Antigen-antibody interactions: principles and methods

6 ASCP MLS practice questions on Antigen-antibody interactions: principles and methods, part of Blood Banking. The first 6 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

1. Question 1 · Hard

   An antibody gives 2+ agglutination at immediate spin at room temperature, does not react at 37 °C, and is negative at the antiglobulin phase. It is most likely:

   1. AA cold-reacting IgM antibody
   2. BA complement-binding IgG
   3. CA warm IgG antibody
   4. DAn antibody showing dosage

   Show answerHide answer

   Answer: A · A cold-reacting IgM antibody

   Reactivity that is strongest at room temperature and disappears at 37 °C points to a cold-reacting IgM antibody, such as anti-M, anti-N, anti-P1 or anti-Lea. Such antibodies are usually clinically insignificant. A warm IgG antibody would react at the 37 °C or antiglobulin phase. Prewarming confirms that the antibody does not react at body temperature.

   • B: A complement-binding IgG would give a positive antiglobulin phase.
   • C: A warm IgG antibody would be positive at the antiglobulin phase.
   • D: Dosage describes reaction strength, not thermal range.

2. Question 2 · Hard

   In an indirect antiglobulin test, the cells are not washed well before anti-human globulin is added. What is the likely result?

   1. AA false positive from leftover plasma antibody
   2. BHemolysis of the cells
   3. CA false negative from neutralized AHG
   4. DNo effect, as long as the test is read quickly

   Show answerHide answer

   Answer: C · A false negative from neutralized AHG

   Unbound immunoglobulin left in the tube reacts with the anti-human globulin and uses it up, so sensitized cells are not agglutinated. Adding IgG-coated check cells to every negative test detects this: if they do not agglutinate, the AHG was neutralized and the test is invalid. Cells are washed at least three times with saline. Delays after washing also cause false negatives as antibody elutes.

   • A: Leftover plasma antibody neutralizes the reagent rather than causing agglutination.
   • B: Poor washing does not hemolyze the cells.
   • D: Speed of reading does not fix a neutralized reagent.

3. Question 3 · Medium

   IgG antibodies usually cannot agglutinate red cells directly in saline. The main reason is:

   1. AIgG molecules have no antigen-binding sites
   2. BSurface negative charge keeps cells too far apart
   3. CSaline gradually breaks down IgG antibodies in the tube
   4. DIgG can bind only to complement components

   Show answerHide answer

   Answer: B · Surface negative charge keeps cells too far apart

   Red cells carry a negative charge from sialic acid, and the cloud of ions around them keeps cells a set distance apart. This distance, related to the zeta potential, is wider than the span of an IgG molecule. IgM is large enough to bridge it. Enhancement media and the antiglobulin reagent help IgG produce visible agglutination.

   • A: IgG has two antigen-binding sites.
   • C: Saline does not destroy antibodies.
   • D: IgG binds antigen directly; complement binding is a separate step.

4. Question 4 · Medium

   Most clinically significant red cell alloantibodies react best at:

   1. ARoom temperature
   2. B4 °C
   3. C56 °C
   4. D37 °C

   Show answerHide answer

   Answer: D · 37 °C

   Clinically significant alloantibodies are mostly IgG and react at body temperature, which is why tests are incubated at 37 °C and read at the antiglobulin phase. Antibodies that react only at room temperature or below are usually IgM and rarely destroy red cells in the body. 56 °C inactivates complement and is used to heat-inactivate serum. Thermal range is a key clue to clinical significance.

   • A: Room temperature favors cold-reacting antibodies of little significance.
   • B: 4 °C favors cold-reacting IgM antibodies.
   • C: 56 °C is used to inactivate complement, not to detect antibodies.

5. Question 5 · Easy

   The first stage of agglutination, in which antibody binds to red cell antigens without visible clumping, is called:

   1. ANeutralization
   2. BPrecipitation
   3. CSensitization
   4. DLattice formation

   Show answerHide answer

   Answer: C · Sensitization

   In sensitization, antibody attaches to antigen on the red cell surface. Lattice formation, the second stage, is when antibodies bridge neighbouring cells to form visible agglutinates. IgG often sensitizes cells without causing lattice formation, which is why the antiglobulin test is needed. Temperature, pH, ionic strength and time affect sensitization.

   • A: Neutralization is inhibition of an antibody by soluble antigen.
   • B: Precipitation involves soluble antigens, not red cells.
   • D: Lattice formation is the second, visible stage.

6. Question 6 · Hard

   During a busy evening shift, a technologist sets up 10 tube LISS antiglobulin screens and a weak anti-D control with D-positive cells. After the 15-minute incubation at 37 °C, all tubes are washed four times with saline and then left on the bench for 30 minutes while she handles an emergency release. She then adds anti-human globulin. Every tube, including the weak anti-D control, is negative; IgG-coated check cells then give 2+ in every tube. The heat block reads 37 °C, the cell suspensions are 3%, and all reagents are in date. The most likely cause of the failed control is:

   1. AThe LISS incubation time was too short
   2. BAHG was neutralized by plasma left after washing
   3. CBound antibody dissociated before AHG was added
   4. DThe red cell suspensions were too heavy

   Answer and explanation in LearnFree account