Question 1

Immunoglobulin G (IgG) cleavage: A specific protease (such as papain) splits an IgG molecule into which defined fragments?

Accepted Answer

Introduction / Context: IgG structure determines how antibodies bind antigen and recruit effector functions. Proteolytic cleavage experiments (papain versus pepsin) are classic tools to map domains and understand function. Given Data / Assumptions: Protease referenced is the classic one used for IgG domain mapping. We consider intact IgG with two antigen-binding arms and one constant Fc region. Concept / Approach: Papain cleaves IgG above the hinge, yielding two monovalent Fab fragments (each with one antigen-binding site) and one Fc fragment (crystallizable fragment mediating effector functions like complement and Fc receptor binding). In contrast, pepsin cuts below the hinge, producing F(ab')2. Step-by-Step Solution: Recall papain action → cleavage above hinge. Products → Fab + Fab + Fc. Choose the option explicitly naming these three fragments. Confirm that other options describe different outcomes (e.g., pepsin makes F(ab')2). Verification / Alternative check: Fab retains antigen-binding without effector function; Fc retains constant-region functions without antigen binding. Why Other Options Are Wrong: Heavy-light dimers not standard nomenclature; random oligopeptides imply nonspecific digestion; F(ab')2 is pepsin product, not papain. Common Pitfalls: Confusing papain with pepsin; mixing up Fab (monovalent) versus F(ab')2 (bivalent). Final Answer: Two Fab fragments and one Fc fragment.

Question 2

Lymphocyte nomenclature: β-lymphocytes are more commonly referred to as which cell type in immunology?

Accepted Answer

Introduction / Context: Understanding lymphocyte classes is central to adaptive immunity. B lymphocytes (B cells) mediate humoral immunity by differentiating into plasma cells that secrete antibodies. Given Data / Assumptions: The term β-lymphocyte is an alternative label pointing toward B lineage. We compare B versus T cell roles. Concept / Approach: B cells recognize antigen via membrane-bound immunoglobulin (BCR). After activation with T-cell help or via T-independent antigens, they proliferate and differentiate into plasma cells producing antibodies with specific Fab regions and effector Fc functions. Step-by-Step Solution: Identify equivalence: β-lymphocyte → B lymphocyte. Recall function: humoral immunity and antibody production. Select the concise term used in practice → B cells. Exclude T-lineage terms (T cells, T lymphocytes). Verification / Alternative check: Standard textbooks and clinical labs refer to B cells (CD19+, CD20+) and T cells (CD3+), confirming the naming convention. Why Other Options Are Wrong: T cells/T lymphocytes are a different lineage mediating cellular immunity; plasma cells are differentiated B cells but not the generic name for β-lymphocytes. Common Pitfalls: Confusing plasma cells (antibody factories) with their progenitors (B cells). Final Answer: B cells.

Question 3

ABO blood group compatibility: Considering the ABO system (with Rh compatibility assumed), type A blood can be donated to which recipient type?

Accepted Answer

Introduction / Context: Safe transfusion requires matching donor red cell antigens to recipient antibodies. In the ABO system, type A red cells express A antigen and will react with anti-A antibodies present in type B or type O recipients. Given Data / Assumptions: ABO compatibility only; Rh factor compatibility is assumed or separately matched. Type A donor has A antigen on red cells and anti-B antibody in plasma. Concept / Approach: Recipients with AB blood express both A and B antigens and have no anti-A or anti-B antibodies; therefore, they can receive A, B, AB, or O red cells (universal recipient for ABO). Type B recipients have anti-A, so A cells would be hemolyzed. Step-by-Step Solution: Determine antigen/antibody profile: A cells carry A antigen. Identify compatible recipients lacking anti-A → AB (and A). Among listed choices, AB is broadly correct. Exclude misleading statements about Rh and incorrect antigen claims. Verification / Alternative check: Crossmatching and ABO charts confirm A → A or AB, O → all types, AB → AB only (for red cells). Why Other Options Are Wrong: Type B recipients have anti-A; saying A blood has B antigens is false; Rh+ statement ignores ABO incompatibility; O recipients would react to A antigen. Common Pitfalls: Confusing plasma compatibility with red cell compatibility; ignoring Rh as a separate issue. Final Answer: Type AB individuals.

Question 4

Human IgG subclasses: Into how many distinct IgG classes (subclasses) is human IgG divided?

