In order to detect a signal (that is, to be a target cell), a neighbor cell must have the right receptor for that signal. When a signaling molecule binds to its receptor, it alters the shape or activity of the receptor, triggering a change inside of the cell. Signaling molecules are often called ligands, a general term for molecules that bind specifically to other molecules (such as receptors).
A chunk paragraph is just a way of developing your skill in writing - it is a style or format which forces you to explain your ideas and arguments. Here's you do chunk writing: Write your topic sentence.
Comments. Comment lines are allowed in the message text file. A semicolon (;) begins a comment that ends at the end of the line. Follow the semicolon with a C/C++ single-line comment indicator (//) so that the header file generated by the message compiler will compile in your application.