USMLE Step 1 Quiz
Practice Innate And Adaptive Immunity in USMLE Step 1 with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.
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A pharmaceutical company is developing a new immunosuppressant that functions as an antagonist to the B7 molecule on antigen-presenting cells (APCs). The drug is intended to prevent the rejection of transplanted organs by modulating T-cell responses.
By blocking the interaction of B7 with its ligand on T cells, this drug would most directly interfere with which of the following events?
This quiz focuses on Innate And Adaptive Immunity, giving you a quick way to practice the rules, question types, and explanations that matter most for USMLE Step 1.
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A pharmaceutical company is developing a new immunosuppressant that functions as an antagonist to the B7 molecule on antigen-presenting cells (APCs). The drug is intended to prevent the rejection of transplanted organs by modulating T-cell responses.
By blocking the interaction of B7 with its ligand on T cells, this drug would most directly interfere with which of the following events?
Explanation: Full activation of a naive T cell requires two signals. Signal 1 is provided by the T-cell receptor (TCR) binding to the peptide-MHC complex on an antigen-presenting cell (APC). Signal 2, known as costimulation, is provided by the interaction of costimulatory molecules. The most important costimulatory interaction is between the B7 molecule (CD80/CD86) on the APC and the CD28 receptor on the T cell. Blocking this interaction prevents full T-cell activation, leading to a state of unresponsiveness called anergy. This is a key target for immunosuppressive drugs. (A) The drug does not affect the TCR-MHC interaction (Signal 1). (B) Adhesion is mediated by molecules like LFA-1 and ICAM-1. (D) Differentiation occurs after T-cell activation.
A 2-year-old boy presents with a history of recurrent severe bacterial infections of the sinuses and lungs, as well as an episode of Pneumocystis jirovecii pneumonia. Laboratory studies show markedly elevated serum IgM levels but almost undetectable levels of IgG, IgA, and IgE. T-cell and B-cell counts are normal.
This patient's clinical presentation is most likely caused by a genetic defect affecting the function of which of the following molecules?
Explanation: This clinical picture is classic for X-linked hyper-IgM syndrome, which is caused by a defect in the gene for CD40 ligand (CD40L or CD154). CD40L is expressed on activated helper T cells and is essential for interacting with the CD40 receptor on B cells. This CD40-CD40L interaction is required for B cells to undergo class switch recombination (isotype switching) from IgM to other isotypes (IgG, IgA, IgE) and to form germinal centers for affinity maturation. Without this signal, B cells can only produce IgM, leading to high levels of IgM and a deficiency of other antibodies, resulting in susceptibility to pyogenic bacteria and opportunistic pathogens. (B) A defect in MHC class II would cause a lack of CD4+ T cells (bare lymphocyte syndrome type II). (C) A B-cell receptor defect would lead to a lack of B cells. (D) RAG deficiency causes severe combined immunodeficiency (SCID) with an absence of both B and T cells.
A 1-month-old infant presents with delayed separation of the umbilical stump, omphalitis, and recurrent skin infections that notably lack pus formation. Laboratory analysis shows a persistent and profound leukocytosis, with a high neutrophil count.
The pathogenesis of this disorder involves a defect in the expression of which class of molecules on the surface of neutrophils?
Explanation: This presentation is characteristic of Leukocyte Adhesion Deficiency Type 1 (LAD-1), an autosomal recessive disorder caused by mutations in the gene encoding the β2-chain (CD18) of integrins. Integrins (like LFA-1 and Mac-1) are crucial for the firm adhesion of neutrophils to the vascular endothelium, a necessary step for extravasation into tissues. Without functional integrins, neutrophils cannot migrate from the bloodstream to sites of infection. This leads to recurrent infections without pus (which is composed mainly of dead neutrophils) and a high circulating neutrophil count (neutrophilia) because they are trapped in the blood. (A) Selectin defects cause a similar but generally milder condition (LAD-2). (B) Immunoglobulins are antibodies. (C) Chemokine receptors are involved in directing migration, but the primary defect in LAD-1 is in firm adhesion.
A developing B lymphocyte in the bone marrow has successfully completed the rearrangement of its immunoglobulin heavy-chain genes. It expresses the heavy chain on its surface in a complex with surrogate light chains. This complex is known as the pre-B cell receptor.
Successful signaling through the pre-B cell receptor is a critical checkpoint that directly triggers which of the following events?
