- A patient receives an infusion of pooled intravenous immunoglobulin (IVIG) prepared from thousands of donors. Several weeks later, the patient develops antibodies that react with the Fc portion of some, but not all, of the infused IgG molecules. These patient-generated antibodies are most likely specific for:
Answer: allotypic determinants on the infused immunoglobulins.
Allotypes are genetically determined variations in the constant regions of antibodies that differ between individuals of the same species. When a patient receives IVIG, they are exposed to allotypes that they may not possess themselves. The patient's immune system can recognize these foreign allotypes as non-self and produce anti-allotype antibodies. These antibodies would react with some (those bearing the foreign allotype) but not all (those matching the patient's own allotype or other non-recognized allotypes) of the infused IgG.
- A fundamental difference between antigen recognition by T-cell receptors (TCRs) and B-cell receptors (BCRs) is that:
Answer: TCRs recognize processed peptide fragments presented by MHC molecules, whereas BCRs can recognize intact antigens.
This is a central tenet of adaptive immunity. B cells use their BCRs to bind directly to epitopes on intact, native antigens, which can be proteins, polysaccharides, or lipids. T cells, via their TCRs, cannot recognize soluble or native antigens. They can only recognize short, processed peptide fragments that are displayed on the surface of other cells by Major Histocompatibility Complex (MHC) molecules.
- A patient presents with recurrent bacterial infections of the respiratory and gastrointestinal tracts. Laboratory tests reveal normal serum levels of IgG and IgM, but a selective deficiency in the polymeric immunoglobulin receptor (pIgR). This patient's susceptibility to mucosal infections is most directly caused by an inability to:
Answer: transport dimeric IgA across epithelial cells into mucosal secretions.
The polymeric immunoglobulin receptor (pIgR) is expressed on the basolateral surface of mucosal epithelial cells. Its function is to bind J-chain-containing dimeric IgA (and pentameric IgM) produced by plasma cells in the underlying tissue (lamina propria) and transport it across the cell to be secreted into the lumen. A defect in pIgR would lead to a lack of secretory IgA, the primary antibody protecting mucosal surfaces, despite normal production of IgA in the tissues and normal levels of other immunoglobulins in the serum.
- A small polysaccharide isolated from a bacterial capsule is found to be readily bound by pre-existing antibodies in an in vitro assay. However, when this purified polysaccharide is injected into a naive animal, it fails to elicit a significant new antibody response. Which of the following statements best describes the properties of this polysaccharide?
Answer: It is antigenic but poorly immunogenic.
Antigenicity is the ability to be recognized and bound by an antibody or TCR. Since pre-existing antibodies bind it, the polysaccharide is antigenic. Immunogenicity is the ability to induce an immune response (e.g., antibody production). Since it fails to elicit a response when injected, it is not immunogenic, or at best, poorly immunogenic. This is a common property of haptens and some T-independent antigens. The statement 'immunogenic but not antigenic' is a logical impossibility.
- Researchers use genetic engineering to create a chimeric antibody. They fuse the variable (V) regions from a mouse monoclonal antibody specific for a bacterial toxin with the constant (C) regions of human IgG1. What is the primary functional advantage of this chimeric antibody for therapeutic use in humans compared to the original mouse antibody?
Answer: It retains toxin-neutralizing specificity while reducing the risk of an anti-antibody immune response in the patient.
The variable regions determine antigen specificity, so the chimeric antibody will bind the toxin just like the original mouse antibody. The constant regions are the primary targets of an immune response against a foreign antibody (a HAMA, or human anti-mouse antibody, response). By replacing the mouse constant regions with human ones, the antibody appears less 'foreign' to the human immune system, reducing its immunogenicity. The human Fc region also allows it to interact effectively with human effector cells and complement.
- An animal is repeatedly injected with dinitrophenol (DNP), a small chemical molecule, and no significant anti-DNP antibody response is detected. Subsequently, the animal is injected with DNP chemically conjugated to bovine serum albumin (BSA). A strong antibody response against DNP is now observed. If the animal is later challenged with only DNP-BSA, which of the following is the most accurate description of the secondary immune response?
Answer: A rapid and robust production of high-affinity, class-switched anti-DNP antibodies will occur.
DNP is a hapten: it is antigenic (can be bound by antibodies) but not immunogenic (cannot elicit a response) on its own. When conjugated to a carrier protein (BSA), it becomes immunogenic. The initial injection of DNP-BSA elicits a primary response, creating memory B cells and T helper cells. A subsequent challenge with DNP-BSA triggers a secondary (anamnestic) response, which is characterized by rapid production of large quantities of high-affinity, isotype-switched (e.g., IgG) antibodies specific for the hapten (DNP).
