/mL}$).
- Cluster 2 (Non-Identity): Central well = Anti-BSA + Anti-Lysozyme; Peripheral Well 1 = BSA; Peripheral Well 2 = Hen Egg White Lysozyme.
- Cluster 3 (Partial Identity): Central well = Polyclonal Anti-BSA; Peripheral Well 1 = BSA; Peripheral Well 2 = Goat Serum Albumin (GSA).
Pipette exactly $10,mutext{L}$ of reagent into each well.
- Incubation: Place slides in a closed, humidified plastic chamber containing moistened cotton balls. Incubate at room temperature ($22^circtext{C}$) for 24 to 48 hours.
- Staining & Permanent Preservation (Optional): Wash the plate in $0.9%,text{NaCl}$ for 24 hours to elute unprecipitated soluble proteins. Dry under filter paper weights, then stain in Coomassie Brilliant Blue R-250 ($0.1%$ in $45%$ methanol, $10%$ acetic acid) for 15 minutes, destain, and evaluate the sharp blue precipitin lines.
Enzyme-Linked Immunosorbent Assay (ELISA): Indirect & Sandwich Microplate Assays
๐ฌ Principle & Molecular Cascades
ELISA (Enzyme-Linked Immunosorbent Assay) was invented independently in 1971 by Peter Perlmann and Eva Engvall in Sweden and Anton Schuurs and Bauke van Weemen in the Netherlands as a non-radioactive alternative to Radioimmunoassay (RIA).
ELISA relies on two essential biochemical properties:
1. Solid-phase adsorption: Polystyrene or polyvinyl chloride 96-well microplates possess hydrophobic surfaces that passively adsorb proteins through hydrophobic and charge interactions.
2. Enzyme amplification: An antibody or antigen is covalently conjugated to an enzyme (Horseradish Peroxidase HRP or Alkaline Phosphatase AP) that catalytically converts a colorless substrate into a chromogenic soluble product. A single bound enzyme molecule turnover converts millions of substrate molecules per minute, granting ELISA femtomolar analytical sensitivity ($10^{-15},text{M}$).
- Indirect ELISA Architecture: Solid Phase → Coated Target Antigen → Blocking Unoccupied Sites (BSA) → Primary Antibody (Patient Serum IgG) → Wash → Enzyme-Conjugated Secondary Antibody (Goat Anti-Human IgG-HRP) → Wash → Chromogenic Substrate (TMB + $text{H}_2text{O}_2$) → Stop Solution ($2,text{N H}_2text{SO}_4$) → Read at $450,text{nm}$. Signal is directly proportional to serum antibody titer.
- Sandwich ELISA Architecture: Solid Phase → Immobilized Capture Antibody → Blocking → Analyte Antigen Sample → Wash → Enzyme-Conjugated Detection Antibody → Wash → TMB Substrate → Read at $450,text{nm}$. The target antigen is “sandwiched” between two distinct antibodies that recognize non-overlapping epitopes. Offers the highest analytical specificity.
๐ Step-by-Step Laboratory Workflow (Sandwich ELISA)
- Coating Microplate Wells: Dilute capture monoclonal antibody to $2-5,mutext{g/mL}$ in carbonate-bicarbonate coating buffer ($0.05,text{M}$, pH 9.6). Dispense $100,mutext{L}$ into each well of a high-binding 96-well polystyrene plate. Seal and incubate at $4^circtext{C}$ overnight (or $37^circtext{C}$ for 2 hours).
- Washing: Aspirate coating buffer. Wash each well 3 times with $300,mutext{L}$ of Wash Buffer (PBST: Phosphate Buffered Saline containing $0.05%$ Tween-20). Invert and firmly slap the plate onto paper towels to remove residual droplets.
- Blocking Non-Specific Sites: Add $200,mutext{L}$ of blocking solution ($1-3%$ Bovine Serum Albumin BSA or $5%$ non-fat dry milk in PBST) to prevent non-specific binding of subsequent reactants. Incubate at room temperature for 1 hour; wash $3times$.
