All these belong to the second line of defense except

A. the macrophage system.

B. natural killer cells.

C. inflammation.

D. the gastric juices.

Option D, Gastric Juices

Explanation:

The second line of defense is a non specific defense mechanism in which no specific individual is targeted but all invaders are killed in general way. Some common mode of second line of defense are - Phagocytic cells (white blood cells), inflammatory responses, macrophage system etc. The second line of defense comes into action when the invader has surpassed the first line of defense which include either physical barrier like skin, mucous membrane, hair, cilia etc. or chemical barriers like gastric juices, saliva, Hyaluronic acid etc.

Hence, gastric juice does not belong to second line of defense.

Thus, option D is correct.