Explanation: The pre-B cell receptor checkpoint is a crucial step in B-cell development. Successful signaling from this receptor confirms that a functional heavy chain has been produced. This signal has several consequences, including inducing proliferation of the pre-B cell and, importantly, enforcing allelic exclusion by shutting down further V(D)J recombination at the immunoglobulin heavy-chain locus. This ensures that each B cell expresses a heavy chain from only one parental chromosome, maintaining clonal specificity. Following this, recombination of the light-chain genes is initiated. (A) Heavy-chain recombination occurs before the formation of the pre-B cell receptor. (C) Somatic hypermutation occurs much later, in mature B cells in germinal centers. (D) Elimination of self-reactive B cells (negative selection) occurs after a complete IgM B-cell receptor is expressed.
A 40-year-old veterinarian steps on a rusty nail and receives a tetanus toxoid booster shot. His last vaccination was 10 years ago. Two weeks later, his serum contains a high concentration of high-affinity IgG antibodies against the tetanus toxoid.
The rapid production of high-affinity IgG antibodies in this patient is best attributed to the activation of which of the following cell types?
Explanation: This scenario describes a secondary (or anamnestic) immune response, which is characterized by being faster, stronger, and more effective than the primary response. This is due to the existence of long-lived memory cells generated during the primary immunization. Upon re-exposure to the tetanus antigen, pre-existing memory B cells are rapidly activated. They quickly proliferate and differentiate into plasma cells that secrete large amounts of high-affinity, isotype-switched antibodies (primarily IgG), providing rapid protection. (A) Naive B cells would mediate a slower primary response, initially producing IgM. (C) Plasmacytoid dendritic cells are key producers of type I interferons in viral infections. (D) Regulatory T cells function to suppress immune responses.
A patient with DiGeorge syndrome, characterized by thymic aplasia and a profound deficiency of T cells, is given a vaccine composed of pneumococcal capsular polysaccharides. Despite the absence of T-cell help, the patient is able to generate a weak antibody response.
Which of the following best explains the antibody response observed in this patient?
Explanation: Some antigens, particularly those with highly repetitive structures like bacterial capsular polysaccharides, are classified as T-independent (TI) antigens. These antigens can activate B cells without the help of T helper cells. They do so by extensively cross-linking the B-cell receptors (BCRs) on the B-cell surface, providing a strong enough signal to induce proliferation and differentiation into IgM-secreting plasma cells. This response is typically weaker, shorter-lived, generates no memory B cells, and does not involve isotype switching or affinity maturation compared to T-dependent responses. (B) NK cells do not provide help for B-cell activation. (C) Affinity maturation requires T-cell help in germinal centers. (D) B cells bind intact antigen directly; they do not require processing and presentation by macrophages.
A 5-year-old boy has a history of recurrent pneumonia, abscesses, and osteomyelitis caused by catalase-positive organisms like Staphylococcus aureus and Serratia marcescens. A functional assay of his neutrophils shows a failure to produce a respiratory burst after stimulation.
This patient's neutrophils most likely have a deficiency in which of the following enzymes?
Explanation: This clinical presentation is classic for Chronic Granulomatous Disease (CGD), an immunodeficiency resulting from a defect in NADPH oxidase. This enzyme complex is essential for the phagocyte respiratory burst, a process that generates superoxide radicals (O2-) from oxygen. Superoxide is then converted to other reactive oxygen species (ROS) that are toxic to ingested microbes. In CGD, phagocytes can ingest but cannot kill pathogens effectively, leading to granuloma formation and recurrent infections, particularly with catalase-positive organisms which can break down the small amount of H2O2 that the host cell might produce. (A) Myeloperoxidase deficiency is less severe, as other killing mechanisms remain intact. (B) G6PD deficiency can impair the respiratory burst by limiting NADPH supply, but it is primarily known for causing hemolytic anemia. (D) Lysozyme is an enzyme in phagocytic granules that degrades bacterial cell walls.
A college student presents with fever, headache, and neck stiffness. A lumbar puncture is performed, and cerebrospinal fluid analysis is consistent with bacterial meningitis. The causative organism is identified as Neisseria meningitidis. A review of the patient's history reveals recurrent sinopulmonary infections and a prior episode of disseminated gonococcal infection.
A deficiency in components of which of the following immune pathways is most strongly associated with susceptibility to recurrent Neisseria infections?