- Sample & Standard Incubation: Add $100,mutext{L}$ of serial standard dilutions and unknown serum samples in duplicate. Incubate at $37^circtext{C}$ for 1 hour; wash $4times$ with PBST.
- Enzyme Conjugate Addition: Add $100,mutext{L}$ of Horseradish Peroxidase-conjugated detection antibody (diluted $1:5000$ in blocking buffer). Incubate at room temperature for 1 hour in the dark; wash $5times$ thoroughly.
- Chromogenic Development: Add $100,mutext{L}$ of freshly prepared Tetramethylbenzidine (TMB) substrate solution containing hydrogen peroxide ($text{H}_2text{O}_2$). Incubate at room temperature for 15-20 minutes in the dark until a blue color develops.
- Reaction Stopping & Absorbance Reading: Add $50,mutext{L}$ of $2,text{N H}_2text{SO}_4$ stop solution. The sulfuric acid protonates the oxidized TMB diamine, instantly halting the peroxidase reaction and shifting the blue product to a bright, stable yellow color. Read optical density (OD) immediately on an ELISA microplate spectrophotometer at $lambda = 450,text{nm}$ (reference filter $620-650,text{nm}$).
Widal Tube Agglutination Titration for Serodiagnosis of Enteric (Typhoid & Paratyphoid) Fever
๐ฌ Principle & Serological Antigens
The Widal test, developed by Georges-Fernand Widal in 1896, is a tube agglutination test that measures circulating agglutinating serum antibodies (agglutinins) against specific antigenic preparations of Salmonella enterica serovars:
- Somatic O-Antigen (TO): Lipopolysaccharide (LPS) endotoxin component of the bacterial outer cell envelope. Because it is non-flagellar, O-agglutination produces a compact, granular, chalky precipitate that is difficult to dislodge upon shaking, settling in flat-bottomed Felix tubes. Antibodies against TO appear first (end of the 1st week of fever) and decline rapidly after recovery. An O-titer $ge 1:160$ strongly indicates active acute infection.
- Flagellar H-Antigen (TH, AH, BH): Heat-labile flagellin protein. Because flagella interlock loosely, H-agglutination produces a loose, cotton-wool-like, fluffy floccose precipitate that forms rapidly and settles in conical-bottomed Dreyer tubes. Antibodies against TH appear later (2nd week of illness) and persist for months to years, often indicating past infection, endemic exposure, or TAB vaccination.
Diagnostic Criteria:
In endemic regions, a single baseline cutoff titer of $ge 1:160$ for TO and $ge 1:160$ for TH is considered clinically suspicious. However, the definitive gold standard confirmation is demonstrating a four-fold or greater rise in antibody titer between paired serum specimens collected 7 to 10 days apart (acute vs. convalescent phase).
๐ Step-by-Step Tube Dilution Protocol
- Tube Rack Setup: Set up 4 rows of 8 small test tubes ($75 times 12,text{mm}$ Dreyer or Kahn tubes) labeled: Row 1 = *S. Typhi* O (TO); Row 2 = *S. Typhi* H (TH); Row 3 = *S. Paratyphi* AH; Row 4 = *S. Paratyphi* BH.
- Master Serial Dilution: Pipette $1.9,text{mL}$ of sterile physiological saline ($0.85%,text{NaCl}$) into Tube 1, and $1.0,text{mL}$ of saline into Tubes 2 through 8 for each row. Add $0.1,text{mL}$ of patient serum to Tube 1 (yielding an initial $1:20$ dilution). Mix thoroughly, transfer $1.0,text{mL}$ to Tube 2 ($1:40$), mix and transfer $1.0,text{mL}$ to Tube 3 ($1:80$), continuing serial doubling dilutions through Tube 7 ($1:1280$). Discard $1.0,text{mL}$ from Tube 7. Tube 8 receives NO serum and serves as the negative antigen control.
- Antigen Addition: Add 1 drop ($~50,mutext{L}$) of standardized killed antigen suspension to all 8 tubes of the corresponding row:
Row 1 = TO antigen; Row 2 = TH antigen; Row 3 = AH antigen; Row 4 = BH antigen.