Accepted Answer

Introduction / Context: Human IgG antibodies are subdivided into subclasses with different hinge regions, serum abundance, and effector functions. Recognizing the number and names aids in interpreting laboratory results and understanding vaccine responses. Given Data / Assumptions: Human IgG subclasses: IgG1, IgG2, IgG3, IgG4. Each subclass has unique properties (e.g., complement activation strength). Concept / Approach: Subclassification is based on differences in heavy chain gamma constant regions (γ1–γ4). These differences affect Fc-mediated functions, placental transfer efficiency, and half-life. Step-by-Step Solution: List subclasses: 1, 2, 3, 4. Confirm no fifth subclass exists in humans. Select the option indicating four. Note: nomenclature sometimes written as IgG1–IgG4. Verification / Alternative check: Immunology references and clinical assay reports (e.g., IgG subclass panels) enumerate four distinct subclasses. Why Other Options Are Wrong: Two or three undercount; five overcounts; one ignores known subclass diversity. Common Pitfalls: Confusing human subclass count with other species or with Ig heavy-chain classes (IgG, IgA, IgM, IgD, IgE). Final Answer: Four IgG classes.

Question 5

History of monoclonal antibodies: Who first developed the hybridoma technique to produce monoclonal antibodies?

Accepted Answer

Introduction / Context: Monoclonal antibodies revolutionized diagnostics and therapeutics. The hybridoma method fuses a specific B cell with a myeloma cell to produce immortal antibody-secreting clones. Given Data / Assumptions: Question asks for the pioneers of the hybridoma technique. Historical timeframe: mid-1970s; later awarded the Nobel Prize (1984) to Köhler and Milstein (with Niels K. Jerne). Concept / Approach: Hybridoma technology enables endless production of a single antibody specificity. It underpins many ELISA kits, imaging agents, and modern biologic drugs (with humanization advances). Step-by-Step Solution: Identify correct pair → Georges Köhler and César Milstein. Exclude unrelated scientists: Waksman (streptomycin), Jenner/Pasteur (vaccination), others not linked to hybridomas. Select answer accordingly. Acknowledge their Nobel recognition for immunology advances. Verification / Alternative check: Primary literature and Nobel archives confirm the discovery and its impact on immunology and biotechnology. Why Other Options Are Wrong: They are notable scientists but not for monoclonal antibody hybridomas. Common Pitfalls: Attributing monoclonal antibodies to general vaccine pioneers or antibiotic discoverers. Final Answer: Georges Köhler and César Milstein.

Question 6

Antibody fragments: How many antigen-binding sites does a single Fab fragment contain?

Accepted Answer

Introduction / Context: IgG antibodies are Y-shaped with two antigen-binding arms. Enzymatic cleavage yields Fab and Fc fragments, each with distinct properties used in research, therapeutics, and diagnostics. Given Data / Assumptions: Fab is the fragment antigen-binding region derived from one arm of IgG. An intact IgG has two Fabs (bivalent overall). Concept / Approach: Each Fab comprises one light chain and the corresponding portion of one heavy chain, forming a single paratope that binds one epitope. Therefore, a single Fab is monovalent for antigen. Step-by-Step Solution: Start from whole IgG: two identical binding sites. Isolate one arm by papain → one Fab. Recognize each arm binds one epitope. Answer: a Fab has one antigen-binding site. Verification / Alternative check: Structural studies (X-ray, cryo-EM) and binding assays demonstrate monovalent Fab interactions, in contrast to bivalent whole IgG or F(ab')2. Why Other Options Are Wrong: Two sites describes whole IgG or F(ab')2; antibody-binding site is not meaningful here; zero sites contradicts function. Common Pitfalls: Confusing bivalent antibody with monovalent Fab; mixing terms Fab versus Fc. Final Answer: One antigen-binding site.

Question 7

Secretory immunity: Which immunoglobulin class is predominantly found in external secretions such as saliva, tears, colostrum, and mucosal fluids?

