In the recognition step of the acquired immune response, the antigen is called the __________ molecule.

• A. Causative
• B. Signal
• C. Reaction
• D. Inducer

Answer: (B)

In the recognition step, lymphocytes identify foreign material through their receptors, and the antigen acts as the signal that prompts the immune cell to respond. When a receptor binds its specific antigen, it sends intracellular signals that initiate activation, proliferation, and the subsequent immune actions. This signaling role is why the antigen is described as a signal in this context—the binding event conveys the essential cue to start the response.

The other terms don’t fit as well. Causative would imply it’s simply the cause of disease, not the immediate cue for activation; Reaction describes what the immune system does after activation, not what triggers it; Inducer is not the standard term used for the molecule recognized by immune receptors in this step.