(Final serum dilutions become $1:40, 1:80, 1:160, 1:320, 1:640, 1:1280, 1:2560$). - Incubation: Shake the racks to mix. Incubate in a water bath at $37^circtext{C}$ for 4 hours (or at $50^circtext{C}$ for 2 hours), then transfer to $4^circtext{C}$ refrigerator overnight for complete sedimentation.
- Reading Agglutination: Examine tubes against a black background using indirect light:
- $++++$ : Complete agglutination; 100% clearing of supernatant with heavy precipitate.
- $+++$ : 75% clearing of supernatant with visible precipitate.
- $++$ : 50% clearing with moderate precipitate (endpoint titer definition).
- $-$ : Uniformly turbid suspension; smooth round button of unagglutinated bacteria.
The titer is defined as the highest serum dilution showing at least $50%$ ($++$) agglutination.
Isolation of Human Peripheral Blood Mononuclear Cells (PBMCs) via Ficoll-Paque Density Gradient & Viability Assaying
๐ฌ Principle & Isopycnic Density Gradient Separation
Whole human peripheral blood contains distinct cellular lineages with characteristic buoyant densities:
- Platelets / Thrombocytes: Density $rho < 1.030,text{g/mL}$. Remain suspended in the plasma layer.
- Mononuclear Cells (PBMCs): Lymphocytes ($rho approx 1.060-1.070,text{g/mL}$) and Monocytes ($rho approx 1.065-1.072,text{g/mL}$).
- Polymorphonuclear Granulocytes (PMNs – Neutrophils, Eosinophils, Basophils): Density $rho > 1.080,text{g/mL}$.
- Erythrocytes (RBCs): Density $rho > 1.090,text{g/mL}$.
Ficoll-Paque is an aqueous solution consisting of Ficoll 400 (a synthetic, highly branched, neutral polymer of sucrose that aggregates erythrocytes into rouleaux, accelerating their sedimentation) and Sodium Diatrizoate (an iodinated radiopaque compound that adjusts the refractive index and osmotic density strictly to $1.077 pm 0.001,text{g/mL}$ at $20^circtext{C}$).
When diluted whole blood is layered gently over Ficoll-Paque and centrifuged at $400times g$ for 30 minutes with the centrifuge brake disengaged, cells migrate based on their buoyant densities:
Erythrocytes and granulocytes are denser than the Ficoll medium ($rho > 1.077,text{g/mL}$) and sediment rapidly to the bottom pellet. Conversely, mononuclear cells (lymphocytes and monocytes) have buoyant densities lower than Ficoll ($rho < 1.077,text{g/mL}$) and are retained as a dense, opaque white ring (the buffy coat ring) at the horizontal interphase between the plasma/saline layer and the Ficoll-Paque layer.
๐ Step-by-Step Isolation Protocol
- Blood Collection & Dilution: Aseptically collect $5,text{mL}$ of human venous blood into an EDTA or heparin vacutainer tube. Dilute the blood $1:1$ with sterile Phosphate-Buffered Saline (PBS, pH 7.4) at room temperature.
- Gradient Layering (Critical Checkpoint): Add $3.0,text{mL}$ of Ficoll-Paque ($1.077,text{g/mL}$) into a sterile $15,text{mL}$ conical polypropylene centrifuge tube. Hold the tube at a $45^circ$ angle. Using a sterile pipette, very slowly and gently layer $6.0,text{mL}$ of diluted blood over the Ficoll surface without disturbing the meniscus or mixing the two phases.
- Density Gradient Centrifugation: Centrifuge at $400times g$ ($~1600,text{rpm}$ in a standard swinging-bucket rotor) for 30 minutes at $20^circtext{C}$. Crucial: Ensure the centrifuge deceleration brake is set to OFF (zero brake) to prevent vibration-induced disruption of the cell interphase.
- Harvesting the PBMC Interphase: Following centrifugation, observe the four distinct layers: (1) Upper straw-yellow plasma/PBS, (2) Opaque white buffy coat interphase, (3) Clear Ficoll layer, (4) Dark red RBC/granulocyte pellet. Carefully aspirate and discard the top $2-3,text{mL}$ of plasma without touching the ring. Using a sterile Pasteur pipette, aspirate the cloudy PBMC buffy coat ring and transfer into a fresh $15,text{mL}$ centrifuge tube.