Accepted Answer

Introduction / Context: Mucosal surfaces are key portals of pathogen entry. Secretory immunoglobulin A (sIgA) protects these surfaces by neutralizing pathogens and toxins without provoking inflammation. Given Data / Assumptions: Focus is on external secretions: saliva, tears, milk/colostrum, respiratory and intestinal mucus. We consider the dominant Ig class present as secretory dimeric IgA. Concept / Approach: sIgA is produced by plasma cells in mucosa, dimerized via J chain, and transported through epithelial cells using the polymeric Ig receptor. The secretory component protects IgA from proteolysis in harsh mucosal environments. Step-by-Step Solution: Identify the immunoglobulin enriched in secretions → IgA. Note structure: dimer with J chain and secretory component. Recall function: neutralization at mucosal surfaces without complement-mediated inflammation. Select IgA among options. Verification / Alternative check: Clinical measurements (e.g., salivary IgA) and mucosal immunology literature confirm IgA predominance in secretions. Why Other Options Are Wrong: IgD is a B-cell receptor isotype; IgM predominates in primary serum responses; IgE mediates allergy and parasite defense; IgG dominates serum and tissues. Common Pitfalls: Confusing serum abundance (IgG) with secretory predominance (IgA). Final Answer: IgA.

Question 8

Immune specificity: Which component primarily confers specificity in the adaptive immune response by recognizing unique epitopes?

Accepted Answer

Introduction / Context: Adaptive immunity achieves exquisite specificity via antigen receptors on B and T lymphocytes. Antibodies (secreted B-cell receptors) selectively bind epitopes with high affinity, directing neutralization and effector mechanisms. Given Data / Assumptions: We focus on the molecular determinant of recognition specificity in humoral immunity. Antibodies are the soluble mediators with unique paratopes recognizing specific epitopes. Concept / Approach: Specificity stems from variable regions (VH/VL) of antibodies generated by V(D)J recombination, junctional diversity, and somatic hypermutation. While T cells also exhibit specificity via TCRs, the question emphasizes the component responsible for specific binding in humoral responses. Step-by-Step Solution: Identify humoral specificity molecule → antibodies (immunoglobulins). Explain generation of diversity and affinity maturation. Select antibodies as the principal specificity-conferring component. Differentiate from antigens (targets) and accessory cells (macrophages). Verification / Alternative check: ELISA and neutralization assays demonstrate that antibodies bind specific antigen epitopes; clonality maps to unique variable region sequences. Why Other Options Are Wrong: Antigens are recognized, not the source of specificity; T lymphocytes are specific via TCR but not the humoral binding molecule; macrophages are innate effectors. Common Pitfalls: Equating antigen presence with specificity; overlooking that specificity resides in receptor/antibody structure. Final Answer: Antibodies.

Question 9

Basic IgA structure: In its monomeric serum form, each IgA antibody molecule consists of how many polypeptide chains and how many antigen-binding sites?

Accepted Answer

Introduction / Context: Immunoglobulin monomers (IgG, IgD, IgE, and serum IgA) share a common architecture: two identical heavy chains and two identical light chains forming a Y-shaped molecule with two antigen-binding sites. Given Data / Assumptions: We refer to the basic monomeric IgA unit found in serum. Secretory IgA in mucosa is commonly a dimer, but the question asks per single antibody molecule. Concept / Approach: Each IgA monomer consists of 2 heavy (alpha) chains and 2 light chains (kappa or lambda). The variable regions at the tips of the two Fab arms provide two identical antigen-binding sites (bivalency). Step-by-Step Solution: Identify chain composition → 2 heavy + 2 light = 4 polypeptide chains. Determine binding capacity → one site at each Fab arm = 2 sites total. Account for secretory forms separately (dimer with J chain), but answer for a monomer. Select the matching option accordingly. Verification / Alternative check: Structural analyses and immunology texts depict Ig monomers as heterotetramers with two combining sites; IgA heavy chains define the class. Why Other Options Are Wrong: Two or three chains do not describe immunoglobulin assembly; one binding site would be a Fab fragment, not whole IgA; six chains would imply multimeric Ig beyond a single monomer. Common Pitfalls: Confusing secretory dimeric IgA with the monomeric structural unit; mixing Fab fragments with whole antibodies. Final Answer: Four polypeptide chains and two antigen-binding sites.

Question 10

Immunology—Enzymatic cleavage of antibodies: When the protease papain digests an IgG molecule (under non-reducing conditions), into which defined fragments is IgG typically split?

