A specific immune response triggered by a pathogen involves several steps.Which of these steps occurs first?

A) R proteins trigger specific immune responses.
B) PR proteins signal cells that have not yet been attacked.
C) Effector binds to cytoplasmic receptor (R protein)inside the cell.
D) Signaling molecules in the cell trigger production of defensive molecules.
E) Substances are produced to strengthen the cell wall and block plasmodesmata.