- Washing: Add $10,text{mL}$ of cold PBS to the harvested PBMCs. Centrifuge at $300times g$ for 10 minutes (washes away residual Ficoll and platelets). Discard supernatant, resuspend pellet in $5,text{mL}$ PBS, and centrifuge at $200times g$ for 10 minutes. Resuspend final cell pellet in $1.0,text{mL}$ complete RPMI-1640 medium.
- Trypan Blue Viability Counting: Mix $20,mutext{L}$ of cell suspension with $20,mutext{L}$ of $0.4%$ Trypan Blue solution. Load $10,mutext{L}$ onto an improved Neubauer hemocytometer. Under $10times$ objective, count viable cells (unstained, clear, glistening) and non-viable cells (dark blue, ruptured membranes) in the 4 large corner squares:
$$text{Viability (%)} = left( frac{text{Total Unstained Viable Cells}}{text{Total Cells (Viable + Blue Dead)}} right) times 100$$
A successful preparation yields $>95%$ cell viability and approximately $1-2 times 10^6,text{PBMCs/mL}$ of whole blood.
Direct & Indirect Immunofluorescence Assay (IFA): Detection of Antinuclear Antibodies (ANA) on HEp-2 Substrates
๐ฌ Principle & Fluorophore Photophysics
Immunofluorescence (IFA) was developed in 1941 by Albert Coons, who demonstrated that antibodies could be covalently conjugated to fluorescent dyes (fluorochromes) without impairing their antigen-binding specificity.
A fluorophore absorbs high-energy photons at a shorter excitation wavelength ($lambda_{text{ex}}$), promoting electrons to an excited singlet state. As electrons relax to the ground state, they release photons of lower energy and longer wavelength ($lambda_{text{em}}$)โa phenomenon termed Stokes Shift. The most common fluorophore in diagnostic immunology is Fluorescein Isothiocyanate (FITC), which absorbs blue light at $lambda_{text{ex}} = 495,text{nm}$ and emits intense apple-green fluorescence at $lambda_{text{em}} = 519,text{nm}$.
- Direct Immunofluorescence (DIF): A single-step assay where a fluorochrome is directly conjugated to the primary antibody that recognizes the tissue-bound antigen. Used primarily for detecting immunoglobulin and complement depositions in renal biopsies (glomerulonephritis) and skin biopsies (pemphigus vulgaris).
- Indirect Immunofluorescence (IIF): A two-step assay where unlabeled patient serum antibody binds to an immobilized substrate (e.g., HEp-2 cells). After washing away unbound serum, a secondary anti-human IgG antibody labeled with FITC is added. Because multiple secondary antibody molecules bind to the constant Fc domain of each primary antibody, IIF provides dramatic 5- to 10-fold signal amplification, making it the international gold standard for screening autoantibodies.
๐ Standard Diagnostic HEp-2 ANA Staining Patterns
| ANA Pattern (HEp-2) | Target Nuclear Autoantigens | Fluorescence Appearance | Associated Autoimmune Disease |
|---|---|---|---|
| Homogeneous (Diffuse) | dsDNA, Histones, Nucleosomes | Uniform solid fluorescence of entire interphase nucleus; mitotic chromosomes stained | Systemic Lupus Erythematosus (SLE), Drug-induced Lupus |
| Speckled (Fine / Coarse) | Extractable Nuclear Antigens: Sm, U1-RNP, SS-A (Ro), SS-B (La) | Punctate fluorescent speckles throughout nucleoplasm; mitotic chromosomes unstained | Sjรถgren’s Syndrome, Mixed Connective Tissue Disease (MCTD), SLE |
| Nucleolar | RNA Polymerase I, Fibrillarin, PM-Scl | Intense fluorescence confined strictly to 1-6 nucleoli inside nucleus | Systemic Sclerosis (Scleroderma), Polymyositis |
| Centromere (Discrete Speckles) | Centromere Proteins CENP-A, B, C | Discrete, uniform 40-60 dots aligned along condensed metaphase plate | CREST Syndrome / Limited Cutaneous Scleroderma |
Comprehensive Practical Viva-Voce Examination & Clinical Immunoassay Bank
Essential diagnostic mechanisms, serological thresholds, calculations, and clinical case checkpoints covering all 7 practical experiments:
Ans: Law 1: If an agglutinogen (A or B) is present on an individual’s RBCs, the corresponding isoantibody must be absent from plasma. Law 2: If an agglutinogen is absent from RBCs, the corresponding isoantibody must be present in plasma.
