Medical Laboratory Technician Exam Medical Laboratory Technician Immunology and Serology 1 — Questions and Answers
Question 1: Which complement activation pathway is triggered when mannose-binding lectin (MBL) recognizes carbohydrate patterns on the surface of microorganisms?
- Classical pathway
- Lectin pathway (Correct answer)
- Alternative pathway
- Terminal pathway
Correct answer: Lectin pathway
The lectin pathway is initiated when mannose-binding lectin binds to mannose residues on microbial surfaces, activating MASP-1 and MASP-2 serine proteases that cleave C4 and C2, similar to the classical pathway but without requiring antibody.
Question 2: In an indirect antiglobulin test (IAT), what is the purpose of incubating patient serum with reagent red blood cells before adding antihuman globulin (AHG)?
- To lyse antibody-coated red cells in vitro
- To allow antibodies in the patient's serum to coat the reagent red cells (Correct answer)
- To activate complement directly on the red cell surface
- To detect IgM antibodies already bound to the patient's own red cells
Correct answer: To allow antibodies in the patient's serum to coat the reagent red cells
In the IAT, patient serum is incubated with reagent red cells so that any clinically significant antibodies present in the serum can sensitize (coat) those cells. AHG is then added to cross-link the bound antibody, producing visible agglutination.
Question 3: A patient is diagnosed with systemic lupus erythematosus (SLE). Which serological marker is considered the most sensitive screening test for this disease?
- Anti-streptolysin O (ASO) titer
- Rheumatoid factor (RF)
- Antinuclear antibody (ANA) (Correct answer)
- C-reactive protein (CRP)
Correct answer: Antinuclear antibody (ANA)
ANA testing detects antibodies directed against nuclear components and is positive in over 95% of SLE patients, making it the preferred initial screening test. More specific markers such as anti-dsDNA and anti-Sm are then used for confirmation.
Question 4: Which immunoglobulin class is produced first during a primary immune response and is the most efficient activator of the classical complement pathway on a per-molecule basis?
- IgA
- IgG
- IgM (Correct answer)
- IgE
Correct answer: IgM
IgM is the first antibody class secreted following initial antigen exposure. Its pentameric structure provides ten antigen-binding sites and multiple Fc regions that very efficiently fix complement via the classical pathway, even with a single molecule bound to antigen.
Question 5: In an enzyme-linked immunosorbent assay (ELISA), where does the detectable signal originate?
- The primary antibody binding directly to antigen on the plate
- An enzyme-conjugated secondary antibody converting a substrate to a colored product (Correct answer)
- Fluorescent-labeled antigen re-binding to a capture antibody
- Complement activation on the solid-phase surface
Correct answer: An enzyme-conjugated secondary antibody converting a substrate to a colored product
In a standard indirect ELISA, a secondary antibody conjugated to an enzyme (such as horseradish peroxidase) binds to the primary antibody. When substrate is added, the enzyme converts it to a colored product whose absorbance is measured spectrophotometrically to quantify the reaction.
Question 6: A laboratory technician notices a false-negative agglutination result in a patient sample despite known antibody presence. Repeating the test after diluting the serum gives a positive result. This finding is most consistent with which phenomenon?
- Hook effect (high-dose hook)
- Prozone phenomenon (Correct answer)
- Postzone phenomenon
- Complement fixation inhibition
Correct answer: Prozone phenomenon
The prozone phenomenon occurs when excess antibody relative to antigen saturates all available antigenic sites, preventing the cross-linking (lattice formation) needed for visible agglutination. Diluting the serum reduces antibody excess and allows lattice formation, producing a positive result.
Medical Laboratory Technician Exam Medical Laboratory Technician Immunology and Serology 1
Which complement activation pathway is triggered when mannose-binding lectin (MBL) recognizes carbohydrate patterns on the surface of microorganisms?