Immunoglobulins: classes, structure and properties

6 ASCP MLS practice questions on Immunoglobulins: classes, structure and properties, 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

   After high-dose intravenous immunoglobulin (IVIG), a group A patient has a positive DAT and mild hemolysis. The most likely cause is:

   1. AWarm autoimmune hemolytic anemia
   2. BA delayed hemolytic transfusion reaction
   3. CPassive anti-A in the IVIG
   4. DA new alloantibody from pregnancy

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   Answer: C · Passive anti-A in the IVIG

   IVIG is pooled from thousands of donors and contains anti-A and anti-B, and sometimes anti-D. In large doses these passive antibodies can coat the red cells of group A, B or AB patients and cause hemolysis. The eluate shows anti-A, and the effect fades as the IVIG is cleared. Knowing about recent IVIG avoids a needless autoimmune workup.

   • A: Autoimmune hemolysis is a diagnosis made after passive antibody is excluded.
   • B: No red cells were transfused, so a transfusion reaction does not fit.
   • D: A new alloantibody would not explain anti-A on a group A patient's cells.

2. Question 2 · Hard

   Anti-D destroys red cells mainly outside the blood vessels, in the spleen, rather than by intravascular hemolysis. The main reason is:

   1. AAnti-D cannot bind to Fc receptors on macrophages
   2. BD sites are too sparse for IgG to fix complement
   3. CAnti-D is an IgM antibody
   4. DAnti-D is mainly IgG4

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   Answer: B · D sites are too sparse for IgG to fix complement

   Complement activation by IgG needs two IgG molecules bound close together. D antigen sites are relatively sparse and spread out, so anti-D rarely activates complement fully. Instead, IgG-coated cells are removed by splenic macrophages through their Fc receptors, which is extravascular hemolysis. The DAT in Rh hemolysis is typically positive with anti-IgG only.

   • A: IgG1 and IgG3 anti-D bind Fc receptors well; that is how the cells are removed.
   • C: Anti-D is usually IgG, not IgM.
   • D: Anti-D is mainly IgG1 and IgG3.

3. Question 3 · Easy

   Which immunoglobulin class agglutinates red cells in saline most efficiently?

   1. AIgM
   2. BIgE
   3. CIgA
   4. DIgG

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   Answer: A · IgM

   IgM is a pentamer with ten antigen-binding sites, so it can bridge the gap between red cells and agglutinate them directly. ABO antibodies are mainly IgM and react at immediate spin. Most IgG antibodies are too small to bridge the gap and need the antiglobulin test. IgM also activates complement efficiently.

   • B: IgE causes allergic reactions, not red cell agglutination.
   • C: IgA is not an important red cell agglutinin.
   • D: IgG usually needs anti-human globulin to show agglutination.

4. Question 4 · Medium

   Which IgG subclasses are most important in red cell destruction, because they bind complement and macrophage Fc receptors best?

   1. AIgG2 alone
   2. BIgG2 and IgG4
   3. CIgG1 and IgG3
   4. DIgG4 alone

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   Answer: C · IgG1 and IgG3

   IgG3 activates complement most strongly, followed by IgG1, and both bind well to macrophage Fc receptors. Red cells coated with IgG1 or IgG3 are therefore removed quickly in the spleen. IgG2 is a weak activator and IgG4 does not activate complement. Most clinically significant alloantibodies are IgG1, IgG3 or both.

   • A: IgG2 is a weak complement activator.
   • B: IgG2 and IgG4 bind complement and Fc receptors poorly.
   • D: IgG4 does not activate complement.

5. Question 5 · Medium

   Compared with IgG, IgM:

   1. ACrosses the placenta
   2. BDominates the secondary response
   3. CActivates complement more efficiently
   4. DHas a longer half-life

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   Answer: C · Activates complement more efficiently

   A single IgM molecule bound to a red cell can activate C1, while IgG needs two molecules close together. IgM antibodies such as anti-A and anti-B can therefore cause fast intravascular hemolysis. IgM has a half-life of about 5 days, compared with about 3 weeks for IgG. IgM does not cross the placenta.

   • A: Only IgG crosses the placenta.
   • B: The secondary response is mainly IgG.
   • D: IgG has the longer half-life.

6. Question 6 · Hard

   A D-positive boy was born at 37 weeks to a mother with anti-D (titer 256). His cord DAT was 4+ with anti-IgG, and he had a double-volume exchange transfusion with group O, D-negative red cells, then phototherapy. At 5 weeks of age, with no bleeding, his hemoglobin has fallen to 6.9 g/dL (69 g/L) and reticulocytes are 0.8%. The DAT is now 1+ with anti-IgG, and his plasma still contains anti-D. He has had no other transfusions and is breastfed. Which property of the antibody best explains these findings?

   1. AThe infant has begun making his own anti-D
   2. BMaternal IgG persists, with a half-life of about 3 weeks
   3. CAnti-D in breast milk is absorbed from his gut
   4. DMaternal IgM persists, because pentamers clear slowly

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