USMLE Step 1 Quiz: Autoimmunity And Self Tolerance

What this quiz covers

This quiz focuses on Autoimmunity And Self Tolerance, giving you a quick way to practice the rules, question types, and explanations that matter most for USMLE Step 1.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

Question 1

1. Expression of tissue-specific antigens in the thymus (correct answer)
2. Development and function of regulatory T cells
3. Fas-mediated apoptosis of peripheral lymphocytes
4. Somatic hypermutation in germinal centers

Explanation: This patient's presentation of chronic mucocutaneous candidiasis, hypoparathyroidism, and adrenal insufficiency is characteristic of Autoimmune Polyendocrinopathy-Candidiasis-Ectodermal Dystrophy (APECED). This syndrome is caused by mutations in the Autoimmune Regulator (AIRE) gene. The AIRE protein is a transcription factor critical for central T-cell tolerance, as it promotes the expression of a wide range of tissue-specific antigens in medullary thymic epithelial cells. This allows for the negative selection (deletion) of developing T cells that are reactive to these self-antigens. Defective AIRE function leads to the escape of autoreactive T cells into the periphery, causing widespread autoimmune disease.

Question 2

1. It will become functionally unresponsive. (correct answer)
2. It will differentiate into a regulatory T cell.
3. It will undergo rapid proliferation.
4. It will initiate apoptosis in the epithelial cell.

Explanation: Full T-cell activation requires two signals. Signal 1 is the binding of the T-cell receptor (TCR) to the antigen-MHC complex. Signal 2 is the co-stimulatory signal provided by the binding of CD28 on the T cell to B7 (CD80/86) on an antigen-presenting cell (APC). When a T cell receives Signal 1 in the absence of Signal 2, as described in this scenario, it does not become activated. Instead, it enters a state of anergy, becoming functionally unresponsive to subsequent antigenic stimulation. This is a critical mechanism of peripheral tolerance.

Question 3

1. Presenting exogenous antigens to CD4+ T cells
2. Presenting endogenous antigens to CD8+ T cells (correct answer)
3. Binding to the inhibitory receptors on NK cells
4. Functioning as a receptor for complement fragments

Explanation: The patient's presentation is consistent with ankylosing spondylitis, a seronegative spondyloarthropathy strongly associated with the HLA-B27 allele. HLA-B alleles encode MHC class I molecules. The primary function of MHC class I molecules is to bind and present endogenous peptide antigens (e.g., viral proteins, tumor antigens, or normal self-proteins) to the T-cell receptors on CD8+ cytotoxic T lymphocytes. While the exact mechanism linking HLA-B27 to disease is debated, it involves the presentation of self-peptides or arthritogenic microbial peptides to CD8+ T cells.

Question 4

1. T-cell sensitization to previously sequestered antigens (correct answer)
2. Deposition of circulating immune complexes in the uvea
3. Cross-reactivity between a viral peptide and an ocular protein
4. Bystander activation from a subclinical systemic infection

Explanation: This clinical scenario describes sympathetic ophthalmia, a rare bilateral uveitis that occurs after trauma to one eye. The eye is an immunologically privileged site, meaning its antigens are normally sequestered from the systemic immune system. The trauma to the right eye releases these previously hidden intraocular antigens (e.g., retinal S-antigen, melanin-associated proteins) into the circulation. These antigens are processed and presented to T cells, leading to sensitization and the generation of an autoimmune response. The activated T cells can then cross the blood-ocular barrier and attack antigens in the uninjured eye, causing inflammation.

Question 5

1. Molecular mimicry
2. Epitope spreading
3. Bystander activation (correct answer)
4. Release of sequestered antigens

Explanation: Bystander activation is a mechanism of autoimmunity where a strong inflammatory response to a pathogen leads to the activation of autoreactive lymphocytes that are not specific for the pathogen itself. The intense local inflammation and cytokine storm (e.g., IL-1, IFN-γ) cause tissue APCs to upregulate co-stimulatory molecules (B7). This environment can provide the necessary co-stimulation to activate nearby autoreactive T cells that recognize self-antigens released from damaged tissue, thereby breaking peripheral tolerance.

Question 6

1. Clonal deletion
2. Anergy induction
3. Receptor editing (correct answer)
4. Isotype switching

Explanation: When an immature B cell in the bone marrow produces a self-reactive BCR, one of the primary mechanisms to establish central tolerance is receptor editing. This process involves the re-expression of the RAG (recombination-activating genes) enzymes, which allows the B cell to rearrange its immunoglobulin light chain genes. This creates a new light chain, which pairs with the existing heavy chain to form a new BCR with a different, hopefully non-self-reactive, specificity. If receptor editing is successful, the cell can survive and mature. If it fails to produce a non-self-reactive receptor, the cell is eliminated via apoptosis (clonal deletion).

