Selective IgA deficiency is defined as serum IgA levels less than 10 mg/dL with normal levels of other immunoglobulins. The diagnosis cannot be confirmed until the patient is at least

Correct Answer: B

Rationale: IgA levels may not fully mature until after infancy, so diagnosis is typically confirmed after 4 years of age when levels should stabilize.

Correct Answer: A

Rationale: Conventional dendritic cells are the most efficient antigen-presenting cells for activating naïve CD8 T cells via cross-presentation on MHC class I.

Correct Answer: C

Rationale: Naïve mature T helper cells (CD4+) reside in secondary lymphoid organs before antigen exposure, distinguishing them from activated or cytotoxic cells.

Correct Answer: D

Rationale: CD46 regulates complement, not LPS recognition, which involves CD14, MD2, and MYD88 in the TLR4 pathway.

Correct Answer: A

Rationale: Neutrophils do not express MHC class II, unlike professional APCs (macrophages, B cells, dendritic cells) or activated endothelial cells.