Describe the first interaction that triggers signaling to begin in a target cell

Ligand-receptor interaction is the first interaction that triggers cell signalling.

Explanation:

Ligands are the hormones, drugs or neurotransmitters that specifically bind with receptors present in cell surface to initiate cell signalling. Ligand binding to the receptor is reversible. Binding of the ligand to the receptor can activate or inactivate receptor. Activation may lead to cell signalling pathways. Inside cells, there are secondary molecules which on activation starts a cascade of reactions. which in-turn regulate the function of the ligand molecule.

Best Choice:

Ligand-receptor interaction is the first interaction that triggers cell signalling.

Explanation : At first, the activated receptor relates with G proteins molecules in the cell.

before the ultimate physiological effect of the ligand on the cell's behavior is produced Therefore, the cellular signaling starts as soon as the first messenger (the ligand) binds to its receptor.