Immunoglobulin structure, isotype classes, class switching, and affinity maturation concepts tested on USMLE Step 1.
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- What is the basic chain structure of an immunoglobulin monomer?
- Two identical heavy chains and two identical light chains linked by disulfide bonds
- What are the two types of light chains found in immunoglobulins?
- Kappa and lambda
- What region of the antibody determines its antigen specificity?
- The variable (V) region, located at the tips of the Fab arms
- What region of the antibody mediates effector functions like complement fixation and Fc receptor binding?
- The constant (Fc) region of the heavy chain
- What is the Fab fragment of an antibody?
- The antigen-binding fragment, containing the variable regions of one heavy and one light chain
- What structure connects the two Fab arms to the Fc region and allows flexibility?
- The hinge region
- Which immunoglobulin isotype is the first antibody produced in a primary immune response?
- IgM
- What is the typical structural form of secreted IgM?
- A pentamer, joined by a J chain
- Why is pentameric IgM particularly effective at complement fixation despite relatively low individual binding affinity?
- Its multivalent structure provides high avidity, and multiple Fc regions in close proximity efficiently activate the classical complement pathway
- Which immunoglobulin isotype is the most abundant in serum and provides long-term humoral immunity?
- IgG
- Which immunoglobulin isotype is the only one that crosses the placenta to provide passive immunity to the fetus?
- IgG
- Which immunoglobulin isotype is the predominant antibody in mucosal secretions such as saliva, tears, and breast milk?
- IgA
- What structural feature allows secretory IgA to resist degradation in mucosal secretions?
- The secretory component, added during transport across epithelial cells
- What is the typical structural form of secretory IgA?
- A dimer, joined by a J chain
- Which immunoglobulin isotype is bound to mast cells and basophils and mediates immediate (type I) hypersensitivity?
- IgE
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