Ans: Group O RBCs lack both A and B antigens, so they cannot be hemagglutinated by recipient anti-A or anti-B antibodies in packed RBC transfusions. Group AB plasma lacks both anti-A and anti-B antibodies, so it will not attack transfused RBCs.
Ans: Forward (cell) grouping detects ABO antigens on RBC membranes using known commercial antisera (Anti-A, Anti-B). Reverse (serum) grouping detects antibodies in patient serum using known A and B reagent RBCs.
Ans: Terminal sugars: H antigen has L-fucose; A antigen has N-acetylgalactosamine (GalNAc) added to H; B antigen has D-galactose added to H.
Ans: As antigen diffuses into the antibody-impregnated agarose gel, it reaches the Zone of Equivalence where the multivalent antigen-antibody ratio crosslinks into an insoluble three-dimensional lattice.
Ans: dยฒ = mยท[Ag] + c, where the square of the precipitin ring diameter (dยฒ) is directly proportional to antigen concentration.
Ans: 1. Reaction of Identity: Smooth fused continuous arc. 2. Reaction of Non-Identity: Crossed independent lines with no interaction. 3. Reaction of Partial Identity: Fused arc with a spur pointing toward the simpler cross-reactive antigen.
Ans: The spur is formed by antibodies directed against unique epitopes present only on the homologous antigen. These antibodies diffuse past the shared arc toward the simpler antigen.
Ans: ELISA does not require hazardous radioactive isotopes, has longer reagent shelf life, is easily automated, and uses stable non-toxic enzymes with femtomolar sensitivity.
Ans: It coats all remaining unbound hydrophobic plastic sites on the microplate well surface, preventing non-specific binding of antibodies in subsequent incubation steps.
Ans: It halts the enzymatic peroxidation by denaturing HRP and acidifies the blue product into a stable yellow diamine that absorbs maximally at 450 nm.
Ans: The antigen must be multivalent or possess at least two distinct, non-overlapping epitopes so that capture and detection antibodies can bind simultaneously.
Ans: TO is the somatic lipopolysaccharide cell envelope antigen (acute infection, appears early). TH is the flagellar protein antigen (indicates past infection, immunization, or convalescent phase).
Ans: A single baseline titer of โฅ 1:160 for TO and โฅ 1:160 for TH in endemic areas, or a four-fold increase in titer between acute and convalescent phase paired sera.
Ans: PBMCs (lymphocytes and monocytes) have a buoyant density of 1.060-1.072 g/mL, which is lower than Ficoll (1.077 g/mL), so they float at the interphase. Granulocytes (>1.080 g/mL) and RBCs (>1.090 g/mL) are denser and pellet to the bottom.
Ans: Braking induces turbulent deceleration currents that disrupt the narrow interphase layer, mixing separated mononuclear cells with the Ficoll or RBC pellet.
Ans: Living cells have intact, selective plasma membranes that exclude the charged dye (remain clear). Dead cells have compromised, leaky membranes that take up Trypan Blue, staining the nucleus dark blue.
Ans: Direct IFA uses a fluorophore conjugated directly to the primary antibody (1 step). Indirect IFA uses an unlabeled primary antibody followed by a fluorophore-conjugated secondary antibody (2 steps), providing 5- to 10-fold signal amplification.
Ans: Fluorescein Isothiocyanate (FITC). Excitation peak is 495 nm (blue light); emission peak is 519 nm (apple-green fluorescence).
Ans: Systemic Lupus Erythematosus (SLE), caused by autoantibodies directed against double-stranded DNA (anti-dsDNA), histones, and nucleosomes.
