(1) Sensitization/Up Regulation: is a process whereby a cell becomes more responsive to a given concentration of a compound. This can occur in the following ways:
Increase in functionality: deals with increased efficiency of the receptors. For example, a receptor might increase functionality upon phosphorylation.

Increase in the # of receptors: This can occur by 1) increasing the expression of receptors, 2) clustering the receptors where they need to be, 3) recruitment of receptors from the cytoplasm to the PM, 4) decreased turnover of receptors or 5) induction which is de novo synthesis of novel receptors via transcriptional activation. An example of de novo induction are B bradykinin receptors which are GPCRs that are detectable only in tissues that have been exposed to inflammatory stimuli or in tissues from individuals with inflammatory diseases like arthritis. One can in fact test for whether someone has some of these diseases by testing for the presence of these receptors.

(2) Desensitization/Down Regulation: is a process whereby a cell becomes less responsive to a given concentration of a compound.
Desensitization typically involves the following:
phosphorylation of receptors
internalization of plasma membrane associated receptors
decreased mRNA and protein synthesis

If the desensitization is receptor specific in that a decrease in responsiveness involves receptors for only one type of agonist, then it is referred to as homologous desensitization.
If the decrease represents a generalized loss that involves receptors for more than one type of agonist, it is called heterologous desensitization.