For medical purposes,<unk> is a method to reduce or eliminate an organism's negative reaction to a substance or stimulus.
For example, if a person with diabetes mellitus has a bad allergic reaction to taking a full dose of beef insulin, the doctor gives the person a very small amount of the insulin at first. Over a period of time, larger doses are given until the person is taking the full dose. This is one way to help the body get used to the full dose and to avoid having the allergic reaction to beef-origin insulin. (See Hyposensitization.)
At the cellular level, administration of small doses of toxin produces an IgG response which eventually overrides the hypersensitive IgE response.
In pharmacology,<unk> is the loss of responsiveness to the continuing or increasing dose of a drug. Also termed tachyphylaxis, down-regulation, fade or drug tolerance. This may be an important area to consider for the future design of safer drugs.
Desensitization 
Emotional contagion
Perspiration
Exaptation
Chemotaxis
Induction 
Blocking 
Autoregulation