Accepted Answer

Introduction / Context: Understanding how antibodies are enzymatically cleaved is foundational in immunology and biotechnology. The classic papain digestion of immunoglobulin G (IgG) produces well-defined fragments that retain specific biological properties used in diagnostics, structural studies, and therapeutics. Given Data / Assumptions: Enzyme: papain (a cysteine protease) acting on IgG. Conditions: non-reducing; disulfide bonds between heavy chains remain relevant for fragment definitions. Goal: identify the named fragments generated from intact IgG. Concept / Approach: Papain cleaves IgG above the hinge region. This releases two identical antigen-binding fragments (Fab) and a single crystallizable fragment (Fc). Each Fab contains one light chain and the corresponding variable and constant domains of a heavy chain, preserving monovalent antigen binding. The Fc fragment contains the effector-mediating CH2–CH3 domains of both heavy chains but lacks antigen-binding sites. Step-by-Step Solution: Recognize papain cleavage site: above the hinge region.Predict products: two Fab (each with one antigen-binding site) + one Fc (effector region).Match to options: two Fab fragments and one Fc fragment. Verification / Alternative check: Contrast with pepsin cleavage (below the hinge) that yields one F(ab')2 fragment plus degraded Fc pieces. This difference confirms the papain-specific pattern. Why Other Options Are Wrong: Three Fab / multiple Fc: IgG has only two antigen-binding arms; cannot produce three intact Fab. Two Fab and two Fc / F(ab')2 only: Do not match papain's above-hinge cleavage signature. Common Pitfalls: Confusing papain with pepsin results; forgetting that Fab is monovalent whereas F(ab')2 is divalent. Final Answer: Two Fab fragments and one Fc fragment.

Question 11

Antibody repertoire and antigen types: Human antibodies can recognize roughly 10^8 distinct antigenic molecules. Which statement best explains this diversity and the typical antigen class recognized?

Accepted Answer

Introduction / Context: The immune system achieves extraordinary recognition breadth with a limited genome. Clarifying how diversity arises and which antigen classes are most commonly targeted helps students link genetics (V(D)J recombination) with immunochemistry. Given Data / Assumptions: Total recognizable specificities ≈ 10^8 (order of magnitude). Normal, healthy immune tolerance reduces effective responses to self. Protein antigens often dominate T-dependent responses. Concept / Approach: Diversity is generated by V(D)J recombination, junctional diversity, and somatic hypermutation—not by possessing 10^8 separate antibody genes. T-dependent antibody responses center on processed protein antigens presented via MHC to T helper cells, amplifying B cell responses to proteins. While haptens (small molecules) can be recognized, they typically require carrier proteins to elicit strong responses. Step-by-Step Solution: Eliminate gene-count explanation (genome lacks 10^8 Ig genes).Account for self-tolerance: strong responses to self are curtailed.Identify dominant antigen class: proteins in typical adaptive responses. Verification / Alternative check: Immunization history and vaccine design overwhelmingly employ proteins or protein subunits; robust T-dependent class switching and affinity maturation support this. Why Other Options Are Wrong: 10^8 genes / fixed at birth: Ig diversity is somatically generated and evolves after birth. Self-antigens: Central and peripheral tolerance limit such responses. Small molecules chiefly: Haptens need carriers; proteins dominate. Common Pitfalls: Equating potential recognition with actual, tolerated responses; ignoring T cell help for strong humoral immunity. Final Answer: Most antibodies in natural responses recognize protein antigens.

Question 12

Definition of a polyclonal antibody response: Which statement correctly describes polyclonal antibodies generated after exposure to a single antigen?

Accepted Answer

Introduction / Context: Polyclonal versus monoclonal antibodies differ in epitope breadth. Clarifying this distinction is essential for interpreting serology, western blots, and immunoassays. Given Data / Assumptions: A single complex antigen usually presents multiple distinct epitopes. Multiple B cell clones can be primed simultaneously. Each B cell clone remains clonal—clonal selection still applies. Concept / Approach: A polyclonal response arises when different B cell clones, each specific for a distinct epitope on the same antigen, expand and secrete antibodies. The resulting serum contains a mixture of immunoglobulins with varying specificities and affinities, improving overall antigen recognition and clearance. Step-by-Step Solution: Identify the unit of specificity: one B cell clone → one epitope.Antigen with many epitopes → multiple clones activated.Serum antibodies are collectively “polyclonal.” Verification / Alternative check: Experimental adsorption against one epitope reduces only a subset of serum binding, confirming multiple specificities in polyclonal sera. Why Other Options Are Wrong: Polymeric-only / lag-phase-only: Polyclonality is not limited to antigen size or a specific kinetic phase. Violates clonal selection: It is built from many independent clonal expansions, so it aligns with clonal selection. Single clone only: Describes monoclonal antibodies. Common Pitfalls: Confusing the serum mixture (polyclonal) with a monoclonal cell line; overlooking epitope diversity on one antigen. Final Answer: Produced by several B cell clones recognizing different epitopes on the same antigen.

