Immunology & Serology Testing Methods Flashcards

Principles of Dilutions and Antibody Titers

• Dilution Definition: An expression of relative concentration representing the amount of substance in a solution rather than total volume.

• Indications for Dilution:

  • Sample analyte concentration exceeds instrument linearity limits (e.g., glucose at $1,300\,\text{mg/dL}$).

  • Insufficient sample volume available for testing.

• Dilution Factor: The reciprocal of the dilution made, serving as the multiplier to adjust the diluted test result back to the undiluted concentration.

• Serial Dilutions: Stepwise dilutions using the same tube dilution factor at each step, yielding increasing dilution ratios across consecutive tubes.

• Antibody Titer: The reciprocal of the highest dilution showing observable reactivity (endpoint), reflecting antibody concentration, immune status, or infection timeline.

• Diagnostic Titer Levels:

  • Acute Titer: Sample collected when disease is first suspected or diagnosed.

  • Convalescent Titer: Sample collected several weeks later during recovery.

  • Diagnostic Significance: A fourfold rise in titer between acute and convalescent specimens confirms an active or recent infection.

• Serological Terminology:

  • Seropositive: Demonstrable antibody present.

  • Seronegative: No demonstrable antibody present.

  • Seroconversion: Transition from seronegative to seropositive status.

Immunoassays and Agglutination Types

• Precipitation: Reaction between a soluble antigen and a soluble antibody that forms an insoluble precipitate.

• Agglutination: Reaction where antibodies crosslink particulate or cellular antigens to form visible aggregates.

• Agglutination Categories:

  • Latex Agglutination: Antigens or antibodies bound to synthetic latex beads.

  • Hemagglutination: Agglutination involving red blood cell antigens.

  • Direct Bacterial Agglutination: Agglutination using intact bacterial cells as the particulate antigen.

  • Flocculation: Precipitate clumps into flake-like structures within a confined area where both antigen and antibody are soluble (e.g., RPR and VDRL tests for syphilis).

• Sources of Error:

  • False Positives: Bacterial or blood contamination, presence of autoantibodies, specimen drying, and overcentrifugation.

  • False Negatives: Failure to add reagent, contaminated reagents, inadequate cell washing, undercentrifugation, and prozone/postzone phenomena.

Stages and Factors Influencing Agglutination

• Two-Step Agglutination Mechanism:

  1. Sensitization: Physical attachment of an antibody to its specific antigen epitope (rapid and reversible).

  2. Lattice Formation: Crosslinking of sensitized particles to create visible complexes (slower process governed by chemical and physical forces).

• Influencing Physical Factors:

  • pH: Optimal range is $6.5$ to $7.5$.

  • Temperature: IgM binds optimally at cold/room temperatures ($\le 22^\circ\text{C}$ )

  • binds optimally at $37^\circ\text{C}$.

  • Incubation Time: Typically ranges from $15\,\text{minutes}$ to $60\,\text{minutes}$.

  • Epitope Density & Hindrance: Antigen density and steric hindrance (competing antibodies or complement coatings) affect binding efficiency.

  • Zeta Potential & Ionic Strength: Negative surface charges repel cells; Low Ionic Strength Solution (LISS) reduces ionic shielding to enhance crosslinking.

  • Antibody Class: IgM is a large pentamer that readily bridges cell distances; IgG is a small monomer requiring enhancement (e.g., Antihuman Globulin/AHG) to form lattices.

• Prozone and Postzone Phenomena:

  • Zone of Equivalence: Optimal antigen-antibody ratio producing maximum lattice formation.

  • Prozone: Antibody excess preventing lattice crosslinking, resulting in false negatives.

  • Postzone: Antigen excess preventing lattice crosslinking, resulting in false negatives.

Agglutination Interpretation, Grading, and Enhancement

• Tube Reaction Grading:

  • $4+$: Single clear cell button/aggregate with clear supernatant.

  • $3+$: Few large agglutinates with clear supernatant.

  • $2+$: Multiple medium-sized agglutinates with clear supernatant.

  • $1+$: Fine granular aggregate with turbid reddish background.