- An antibody is generated in a rabbit by immunizing it with a purified mouse monoclonal antibody (mAb1). The resulting rabbit antiserum is found to bind only to mAb1 and not to other mouse monoclonal antibodies of the same IgG2a class. This rabbit antiserum is therefore specific for which type of determinant on mAb1?
Answer: Idiotypic
Determinants on immunoglobulins are classified as isotypes (constant region differences defining classes like IgG, IgM), allotypes (allelic variants of constant regions within a species), and idiotypes (unique structures of the variable region's antigen-binding site). Since the rabbit antibody binds only to mAb1 and not other antibodies of the same class (isotype), it must be recognizing the unique antigen-binding site of mAb1. This is, by definition, an anti-idiotypic antibody.
- The B-cell receptor (BCR) complex on a mature naive B cell consists of a membrane-bound immunoglobulin (mIg) and the Igα/Igβ heterodimer. What is the primary function of the Igα/Igβ component upon antigen binding to the mIg?
Answer: To transduce a signal into the cell via its intracellular immunoreceptor tyrosine-based activation motifs (ITAMs).
The membrane-bound immunoglobulin (mIg) component of the BCR has a very short cytoplasmic tail and cannot signal effectively on its own. The associated Igα/Igβ (also known as CD79a/CD79b) heterodimer contains ITAMs in its cytoplasmic domains. Upon antigen-induced cross-linking of the BCRs, these ITAMs become phosphorylated, initiating an intracellular signaling cascade that leads to B-cell activation.
- A patient recovers from an infection with Streptococcus pyogenes. Later, they are exposed to a different bacterial species that shares a structurally similar M protein epitope. The patient's immune system mounts an accelerated response to the new bacterium. This phenomenon is best explained by:
Answer: Cross-reactivity, where antibodies generated against the S. pyogenes epitope bind to the similar epitope on the new bacterium.
Cross-reactivity occurs when an antibody or T-cell receptor specific for one antigen binds to a different, structurally similar antigen. In this case, memory cells and antibodies from the S. pyogenes infection recognize the similar epitope on the new bacterium, leading to a rapid and specific secondary-like response. This is the basis for some autoimmune diseases (e.g., rheumatic fever following strep throat) and for protection conferred by some vaccines against related pathogens.
- The J (joining) chain is a 15-kDa polypeptide that plays a critical role in the structure of certain immunoglobulin isotypes. A genetic defect that prevents the synthesis of a functional J chain would most directly impair the structure and function of which of the following?
Answer: Secretory dimeric IgA and pentameric IgM.
The J chain is essential for the polymerization of IgA and IgM. It covalently links two monomers of IgA to form the dimer found in secretions, and it links five monomers of IgM to form the circulating pentamer. Immunoglobulins G, D, and E are monomers and do not contain a J chain. Therefore, a defect in the J chain would specifically disrupt the formation of functional dimeric IgA and pentameric IgM.
- A solution of purified human IgG1 is divided into two aliquots. Aliquot 1 is treated with the enzyme papain, and Aliquot 2 is treated with pepsin. Which statement accurately compares the major antigen-binding products of these reactions?
Answer: Papain yields two separate Fab fragments, while pepsin yields one F(ab')₂ fragment.
Papain cleaves the IgG molecule above the hinge region's disulfide bonds, resulting in two identical, monovalent Fab (fragment, antigen-binding) fragments and one Fc (fragment, crystallizable) fragment. Pepsin cleaves below the disulfide bonds of the hinge region, resulting in a single, bivalent F(ab')₂ fragment (the two Fab arms remain linked) and a degraded Fc portion (pFc').
- The ability of an antibody to agglutinate particulate antigens, such as bacteria or red blood cells, is highly dependent on its valency. Which antibody isotype is generally the most efficient at agglutination in the primary immune response?
Answer: IgM, because its pentameric structure provides ten antigen-binding sites.
Agglutination requires the cross-linking of multiple particles. The efficiency of this process is directly related to the number of antigen-binding sites (valency) an antibody has. IgM is a pentamer held together by a J chain, giving it 10 antigen-binding sites. This high valency makes it exceptionally efficient at agglutinating particulate antigens, which is a key function during the early stages of a primary immune response when IgM is the predominant isotype.
- Consider two scenarios involving a dendritic cell (DC): (1) The DC is infected by a measles virus, which replicates in the cytoplasm. (2) The DC phagocytoses an extracellular Staphylococcus aureus bacterium. How will antigens from these two pathogens primarily be processed and presented to T cells?
Answer: Viral antigens on MHC class I to CD8+ T cells; bacterial antigens on MHC class II to CD4+ T cells.
The two major pathways of antigen presentation are distinct. Endogenous antigens (like viral proteins made inside the cell) are processed by the proteasome and presented on MHC class I molecules to CD8+ cytotoxic T cells. Exogenous antigens (like bacteria taken up from outside the cell) are processed in phagolysosomes and presented on MHC class II molecules to CD4+ helper T cells. While exceptions like cross-presentation exist, these are the primary, canonical pathways.
