Diagram how an antigen can only bind to a specific receptor, due to the specific shape of the receptor protein. The binding is the next step in the process which started due to a stimulus which caused the message to be produced. Illustrate the "lock and key" utilizing snap blocks. Using a pathogen antigen as an example, list the steps which occur in the process to neutralize and kell the pathogen. Define and differentiate between specificity and affinity.