Explanation: The terminal complement components (C5-C9) assemble to form the membrane attack complex (MAC), which creates pores in the outer membranes of pathogens, leading to their lysis. The MAC is particularly important for defense against encapsulated bacteria, especially Neisseria species. Individuals with deficiencies in any of the terminal complement components (C5, C6, C7, C8, or C9) are highly susceptible to recurrent infections with Neisseria meningitidis and Neisseria gonorrhoeae. (A, B, C) While the classical, lectin, and alternative pathways are all ways to activate the complement cascade and lead to the formation of opsonins (C3b) and anaphylatoxins (C3a, C5a), the specific vulnerability to Neisseria is most directly linked to the failure to form the lytic MAC.
A thymocyte is identified in the cortex of the thymus. Analysis of its cell surface markers reveals expression of the T-cell receptor, CD3, CD4, and CD8. This cell is currently interacting with cortical thymic epithelial cells.
This cell is best described as being at which of the following stages of T-cell development?
Explanation: During T-cell development in the thymus, thymocytes progress through several stages defined by the expression of CD4 and CD8 coreceptors. The stage at which a thymocyte expresses both CD4 and CD8 simultaneously is known as the double-positive stage. This is the stage where positive selection occurs in the thymic cortex. During positive selection, these double-positive cells are tested for their ability to recognize self-MHC. Cells that successfully recognize self-MHC will survive and differentiate into single-positive (either CD4+ or CD8+) T cells. (A) Double-negative thymocytes are an earlier stage and lack both CD4 and CD8. (C) Single-positive thymocytes are a later stage and express either CD4 or CD8, but not both. (D) Mature naive T cells are single-positive cells that have exited the thymus and circulate in the periphery.
A 45-year-old man is brought to the intensive care unit with fever, hypotension, and altered mental status. Blood cultures grow Escherichia coli. The patient's severe systemic inflammatory response is triggered by lipopolysaccharide (LPS), a component of the bacterial outer membrane. This molecule is recognized by a specific receptor on innate immune cells such as macrophages.
Which of the following pattern recognition receptors is primarily responsible for initiating the inflammatory cascade in response to the component described?
Explanation: Toll-like receptor 4 (TLR4) is the primary pattern recognition receptor that recognizes lipopolysaccharide (LPS), a major component of the outer membrane of Gram-negative bacteria. Binding of LPS to TLR4 on macrophages and dendritic cells initiates a signaling cascade that leads to the production of pro-inflammatory cytokines like TNF-α, IL-1, and IL-6, causing the systemic inflammatory response seen in septic shock. (A) TLR3 recognizes double-stranded RNA, which is associated with viral infections. (C) NOD2 is an intracellular receptor that recognizes muramyl dipeptide, a component of bacterial peptidoglycan found in both Gram-positive and Gram-negative bacteria. (D) RIG-I-like receptors are intracellular sensors that detect viral RNA in the cytoplasm.
A research study is investigating the mechanisms of immune surveillance against tumors. Certain tumor cells evade the immune system by downregulating the expression of specific surface molecules, making them poor targets for cytotoxic T lymphocytes. However, another type of lymphocyte can recognize and kill these altered tumor cells.
The cytotoxic activity of these specialized lymphocytes is triggered by the absence of which of the following molecules on the target cell surface?
Explanation: Natural killer (NK) cells are part of the innate immune system and play a crucial role in eliminating virally infected cells and tumor cells. NK cells possess both activating and inhibitory receptors. Their inhibitory receptors recognize MHC class I molecules, which are expressed on almost all healthy nucleated cells. When a tumor cell or virally infected cell downregulates MHC class I expression to evade cytotoxic T cells, the inhibitory signal to the NK cell is lost. This 'missing-self' signal, combined with signals from activating receptors, triggers the NK cell to release cytotoxic granules and kill the target cell. (B) MHC class II is primarily expressed on professional antigen-presenting cells. (C) CD80 is a costimulatory molecule on APCs that binds to CD28 on T cells. (D) Fas ligand is expressed on activated T cells and NK cells to induce apoptosis in target cells expressing Fas.
A 4-year-old boy presents with a history of recurrent bacterial and fungal infections. Laboratory analysis reveals a normal number of B cells and T-cell precursors, but a profound lack of mature T lymphocytes in the peripheral blood. A biopsy of the thymus shows a specific defect in the cortical epithelial cells, which are crucial for T-cell maturation.
The function of these defective cells is essential for which of the following processes in T-cell development?
Explanation: Positive selection is the process where developing T cells (thymocytes) are tested for their ability to recognize self-MHC molecules. This occurs in the thymic cortex and is mediated by cortical thymic epithelial cells, which present self-antigens on MHC class I and class II molecules. Thymocytes that can weakly bind to these self-MHC molecules receive a survival signal and are 'positively selected'. A defect in these epithelial cells would impair this process, leading to the death of most thymocytes and a profound T-cell deficiency. (A) Negative selection occurs in the thymic medulla and eliminates T cells that bind too strongly to self-antigens, preventing autoimmunity. (B) Somatic hypermutation is a process that occurs in B cells within germinal centers to increase antibody affinity. (D) Isotype switching is also a B-cell process that changes the constant region of the antibody heavy chain.