Question 7

1. Formation of the membrane attack complex
2. Clearance of immune complexes and apoptotic debris (correct answer)
3. Activation of B lymphocytes
4. Chemotaxis of neutrophils

Explanation: Deficiencies in the early components of the classical complement pathway (C1q, C1r, C1s, C4, C2) are strongly associated with an increased risk of developing SLE. A key function of this pathway is to opsonize apoptotic bodies and immune complexes by coating them with complement fragments (e.g., C3b). This tagging facilitates their recognition and clearance by phagocytes. When this process is defective, apoptotic debris containing nuclear autoantigens (e.g., DNA, histones) persists in the circulation, providing a continuous source of antigen that can stimulate and sustain an autoimmune response.

Question 8

1. Apoptosis; Anergy
2. Anergy; Apoptosis (correct answer)
3. Activation; Differentiation into a memory cell
4. Differentiation into a regulatory T cell; Anergy

Explanation: These two experiments illustrate distinct mechanisms of peripheral tolerance. In the first scenario, T-cell receptor engagement without co-stimulation (Signal 1 without Signal 2) results in T-cell anergy, a state of functional unresponsiveness. In the second scenario, repeated stimulation of T cells leads to the upregulation of both the death receptor Fas (CD95) and its ligand, FasL. The interaction of Fas and FasL on the same or adjacent T cells triggers an apoptotic cascade, a process known as activation-induced cell death (AICD). AICD is crucial for deleting chronically activated T cells, including potentially self-reactive ones.

Question 9

1. Autoantibodies that block the insulin receptor
2. Deposition of immune complexes in the islet microvasculature
3. Cytotoxic T lymphocytes that recognize β-cell antigens (correct answer)
4. Mast cell degranulation triggered by IgE autoantibodies

Explanation: Type 1 diabetes mellitus is a classic example of a cell-mediated autoimmune disease, primarily driven by T lymphocytes. The pathogenic process involves the infiltration of pancreatic islets by immune cells (insulitis). CD4+ helper T cells become activated by β-cell antigens (such as GAD65, proinsulin) presented by APCs. These helper T cells then provide help to autoreactive B cells and, most importantly, activate β-cell-specific CD8+ cytotoxic T lymphocytes (CTLs). These CTLs recognize β-cell peptides presented on MHC class I molecules and directly kill the pancreatic β-cells, leading to insulin deficiency.

Question 10

1. It will undergo receptor editing.
2. It will become anergic and have a shortened lifespan. (correct answer)
3. It will undergo apoptosis immediately upon antigen binding.
4. It will differentiate into a short-lived plasma cell.

Explanation: This scenario describes a mechanism of peripheral B-cell tolerance. When a mature B cell recognizes a self-antigen in the periphery without receiving the necessary second signal from a corresponding T helper cell (because autoreactive T cells are typically deleted centrally), it does not become activated. Instead, it enters a state of anergy. Anergic B cells have downregulated surface IgM, are impaired in their signaling capacity, and are excluded from entering lymphoid follicles. This exclusion prevents them from receiving survival signals, leading to a significantly shortened lifespan and eventual elimination.

Question 11

1. High local concentrations of systemic lymph
2. Low expression of MHC class I molecules
3. Expression of apoptosis-inducing ligands by parenchymal cells (correct answer)
4. Absence of antigen-presenting cells like microglia

Explanation: Immune privilege is maintained by a combination of factors. One of the most important is the active induction of apoptosis in infiltrating immune cells. Parenchymal cells in privileged sites (e.g., neurons, retinal cells, Sertoli cells) constitutively express Fas ligand (FasL). When activated T lymphocytes, which express the Fas receptor, enter these tissues, they are killed by Fas-FasL-mediated apoptosis. This actively eliminates potentially damaging inflammatory cells. Other contributing factors include physical barriers (e.g., blood-brain barrier), an anti-inflammatory microenvironment (e.g., high levels of TGF-β), and reduced lymphatic drainage.

Question 12

1. Promote negative selection in the thymus
2. Provide co-stimulation for T-cell activation
3. Attenuate T-cell responses after initial activation (correct answer)
4. Mediate activation-induced cell death

Explanation: CTLA-4 is an inhibitory receptor expressed on activated T cells and regulatory T cells. It binds to B7 molecules (CD80/86) on antigen-presenting cells with much higher affinity than the co-stimulatory receptor CD28. This binding provides an inhibitory signal that attenuates or terminates the T-cell response, acting as a crucial 'brake' on the immune system to maintain peripheral tolerance. By blocking CTLA-4, checkpoint inhibitors like ipilimumab remove this brake, leading to enhanced T-cell activation against both tumor cells (therapeutic effect) and self-antigens (autoimmune adverse effect).

Question 13

1. Bystander activation of self-reactive T cells
2. Release of sequestered cardiac antigens
3. Molecular mimicry between bacterial and host proteins (correct answer)
4. Defective clearance of apoptotic cells

Explanation: The patient's presentation is classic for acute rheumatic fever, a complication of a preceding Group A Streptococcus pharyngitis. The underlying mechanism is molecular mimicry. Antibodies produced against streptococcal M protein cross-react with structurally similar self-antigens in the heart, such as myosin and valvular proteins. This cross-reactivity leads to inflammation of the endocardium, myocardium, and pericardium, resulting in carditis and potential long-term valvular damage.