Question 13

Immunoglobulin (Ig) fold architecture: Which description best characterizes the Ig fold found in immunoglobulins and many Ig-superfamily proteins?

Accepted Answer

Introduction / Context: The immunoglobulin fold is a widely used structural motif across immune receptors and cell-adhesion molecules. Recognizing its β-sandwich architecture helps connect sequence domains to 3D structure and function. Given Data / Assumptions: Ig domains exist in heavy and light chains and many non-Ig proteins. Typical domain: two antiparallel β-sheets forming a β-sandwich (β-barrel-like). Strand counts commonly described as 3+4. Concept / Approach: Each variable (V) or constant (C) domain adopts the Ig fold. The motif is not exclusive to IgG and appears throughout the Ig superfamily (for example, TCR, MHC-related, cell adhesion molecules). An IgG molecule contains multiple Ig domains (more than six), so fixed “six only” is incorrect. Step-by-Step Solution: Identify architectural hallmark: two antiparallel β-sheets.Note common strand count: one sheet ~4 strands, the other ~3.Exclude exclusivity to IgG and incorrect copy numbers. Verification / Alternative check: Structural databases and textbook schematics depict the conserved 3+4 β-sandwich with intradomain disulfide bridging. Why Other Options Are Wrong: Only in IgG / α-helical bundle: Contradicts known β-sheet nature and distribution. Exactly six times in IgG: IgG typically has 12 Ig domains (8 heavy, 4 light). Common Pitfalls: Confusing “β-barrel” terminology with porins; here it refers to the β-sandwich architecture of Ig domains. Final Answer: A β-barrel composed of two antiparallel β-sheets (typically one with three and one with four strands).

Question 14

Core role of antibodies in host defense: What is the primary function of antibodies generated by the adaptive immune system?

Accepted Answer

Introduction / Context: Antibodies (immunoglobulins) are central effectors of humoral immunity. They recognize antigens with high specificity and mediate multiple protective mechanisms against pathogens and toxins. Given Data / Assumptions: Antibodies bind epitopes on antigens. Effector functions require Fc interactions (for example, complement, Fc receptors). Direct “killing” by antibodies alone is uncommon; they recruit other systems. Concept / Approach: Binding leads to neutralization (blocking attachment/toxin function), opsonization (facilitating phagocytosis), complement activation, and antibody-dependent cellular cytotoxicity. Antibodies do not generate antigens nor restrict protection only to blood; mucosal and tissue compartments are protected via isotype distribution. Step-by-Step Solution: Identify core function: specific antigen binding.Map major outcomes: neutralization, opsonization, complement.Select option that best encompasses these functions. Verification / Alternative check: Clinical uses of therapeutic antibodies (neutralizing toxins/viruses) mirror these mechanisms. Why Other Options Are Wrong: Direct killing / fever induction: Not primary antibody roles. Generate antigens: Antibodies recognize, not create, antigens. Circulatory-only: IgA in mucosa, IgG extravasation show broader protection. Common Pitfalls: Assuming antibodies alone lyse pathogens without complement or cells; ignoring isotype-specific locations. Final Answer: Bind specifically to the inducing antigen, neutralize or inactivate it, and help protect the body from disease.

Question 15

Immunoglobulin E (IgE) function: Which statement best describes the principal immunological role of IgE in humans?