A 12-year-old girl is diagnosed with autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APECED). This condition is characterized by the development of autoimmunity against multiple endocrine organs. The disease is caused by mutations in the autoimmune regulator (AIRE) gene.
The AIRE protein plays a critical role in which of the following immunologic processes?
Explanation: The AIRE (autoimmune regulator) protein is a transcription factor expressed in medullary thymic epithelial cells (mTECs). Its primary function is to promote the expression of a wide range of tissue-specific antigens that are normally restricted to peripheral organs (e.g., insulin, thyroglobulin). This allows developing T cells to be exposed to these self-antigens within the thymus. T cells that bind too strongly to these antigens are eliminated via negative selection, establishing central tolerance. Mutations in AIRE lead to a failure of this process, allowing self-reactive T cells to escape into the periphery and cause autoimmune disease. (A) Peripheral anergy is a mechanism of peripheral tolerance. (C) While important for tolerance, the primary role of AIRE is not the generation of Tregs but the expression of self-antigens for negative selection. (D) Affinity maturation is a B-cell specific process.
A scientist is investigating the molecular basis of immune receptor diversity. They focus on an enzyme that adds non-template encoded nucleotides to the ends of DNA segments during the rearrangement of T-cell receptor genes. This process significantly increases the variability of the antigen-binding site.
This addition of N-nucleotides at gene segment junctions is catalyzed by which of the following enzymes?
Explanation: Terminal deoxynucleotidyl transferase (TdT) is a specialized DNA polymerase that is expressed in developing lymphocytes. During V(D)J recombination, after the RAG enzymes have cleaved the DNA, TdT randomly adds non-template encoded nucleotides (N-nucleotides) to the free 3' ends of the DNA at the junctions between the V, D, and J segments. This process, known as junctional diversity, creates immense variability in the third hypervariable region (CDR3) of T-cell and B-cell receptors, which is the primary site of antigen contact. (A) RAG recombinase is responsible for recognizing recombination signal sequences and making the initial DNA breaks. (B) DNA ligase IV is part of the non-homologous end joining pathway that repairs the DNA breaks. (D) AID is involved in somatic hypermutation and class switching in mature B cells, not initial receptor generation.
A geneticist creates a knockout mouse model that lacks the transcription factor Foxp3. The mice are born healthy but soon develop severe, lethal autoimmune conditions affecting multiple organs, including lymphoproliferation and inflammatory infiltrates.
The T-cell subset that is deficient in these mice normally functions to do which of the following?
Explanation: Foxp3 is the master transcription factor for the development and function of regulatory T cells (Tregs). Tregs are a specialized subset of CD4+ T cells whose primary role is to maintain immunologic self-tolerance and suppress excessive immune responses. They exert their function by producing inhibitory cytokines like IL-10 and TGF-β, and through cell-contact dependent mechanisms. The absence of functional Tregs, as in Foxp3-deficient mice or humans with IPEX syndrome, leads to uncontrolled activation of self-reactive T cells and severe systemic autoimmune disease. (A) Activating macrophages is a function of Th1 cells. (B) Providing help to B cells is a function of follicular helper T cells (Tfh) and Th2 cells. (D) Recruiting neutrophils is a function of Th17 cells.
A patient with a rare immunodeficiency is found to have a defect in the enzyme activation-induced deaminase (AID). Following vaccination, the patient is able to produce an initial IgM response, but the subsequent antibody response is weak, and the antibodies produced have low binding affinity for the antigen.
The defective enzyme in this patient is directly required for which of the following B-cell processes?
Explanation: Activation-induced deaminase (AID) is a crucial enzyme expressed in activated B cells within germinal centers. It is required for two key processes that refine the antibody response: somatic hypermutation and class switch recombination. Somatic hypermutation introduces point mutations into the variable regions of immunoglobulin genes, leading to the selection of B cells producing higher-affinity antibodies (affinity maturation). A defect in AID prevents this process, resulting in the production of only low-affinity antibodies. AID is also required for class switching from IgM to IgG, IgA, or IgE. (A) V(D)J recombination generates initial B-cell receptor diversity and is mediated by RAG1/RAG2 enzymes. (C) Expression of membrane IgM is determined by alternative mRNA splicing. (D) Negative selection eliminates self-reactive B cells in the bone marrow.
