The question of what occurs after an endogenous messenger chemical
is bonded to a receptor according to specificity and affinity was presented. We determined that a response is brought about by target cells. A partial list of possible responses was outlined on the board. Using lego type blocks, the specificity of fit between messenger and receptor was demonstrated. How Exogenous chemicals perform was demonstrated using the block to show that the structure of the exogenous chemical (drug) was geometrically like the endogenous produced chemical. The concept of an agonist and an antagonist response was demonstrated and discussed. The thyroid hormone, introduced as an exongenous drug, was used as an example of an agonist response.