- A cytotoxic T lymphocyte (CTL) clone is isolated that is specific for a peptide derived from a viral polymerase, an enzyme synthesized within an infected host cell. For this CTL to recognize and kill the infected cell, which of the following is an absolute requirement?
Answer: The infected cell must present the viral peptide on an MHC class I molecule on its surface.
CTLs (which are typically CD8+) recognize endogenous antigens—proteins made within a cell, such as viral proteins or tumor antigens. These proteins are degraded by the proteasome, and the resulting peptides are transported to the endoplasmic reticulum, loaded onto MHC class I molecules, and presented on the cell surface. This is the signal for CTLs to kill the cell. MHC class II presents exogenous antigens to helper T cells.
- A patient infected with a parasitic helminth develops a robust IgE response. Eosinophils are recruited to the site of infection and are observed attached to the surface of the worm. Which of the following describes the most likely mechanism linking these observations?
Answer: IgE binds to the parasite, and its Fc portion is recognized by Fcε receptors on eosinophils, triggering degranulation.
This describes antibody-dependent cell-mediated cytotoxicity (ADCC), a key defense mechanism against large parasites like helminths. IgE antibodies bind to antigens on the parasite's surface. Eosinophils express high-affinity Fc receptors for IgE (FcεRI). The cross-linking of these receptors by parasite-bound IgE triggers the eosinophil to release the cytotoxic contents of its granules (e.g., major basic protein, eosinophil cationic protein) directly onto the parasite's surface, damaging it.
- A laboratory is developing a sandwich ELISA to detect a large, complex bacterial toxin. They are considering using either a polyclonal antiserum or a monoclonal antibody as the detection antibody. A key advantage of using the polyclonal antiserum for detection in this assay is that it:
Answer: is more robust to minor denaturation or modification of a single epitope on the toxin.
A polyclonal antiserum contains a mixture of antibodies that recognize many different epitopes on the same antigen. A monoclonal antibody recognizes only a single epitope. If that one specific epitope is damaged, modified, or masked, the monoclonal antibody will fail to bind. The polyclonal antiserum can still bind to the numerous other intact epitopes on the antigen, making the assay more robust and less prone to failure due to subtle changes in the antigen. It can also amplify the signal by having multiple antibodies bind to a single captured antigen molecule.
- A monoclonal antibody is generated against a native, folded viral capsid protein. This antibody shows strong binding in an enzyme-linked immunosorbent assay (ELISA) where the native protein is immobilized. However, the same antibody fails to detect the protein in a Western blot analysis following sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). What is the most likely explanation for this discrepancy?
Answer: The antibody recognizes a conformational epitope that is destroyed by the denaturing conditions of SDS-PAGE.
SDS-PAGE is a denaturing technique that unfolds proteins into linear polypeptide chains. A Western blot detects these linearized proteins. An ELISA, particularly a direct or sandwich ELISA, can immobilize proteins in their native, folded conformation. The antibody's ability to bind in ELISA but not in Western blot strongly suggests it recognizes a conformational (or discontinuous) epitope, which is formed by the 3D folding of the protein and is lost upon denaturation.
- A researcher is studying the interaction between a novel bacterial surface antigen and the host's antibody response. The antigen consists of a single, repeating epitope. They compare the binding of a standard pentameric IgM molecule to an engineered monomeric IgM molecule, both specific for this epitope. The individual antigen-binding sites (Fab regions) of both molecules are identical. Which of the following best describes the expected binding characteristics?
Answer: The pentameric IgM will have a much higher avidity and a similar affinity compared to the monomeric IgM.
Affinity refers to the binding strength of a single antigen-binding site (Fab region) with a single epitope. Since the Fab regions are identical, the affinity is the same for both molecules. Avidity refers to the overall binding strength of a multivalent antibody to a multivalent antigen. The pentameric structure of IgM provides 10 binding sites, allowing for a much stronger overall interaction (higher avidity) with the repeating epitopes on the bacterial surface compared to the two sites on the monomeric version.
- During a secondary immune response to a protein antigen, the antibodies produced typically exhibit a higher average affinity for the antigen than those produced during the primary response. This phenomenon, known as affinity maturation, is a direct result of:
Answer: somatic hypermutation in variable region genes of B cells, followed by selection for high-affinity variants.
Affinity maturation occurs within the germinal centers of secondary lymphoid organs. The enzyme activation-induced deaminase (AID) introduces point mutations into the variable region genes of proliferating B cells (somatic hypermutation). B cells whose mutated receptors bind antigen with higher affinity receive survival signals from follicular helper T cells and are selected to proliferate, while those with lower affinity undergo apoptosis. This selective process results in a B cell population that produces antibodies with progressively higher affinity.