Question 14

1. T-cell mediated destruction of myelin sheaths
2. Antibody-mediated modulation of postsynaptic receptors (correct answer)
3. Immune complex deposition in the neuromuscular junction
4. Granulomatous inflammation of skeletal muscle

Explanation: This patient's symptoms are characteristic of myasthenia gravis, an autoimmune disorder of the neuromuscular junction. The disease is caused by autoantibodies directed against the nicotinic acetylcholine (ACh) receptors on the postsynaptic membrane of the motor endplate. These antibodies interfere with neuromuscular transmission by blocking ACh binding, activating complement-mediated damage to the endplate, and, most significantly, causing cross-linking, internalization, and degradation of the ACh receptors. This reduction in available receptors leads to muscle weakness that worsens with repetitive use.

Question 15

1. Somatic hypermutation
2. Epitope spreading (correct answer)
3. Clonal anergy
4. Molecular mimicry

Explanation: Epitope spreading refers to the diversification of the immune response to target additional epitopes as an autoimmune disease progresses. The initial tissue damage caused by the autoimmune response to one self-antigen (e.g., Sm/RNP) can expose other previously cryptic or sequestered self-antigens (e.g., dsDNA, phospholipids). The immune system then mounts a response to these newly exposed antigens, leading to a broader and often more severe autoimmune disease. This process contributes to the relapsing-remitting nature and progression of many autoimmune disorders like SLE and multiple sclerosis.

Question 16

1. Androgens increase the activity of regulatory T cells.
2. Estrogen promotes a Th1-mediated cellular immune response.
3. Estrogen enhances B-cell activation and antibody production. (correct answer)
4. Progesterone suppresses antigen presentation by dendritic cells.

Explanation: The female predominance in many autoimmune diseases is partly attributed to the immunomodulatory effects of sex hormones. Estrogen, in particular, is generally considered to be immune-enhancing. It can promote B-cell activation, enhance immunoglobulin production, and polarize the T-helper cell response towards a Th2 phenotype, which supports humoral (antibody-mediated) immunity. This can increase susceptibility to antibody-mediated autoimmune diseases like SLE. Conversely, androgens are generally considered to be immunosuppressive.

Question 17

1. Cytotoxic T lymphocytes
2. Regulatory T cells (correct answer)
3. B lymphocytes
4. Neutrophils

Explanation: The clinical triad of severe enteropathy (diarrhea), endocrinopathy (type 1 diabetes), and dermatitis (eczema) in an infant boy is characteristic of IPEX syndrome (Immune dysregulation, Polyendocrinopathy, Enteropathy, X-linked). IPEX is caused by mutations in the FOXP3 gene, a master transcription factor essential for the development and suppressive function of regulatory T cells (Tregs). Without functional Tregs, peripheral tolerance is compromised, leading to unchecked activation of self-reactive lymphocytes and severe multisystem autoimmune disease.

Question 18

1. Age-related decline in immune tolerance
2. Exposure to an environmental trigger
3. Drug-induced modification of self-antigens (correct answer)
4. A primary immunodeficiency disorder

Explanation: This patient is presenting with classic signs of drug-induced lupus erythematosus (DILE). Procainamide, along with hydralazine, is a well-known cause. The presence of anti-histone antibodies is highly characteristic of DILE, whereas anti-dsDNA antibodies are more specific for idiopathic SLE. The mechanism by which these drugs induce autoimmunity involves chemical modification of self-antigens (particularly histones), making them appear foreign to the immune system and breaking tolerance. This differs from idiopathic SLE and explains the distinct antibody profile.

Question 19

1. Activation-induced cell death (correct answer)
2. Negative selection in the thymus
3. Receptor editing in the bone marrow
4. Co-stimulatory signal blockade

Explanation: This patient has Autoimmune Lymphoproliferative Syndrome (ALPS), a disorder caused by defective apoptosis of lymphocytes. The most common cause is a mutation in the FAS gene (CD95). The Fas receptor and its ligand (FasL) are critical for inducing activation-induced cell death, a process that eliminates chronically activated lymphocytes to maintain homeostasis and peripheral tolerance. Failure of this pathway leads to the accumulation of lymphocytes, particularly double-negative T cells, resulting in lymphadenopathy, splenomegaly, and autoimmune cytopenias.

Question 20

1. Stimulation of a cell surface receptor (correct answer)
2. Blockade of a neurotransmitter receptor
3. Opsonization of cells for phagocytosis
4. Fixation of complement on the basement membrane

Explanation: This patient has Graves' disease, the most common cause of hyperthyroidism. The pathophysiology involves a type II hypersensitivity reaction mediated by autoantibodies (thyroid-stimulating immunoglobulins, TSI) that bind to the TSH receptor on thyroid follicular cells. Unlike many autoantibodies that block or destroy their targets, these antibodies act as agonists, stimulating the receptor in the same way as TSH. This leads to unregulated production and release of thyroid hormones (T3 and T4), causing the clinical signs of hyperthyroidism.