Fab Fragment?

Correct Answer: C

Rationale: The Fab fragment, produced by papain digestion, retains the antigen-binding site of an antibody, consisting of one light chain and part of a heavy chain.

Correct Answer: D

Rationale: Anaphylaxis is triggered by allergen-induced cross-linking of IgE on mast cells, leading to degranulation and histamine release.

Correct Answer: A

Rationale: B cells and dendritic cells are professional antigen-presenting cells (APCs) that express MHC class II and present antigens to helper T cells.

Correct Answer: D

Rationale: Properdin deficiency, an X-linked condition affecting males, impairs the alternative complement pathway, increasing susceptibility to fatal meningococcal infections. Terminal complement deficiencies (C5-9) also cause neisserial infections but are not sex-linked.

Correct Answer: C

Rationale: Reticular dysgenesis is a severe condition with both lymphopenia and neutropenia due to failure of lymphoid and myeloid precursors.