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Immunology USMLE Step 1 Questions and Answers | Latest Version | 2025/2026 | Correct & Verified
Which cells are primarily responsible for producing antibodies during a primary immune response?✔✔ Plasma cells
Which immunoglobulin is most effective at activating the classical complement pathway?✔✔ IgM
Which immunoglobulin is found in secretions such as saliva, tears, and breast milk?✔✔ IgA
Which T cell surface protein interacts with MHC class II on antigen-presenting cells?
✔✔ CD4
Which T cell surface protein interacts with MHC class I on infected cells?
✔✔ CD8 1 / 4
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Which cell type expresses both CD4 and CD8 during development in the thymus?✔✔ Double-positive thymocytes
Which cytokine is essential for differentiation of Th1 cells?
✔✔ IL-12
Which cytokine promotes class switching to IgE in B cells?
✔✔ IL-4
Which type of hypersensitivity reaction involves IgE and mast cell degranulation?✔✔ Type I hypersensitivity
Which immunoglobulin mediates type I hypersensitivity reactions?✔✔ IgE
Which hypersensitivity reaction is responsible for Goodpasture syndrome?✔✔ Type II hypersensitivity 2 / 4
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Which hypersensitivity reaction is seen in systemic lupus erythematosus?✔✔ Type III hypersensitivity
Which hypersensitivity type is mediated by T cells rather than antibodies?✔✔ Type IV hypersensitivity
Which cytokine is most associated with fever and the acute-phase response?
✔✔ IL-1
Which complement component is responsible for opsonization?✔✔ C3b
Which complement component is responsible for chemotaxis of neutrophils?✔✔ C5a
Which organ is the site of T cell maturation?✔✔ Thymus 3 / 4
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Which organ is the major site of B cell activation in response to blood-borne antigens?✔✔ Spleen
Which region of the antibody determines its isotype (IgG, IgA, etc.)?✔✔ Constant region of the heavy chain
Which region of the antibody determines antigen specificity?✔✔ Variable region of the Fab fragment
Which major histocompatibility complex presents endogenous antigens to CD8+ T cells?✔✔ MHC class I
Which major histocompatibility complex presents exogenous antigens to CD4+ T cells?✔✔ MHC class II
Which molecule is required for T cell activation and functions as a co-stimulatory signal?✔✔ CD28
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