  • $0$: Smooth homogeneous suspension (negative).

• Gel Column Agglutination: Strong aggregates remain trapped at the top of the gel matrix ($4+$), while unagglutinated cells pass through to form a pellet at the bottom ($0$).

• Rouleaux Formation (Pseudoagglutination):

  • False clumping caused by red cells stacking like coins due to high serum protein (e.g., Multiple Myeloma or Waldenström's macroglobulinemia).

  • Saline Replacement Technique: Differentiates Rouleaux from true agglutination; saline disperses Rouleaux but leaves true antigen-antibody agglutination intact.

• Enhancement Techniques: Centrifugation, proteolytic enzymes (pepsin, papain), colloids (albumin, PEG), and Antihuman Globulin (AHG).

Direct vs. Indirect Antiglobulin Testing

• Direct Antiglobulin Test (DAT / Direct Coombs):

  • Purpose: Detects in vivo sensitization of red blood cells by antibodies or complement.

  • Clinical Applications: Autoimmune hemolytic anemia, transfusion reactions, drug-induced hemolysis, and hemolytic disease of the newborn.

  • Procedure: Patient EDTA red cells are washed to remove unbound globulins, then mixed with AHG reagent.

• Indirect Antiglobulin Test (IAT / Indirect Coombs):

  • Purpose: Detects in vitro free red blood cell antibodies in patient serum or plasma.

  • Clinical Applications: Antibody screening, crossmatching, donor blood testing, and prenatal screening.

  • Procedure: Patient serum/plasma is incubated with reagent screening cells, washed to remove unbound antibodies, and mixed with AHG reagent.

Immunodiffusion and Electrophoresis Methods

• Nephelometry: Measures light scattered by insoluble immune complexes at specific angles; quantitatively measures immunoglobulins, complement components ($C3$, $C4$), and acute-phase proteins (CRP, haptoglobin, fibrinogen).

• Radial Immunodiffusion (RID / Mancini Method):

  • Single quantitative immunodiffusion where antibody is incorporated into gel and antigen diffuses outward from wells.

  • Precipitation ring diameter is directly proportional to antigen concentration.

• Double Immunodiffusion (Ouchterlony Method):

  • Qualitative method where both antigen and antibody diffuse through gel to form precipitate lines at the zone of equivalence.

  • Identity: Smooth continuous arc (identical epitopes).

  • Nonidentity: Crossed precipitation lines with spurs (unrelated epitopes).

  • Partial Identity: Merged arc with a single spur (shared epitopes).

• Serum Protein Electrophoresis (SPEP):

  • Separates serum proteins into five major fractions based on electrical charge: Albumin, $\alpha_1$, $\alpha_2$, $\beta$, and $\gamma$ (gamma globulins/immunoglobulins).

• Immunofixation Electrophoresis (IFE):

  • Combines electrophoresis with immunoprecipitation using monospecific antisera ($\text{IgG}$, $\text{IgA}$, $\text{IgM}$, $\kappa$, $\lambda$).

  • Identifies monoclonal protein spikes ($M$ spike) and differentiates antibody heavy/light chain classes.

• Gammopathy Types:

  • Monoclonal Gammopathy: Single plasma cell clone produces excessive discrete immunoglobulin ($M$ protein / paraprotein) appearing as a narrow, dense band (e.g., Multiple Myeloma, MGUS).

  • Polyclonal Gammopathy: Multiple plasma cell clones produce elevated levels of several immunoglobulin classes appearing as a broad diffuse band (e.g., liver disease, chronic inflammation).

Questions & Discussion

• Question: When performing a saline replacement, will red cells remain aggregated or disperse if the reaction is due to Rouleaux?

  • Answer: If Rouleaux is present, the pseudoagglutination will disperse upon replacing plasma with saline. If true antigen-antibody agglutination is present, the clumping will persist.

• Question: What are the optimal reacting temperatures for IgM and IgG antibodies?

  • Answer: IgM antibodies react optimally at cold or room temperatures ($\le 22^\circ\text{C}$), whereas IgG antibodies react optimally at body temperature ($37^\circ\text{C}$).