Final answer:
The immediate result of ligand binding to receptor protein-tyrosine kinases is receptor dimerization, leading to autophosphorylation and activation of downstream cellular responses.
Step-by-step explanation:
The immediate result once more receptor protein-tyrosine kinases bind to their ligand is receptor dimerization. When the ligand binds to its receptor, conformational changes occur that propagate through the membrane region of the receptor leading to the activation of the intracellular domain or its associated proteins. Specifically, in the case of receptor tyrosine kinases, ligand binding results in dimerization, where two receptors bind together forming a stable complex known as a dimer. This dimerization activates the intracellular domain, autophosphorylation of tyrosine residues occurs, and a phosphorylation cascade is initiated, which is a series of enzyme phosphorylations resulting in a downstream cellular response. Internalization and trimerization are different processes that are not directly related to the immediate result of protein-tyrosine kinase receptor-ligand binding.