In this task, you are given a multiple-choice question about healthcare. Answer the question based on your information and classify your answers into '1', '2', '3', and '4'.
Q: Question: In mammals, Toll-like receptors (TLRs) are receptors:
Options: <1> Signaling. <2> Antigenic <3> Of histocompatibility alleles. <4> Of phagocytosis. <5> Of chemokines.
A:
1