Cholinesterase is an enzyme that breaks down acetylcholine, a chemical neurotransmitter that stimulates muscles, nerves, and glands: 

    Function
    Cholinesterase breaks down acetylcholine so that it doesn't over-stimulate the body's systems. This allows cholinergic neurons to return to a resting state after activation. 

Types
There are two main types of cholinesterase: acetylcholinesterase (AChE) and pseudocholinesterase. AChE is the primary cholinesterase in the nervous system. 
Blood test
A serum cholinesterase blood test measures the levels of these two substances in the blood. A healthcare provider may order this test if they suspect someone has been exposed to organophosphates, chemicals found in pesticides. Symptoms of overexposure include slow heart rate, blurred vision, and difficulty breathing. 
Cholinesterase inhibitors
These chemicals bind to cholinesterase and prevent it from breaking down acetylcholine. They are used in pesticides to neutralize the nervous systems of insects. They are also prescribed to treat mild to moderate Alzheimer's disease.