A 58-year-old man with a history of alcoholism presents with an exquisitely painful, swollen, and red great toe. Joint aspiration reveals needle-shaped, negatively birefringent crystals. These monosodium urate crystals are known to be phagocytosed by macrophages, triggering an intense inflammatory response through the activation of a multiprotein cytosolic complex.
Activation of this intracellular complex directly leads to the enzymatic cleavage and release of which potent pro-inflammatory cytokine?
Explanation: The intracellular multiprotein complex described is the inflammasome (specifically, the NLRP3 inflammasome), which is a key component of the innate immune system. It is activated by various stimuli, including crystalline substances like monosodium urate. Upon activation, the inflammasome activates caspase-1. Activated caspase-1 then cleaves pro-IL-1β into its active form, IL-1β, which is a potent pro-inflammatory cytokine that recruits neutrophils and mediates the acute inflammation characteristic of a gout attack. (A) IL-2 is a key cytokine for T-cell proliferation. (B) TNF-α is produced via other pathways, such as TLR signaling. (C) IFN-γ is the signature cytokine of Th1 cells and NK cells.
A 25-year-old woman receives a vaccine containing a protein antigen. Following injection, specialized antigen-presenting cells in the skin capture the antigen, process it, and migrate to a nearby lymph node. In the lymph node, these cells are essential for initiating the primary adaptive immune response.
Which of the following best describes the primary role of these migratory cells once they reach the lymph node?
Explanation: The cells described are dendritic cells (specifically Langerhans cells in the skin). They are the most potent antigen-presenting cells (APCs) for activating naive T cells. After capturing an antigen in the periphery, they mature and migrate to the T-cell zones of draining lymph nodes. Here, they present processed peptide antigens on MHC class II molecules to naive CD4+ T cells and on MHC class I molecules to naive CD8+ T cells, providing the first signal for T-cell activation and initiating the adaptive immune response. (A) Secreting antibodies is the function of plasma cells. (C) B cells undergo proliferation to form germinal centers. (D) Killing infected cells is a function of cytotoxic T lymphocytes and NK cells.
A 22-year-old male is diagnosed with Crohn's disease, an inflammatory bowel disease. Genetic testing reveals a loss-of-function mutation in the NOD2 gene, a known susceptibility factor for the disease. The protein product of this gene is an important intracellular sensor of bacterial products.
The NOD2 protein normally functions to recognize which of the following microbial components, leading to activation of NF-κB?
Explanation: NOD2 (Nucleotide-binding oligomerization domain-containing protein 2) is an intracellular pattern recognition receptor. It specifically recognizes muramyl dipeptide, a component of peptidoglycan found in the cell walls of most bacteria. Upon binding its ligand, NOD2 activates the NF-κB pathway, leading to the production of inflammatory cytokines and defensins, which are important for maintaining the gut mucosal barrier. Defects in NOD2 are associated with an abnormal inflammatory response to gut commensal bacteria, contributing to the pathogenesis of Crohn's disease. (A) Lipopolysaccharide is recognized by the extracellular receptor TLR4. (B) Flagellin is recognized by TLR5. (C) Unmethylated CpG DNA, characteristic of bacteria, is recognized by the endosomal receptor TLR9.
A 30-year-old man from Southeast Asia is diagnosed with leprosy. A skin biopsy shows numerous acid-fast bacilli within macrophages and few lymphocytes, consistent with lepromatous leprosy. This clinical form is associated with a weak cell-mediated immune response and a dominant antibody response.
The differentiation of naive CD4+ T cells into the helper T-cell subset that predominates in this patient's condition is primarily driven by which cytokine?
Explanation: Lepromatous leprosy is characterized by a weak cell-mediated (Th1) response and a strong humoral (Th2) response, which is ineffective at controlling the intracellular Mycobacterium leprae. The differentiation of naive CD4+ T cells into the Th2 subset is primarily driven by the cytokine Interleukin-4 (IL-4). Th2 cells produce IL-4, IL-5, and IL-13, which promote B-cell proliferation, class switching to IgE, and activation of eosinophils, but they fail to activate macrophages to kill the intracellular bacilli. (A) IFN-γ is the signature cytokine of Th1 cells. (B) IL-12, produced by macrophages and dendritic cells, is the primary cytokine that drives differentiation into Th1 cells. (D) TGF-β is involved in the differentiation of regulatory T cells (Tregs) and Th17 cells.