ELISA? Description please....

Dr E. Buxbaum EB15 at le.ac.uk
Tue Jul 16 04:36:54 EST 1996

ELISA is a method for the determination of the concentration of an antigen or antibody in a 
sample. The HIV-antibody test is a well known example. Proteins from the virus are adsorbed (or 
chemically bound in some cases) to the walls of a reaction tube (usually 96 such tubes are 
fused together to form a plate, that simplifies handling). Then a serum sample is added into 
the tube and incubated for some time. If that sample contains antibodies against HIV proteins, 
they will bind. The serum is then discarded and the tube washed a couple of times, to remove 
all antibodies that did not bind to antigen. Then, the tube is filled with a solution of 
antibodies directed to the constant part of human antibodies (raised in sheep, horse, donkey or 
similar animals). This second antibody is chemically linked to an enzyme like alkaline 
phosphatase or horseraddish peroxidase. If any human antibodies are present, the second 
antibody will bind to it. After washing away unbound second antibody the amount of bound enzyme 
is determined by a colour reaction. So you get a kind of sandwich: the enzyme is chemically 
bound to the second antibody, which is immuno adsorbed to the human antibody, which is 
immunoabsorbed to the virus protein on the walls of the tube. There are modifications to this 
principle, but that is the basic form.

The advantages of the ELISA are as follows:
- no radioactivity is needed (unlike the radioimmunoassay or RIA)
- it is very sensitive (down to concentrations of about 10^-15 M)
- it is reasonably specific (allthough if the result is important, than a positive outcome  has 
  to be verified by other methods)
- it is fairly cheap, because a large number of samples can be analysed in parallel

Any textbook on Immunology will give you further details, if you require them. 

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