Immunoglobulins

5 ASCP MLS practice questions on Immunoglobulins, part of Immunology. The first 5 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 · Easy

   Which immunoglobulin class appears with IgM on the surface of mature naive B cells but is present at very low levels in serum?

   1. AIgG
   2. BIgA
   3. CIgE
   4. DIgD

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   Answer: D · IgD

   Mature naive B cells carry both IgM and IgD as antigen receptors, made from the same rearranged variable gene by alternative RNA splicing. Serum IgD is under 1% of total immunoglobulin, and its role in serum is unclear. IgE is also very low in serum but is mostly bound to mast cells and basophils. After activation and class switching, B cells lose surface IgD.

   • A: IgG is the most abundant serum class and appears after class switching.
   • B: IgA is the second most abundant serum class and is made after class switching.
   • C: IgE is low in serum but is bound to mast cells and basophils, not paired with IgM on naive B cells.

2. Question 2 · Easy

   Papain digestion of an IgG molecule yields:

   1. ATwo Fab fragments and one Fc fragment
   2. BTwo heavy chains and two light chains
   3. COne Fab fragment and two Fc fragments
   4. DOne F(ab')2 fragment and small Fc pieces

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   Answer: A · Two Fab fragments and one Fc fragment

   Papain cuts IgG on the N-terminal side of the hinge disulfide bonds, releasing two separate antigen-binding Fab fragments and one Fc fragment. Pepsin cuts below the hinge, so the two Fab arms stay joined as F(ab')2 and the Fc is broken into small pieces. Reducing agents such as 2-mercaptoethanol separate the heavy and light chains. The Fc part carries the effector functions, such as complement and Fc receptor binding.

   • B: Separate heavy and light chains come from reducing the disulfide bonds.
   • C: Each IgG has two antigen-binding arms and only one Fc.
   • D: F(ab')2 with small Fc pieces is the product of pepsin.

3. Question 3 · Medium

   Why is IgG the only immunoglobulin class that crosses the placenta in useful amounts?

   1. APlacental FcRn binds and transports its Fc
   2. BIt is small enough to diffuse across freely
   3. CIts Fab arms bind antigens on trophoblast
   4. DIt is the only monomeric immunoglobulin

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   Answer: A · Placental FcRn binds and transports its Fc

   The neonatal Fc receptor (FcRn) on the syncytiotrophoblast binds the Fc of IgG inside acidic vesicles and releases it on the fetal side. This active transport gives the newborn passive protection for several months, and it also lets maternal IgG antibodies cause HDFN or neonatal Graves disease. FcRn also rescues IgG from breakdown, which explains its long half-life of about 3 weeks. Other classes are not carried by FcRn.

   • B: Transfer is an active receptor process; IgD and IgE are also small but do not cross.
   • C: Fab binding to placental antigens does not move IgG across.
   • D: IgD, IgE and most serum IgA are also monomers.

4. Question 4 · Hard

   A febrile farm worker's Brucella agglutination titer is 1:640. After the serum is treated with 2-mercaptoethanol, the titer is 1:20. What is the best interpretation?

   1. AThe treatment destroyed the bacterial antigen
   2. BThe first titer was falsely raised by prozone
   3. CMost of the agglutinating antibody is IgM
   4. DMost of the agglutinating antibody is IgG

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   Answer: C · Most of the agglutinating antibody is IgM

   2-Mercaptoethanol and dithiothreitol break the disulfide bonds that hold the IgM pentamer together, so IgM loses its agglutinating activity. IgG is largely resistant, so the titer left after treatment reflects IgG. A fall from 1:640 to 1:20 shows that nearly all the activity was IgM, which fits an early or recent infection. A titer that stays high after treatment points to IgG, as seen later in infection or in chronic disease.

   • A: Only the serum is treated; the antigen suspension is added afterward.
   • B: Prozone makes undiluted results falsely low, not titers falsely high.
   • D: IgG resists 2-mercaptoethanol, so mainly IgG activity would keep the titer high.

5. Question 5 · Easy

   Secretory IgA in saliva and breast milk is protected from digestive enzymes mainly by the:

   1. ASecretory component
   2. BJ chain
   3. CKappa light chain
   4. DHinge region

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   Answer: A · Secretory component

   IgA made by plasma cells under the mucosa is a dimer joined by a J chain. The J chain lets the dimer bind the polymeric immunoglobulin receptor, which carries it across the epithelial cell; part of the receptor stays attached as the secretory component. This piece shields the hinge region from bacterial and digestive proteases. Secretory IgA is the main antibody in tears, saliva, colostrum and gut secretions.

   • B: The J chain joins the two IgA units and allows transport, but is not the main protection from proteases.
   • C: Light chain type does not protect IgA from proteolysis.
   • D: The hinge region is the part most open to protease attack.