AP-Micro Immunology and Host Defense

Question 1

Which complement pathway is activated by mannose-binding lectin (MBL) recognizing carbohydrate patterns on microbial surfaces?

Accepted Answer

Lectin pathway

Answer

The lectin pathway is activated when MBL binds mannose residues on microbial surfaces, triggering MASP proteases.

Question 2

Which leukocyte is the predominant phagocyte recruited first during an acute bacterial infection?

Accepted Answer

Neutrophil

Answer

Neutrophils are the most abundant granulocytes and the first phagocytes to arrive at sites of acute bacterial infection.

Question 3

Which toll-like receptor (TLR) recognizes lipopolysaccharide (LPS) from gram-negative bacteria?

Accepted Answer

TLR-4

Answer

TLR-4, in complex with MD-2, recognizes LPS (endotoxin) and triggers innate immune signaling via MyD88.

Question 4

Natural killer (NK) cells preferentially kill target cells that:

Accepted Answer

Lack or downregulate MHC class I

Answer

NK cells use 'missing self' recognition — they kill cells with absent or reduced MHC class I, which signals viral infection or malignancy.

Question 5

Which cytokine is the primary endogenous pyrogen responsible for inducing fever via the hypothalamus?

Accepted Answer

IL-1β

Answer

IL-1β (along with IL-6 and TNF-α) acts on the hypothalamus to raise the thermal set point, producing fever.

AP Practice Test