Accepted Answer

Introduction / Context: Different antibody isotypes specialize for distinct immune niches. IgE is tightly linked to antiparasitic defense and allergic responses via mast cells and basophils. Given Data / Assumptions: IgE binds Fcε receptors on mast cells and basophils. Cross-linking by antigen triggers degranulation. Secretory dominance is associated with IgA, not IgE. Concept / Approach: IgE arms FcεRI-expressing cells. Upon exposure to helminth antigens or allergens, cross-linking causes mediator release (histamine, leukotrienes), promoting eosinophil recruitment and barrier responses helpful in anti-helminth immunity but also causing allergy symptoms. Step-by-Step Solution: Associate IgE with mast cells/basophils and eosinophils.Identify physiologic role: antiparasitic defense; pathologic role: immediate hypersensitivity.Exclude complement-dominant and secretory-dominant roles (belong to IgG/IgM and IgA). Verification / Alternative check: Elevated IgE and eosinophilia correlate with helminth infections; allergen skin testing depends on IgE-mediated mast cell degranulation. Why Other Options Are Wrong: Complement killing/opsonization: Primarily IgG and IgM. Secretions: Predominantly dimeric IgA with secretory component. All of the above: Overgeneralizes beyond IgE’s niche. Common Pitfalls: Confusing isotype locations and effector mechanisms (IgA vs IgE). Final Answer: Immunity against some parasites (and mediation of immediate hypersensitivity).

Question 16

Vaccine adjuvants—mechanism of alum: Why is alum (aluminum salts) considered an effective adjuvant in many human vaccines?

Accepted Answer

Introduction / Context: Adjuvants amplify vaccine responses by improving antigen presentation and innate signaling. Alum is the most widely used human adjuvant and exemplifies the depot and inflammasome-activating concepts. Given Data / Assumptions: Alum forms a particulate matrix with antigen. Depot effect prolongs antigen availability at the injection site. Innate pathways (for example, inflammasome) can be engaged. Concept / Approach: By adsorbing antigens and releasing them slowly, alum sustains antigen exposure to antigen-presenting cells. Particulate form promotes uptake, and associated innate cues drive helper T cell responses, often skewing toward Th2 and strong antibody production. Step-by-Step Solution: Recognize alum as a particulate adjuvant.Identify depot effect: slow antigen release.Link to enhanced APC uptake and humoral responses. Verification / Alternative check: Clinical vaccines (for example, toxoids) achieve higher antibody titers when formulated with alum compared to antigen alone. Why Other Options Are Wrong: Disaggregation / destruction: Opposite of alum’s protective adsorption. Direct immunogenicity for stem/T cells: Adjuvants enhance responses; they are not themselves specific antigens for these cells. Common Pitfalls: Assuming alum promotes robust cellular (Th1/CTL) responses; it is classically Th2-biased. Final Answer: It slows the release of antigen (depot effect) and enhances immune activation.

Question 17

Where do epitopes bind on antibodies? Within the antibody structure, antigenic determinants (epitopes) interact with which regions?

Accepted Answer

Introduction / Context: Antibody specificity resides in defined structural loops at the tips of the Fab domains. Knowing where epitopes bind is essential for understanding affinity maturation and antibody engineering. Given Data / Assumptions: Antibodies have variable (V) and constant (C) regions on both heavy and light chains. Complementarity-determining regions (CDRs) within V domains form the paratope. Constant regions mediate effector functions via Fc. Concept / Approach: Epitopes bind the paratope built from the VH and VL variable domains, especially the CDR loops (CDR1–3 from each chain). Constant domains (CH, CL) provide structural support and Fc effector interface, not antigen recognition. Step-by-Step Solution: Identify binding site: paratope in variable regions.Acknowledge contributions from both heavy and light chain CDRs.Exclude constant and hinge regions as primary binding sites. Verification / Alternative check: Mutations in CDRs alter specificity/affinity; constant region swaps do not change epitope specificity. Why Other Options Are Wrong: Constant / hinge: Effector or flexibility roles rather than binding. Only light or only heavy chains: Both contribute to the paratope in typical IgG. Common Pitfalls: Overlooking heavy-chain-only antibodies in some species; in human IgG, both chains contribute. Final Answer: Variable regions.

Question 18

For a hapten like PCP (phencyclidine), which interaction dominates tight antibody binding?

Accepted Answer

Introduction / Context: Hapten–antibody interactions reveal how small, nonpolar molecules are recognized. PCP (phencyclidine) is largely hydrophobic, making it a classic example to illustrate dominant binding forces. Given Data / Assumptions: PCP is predominantly nonpolar and aromatic/alicyclic. Antibody combining sites can create hydrophobic pockets. Multiple noncovalent forces contribute to affinity. Concept / Approach: For nonpolar haptens, hydrophobic interactions drive binding by displacing ordered water and fitting the hapten into a complementary nonpolar pocket, yielding favorable entropy and enthalpy. Electrostatic contacts are limited when the ligand lacks strong charges. Van der Waals forces contribute but usually in combination with hydrophobicity rather than alone. Step-by-Step Solution: Characterize PCP as hydrophobic.Map antibody pocket: nonpolar residues line the paratope.Conclude hydrophobic forces dominate tight binding. Verification / Alternative check: Solvent isotope effects and pocket mutagenesis often show decreased affinity when hydrophobic contacts are disrupted, supporting the dominant role of hydrophobic interactions. Why Other Options Are Wrong: Electrostatic forces: Less relevant for largely uncharged ligands. Van der Waals only: Present but not typically the principal driver alone. Covalent bonding: Antigen–antibody binding is noncovalent under physiological conditions. Common Pitfalls: Assuming a single force explains all binding; in reality, multiple weak forces sum, with hydrophobic effects often leading for nonpolar haptens. Final Answer: Hydrophobic forces.

Question 19

Practical uses of antibodies in the laboratory and medicine: Which of the following represents legitimate applications of antibodies?

Accepted Answer

Introduction / Context: Antibodies are versatile tools beyond immunity. Their specificity enables detection, purification, and even catalysis when engineered appropriately. Given Data / Assumptions: High specificity and affinity for target epitopes. Antibodies can be immobilized on solid supports. Catalytic antibodies (abzymes) can be selected to stabilize transition states. Concept / Approach: For localization, labeled antibodies reveal protein distribution within cells or tissues. For purification, immunoaffinity chromatography uses immobilized antibodies to capture targets from mixtures. For catalysis, antibodies raised against transition-state analogs can catalyze reactions by preferentially binding transition-state-like structures. Step-by-Step Solution: A: Use fluorescent or enzyme-conjugated antibodies to visualize proteins.B: Couple antibody to resin; bind and elute target protein.C: Generate abzymes via transition-state analog immunization; test catalytic rate enhancement. Verification / Alternative check: Common kits (IHC/IF, IP/Co-IP, immunoaffinity columns) and literature on abzymes validate these uses. Why Other Options Are Wrong: None of the above: Contradicts standard lab practice and research applications. Common Pitfalls: Overlooking antibody orientation and elution conditions in purification; misinterpreting nonspecific staining without proper controls. Final Answer: All of the above.

Question 20

Immunology concept check: what is a secondary antibody in laboratory diagnostics? Select the statement that best defines a secondary antibody used in ELISA, Western blot, and immunofluorescence workflows.

Accepted Answer

Introduction / Context: In immunoassays such as ELISA, Western blotting, and immunofluorescence, secondary antibodies are indispensable reagents. They provide signal amplification, enable flexible detection chemistries, and allow a single labeled reagent to recognize many primary antibodies that share a common Fc region. Given Data / Assumptions: The question concerns the role and definition of a secondary antibody in common laboratory assays. Primary antibodies bind the target antigen. Secondary antibodies recognize the primary antibody. Signal is often carried by an enzyme, fluorophore, or radionuclide linked to the secondary antibody. Concept / Approach: A secondary antibody is raised against the constant region (Fc) of a primary antibody from a given host species and is typically isotype specific (for example, anti-mouse IgG). Because each secondary carries multiple labels or catalytic activity, it amplifies detection of the antigen–primary complex. Step-by-Step Solution: Identify the object of binding for the secondary: it targets the primary antibody, not the antigen.Recognize typical labels: enzymes (HRP, alkaline phosphatase), fluorophores, or biotin.Conclude that the defining feature is binding to another antibody, enabling amplification and versatile detection. Verification / Alternative check: Protocol schematics for indirect ELISA show antigen → primary antibody → labeled secondary antibody; Western blots show the same hierarchy, confirming the role of the secondary antibody. Why Other Options Are Wrong: “Has been used in prior experiments” describes history, not definition. “Is synthetically produced” is not defining; most are biologically produced and then conjugated. “Produced only after boosters” refers to immunization strategy, not function. “Neutralizes toxins” describes certain primary antibodies, not the generic role of secondary antibodies. Common Pitfalls: Confusing secondary antibodies with antigen-specific primaries; overlooking host species and isotype compatibility, which are crucial for clean results. Final Answer: Binds to another antibody.

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