Figure 1: Interaction between antigen and antibody
(www.scizmic.net/downloads/vaccinations/3_vaccinedef.pdf)
Enzyme-linked immunosorbent assay is a type of assay that depends on the reaction
between antigen and antibody in vitro. It is possible to detect and quantify antigens or antibodies
with the aid of this method. This test was firstly used for HIV screening test. Today, this test is
used to[2]
for measuring antibody level for allergy and vaccines cases
for detecting viruses such as hepatitis and HIV
for detecting hormonal changes such as in pregnancy,
for detecting circulatory inflammatory markers such as cytokines.
Generally ELISA is consisted of five main steps. In the first step, plate is coated with
antibody. In the second step, blocking solution is used for blocking all unbound sites. With the
aid of using blocking solution, we decrease the possibility of seeing false positive results. After
blocking all unbound sites with blocking solution, antigen is added to wells. Then anti-mouse
antibody(IgG) which is conjugated with an enzyme is added to wells. At the end of ELISA,
with the activity of enzyme, it is possible to see a colored product[3].
There are three types of ELISA which are Competitive ELISA, Sandwich ELISA and
Indirect ELISA. In this laboratory section, competitive ELISA was performed. In competitive
assay, specific antibody is used to coat the plate. Then test antigen and labelled antigen are
added to wells and these compete for binding to antibodies. If the amount of the test antigen is
greater than labelled antigen, less amount of labelled antigen can bind to antibodies. If our
sample is consisted of more antigens then, less labeled antigen is remained in the well.
Therefore weak signal can be detected. The following figure shows us the competitive ELISA.
The main steps are the same for all competitive ELISA assays, only the names of sample,
primary and secondary antibodies are different[4].
Figure 2: Example of Competitive ELISA
(
www.scizmic.net/downloads/vaccinations/4_vaccinedef.pdf
)
Competitive ELISA has several advantages. It does not require sample processing. It
also less sensitive to sample dilutions and sample matrix effects. Therefore with the use of this
technique, it is possible to get lower deviation or variability between repeated samples and
repeated assays. Briefly, it can be said that it has high precision, accuracy and reproducibility.
The disadvantages of this method according to sandwich assay are
lower sensitivity
lower specificity[5].
The main steps of our competitive ELISA: In this experiment, BIOXYTECH 8-OHdG-
EIA Kit was used. This is used for quantitative measurment of 8-hydroxy-2'-deoxyguanosine
(8-OHdG) in tissue. In addition 8-hydroxy-2'-deoxyguanosine is one of the important
biomarker for oxidative stress and carcinogenesis. In this kind of ELISA, 8-0HdG is used to
coat plate wells. And these ones and the other 8-0HdG in our sample compete for primary
antibody binding sites. As the concentration of 8-0HdG in sample increases, there will be
reduction in the binding of antibodies. After binding antibodies that bound to 8-OhdG in the
sample are washed out of the well and only the ones that are bound to the plate will remain in
the plate after this washing step. After that enzyme-labelled secondary antibody is added to well
and in these well, it will bind to the primary antibody. To remove unbounded ones, washing
step is performed and unbounded secondary antibodies are eliminated from the
sample.Chromogen is added in to wells and with the aid of enzyme that is conjugated to
secondary antibodies give reaction and turns the color of the solution into yellow. After that to
stop the reaction stop solution is added and the absorbance values are read with the aid of the
spectrophotometer. Here is the flow chart of this procedure[6]:
Figure 3: The main steps of our competitive ELISA
(www.scizmic.net/downloads/vaccinations/5_vaccinedef.pdf)
There are also differences in antibodies: Polyclonal and monoclonal. Polyclonal antibodies
are the ones that:
Their production is not so expensive.
Not so much technology or technique requiring assay,
Its time scale is short.
During production of polyclonal antibodies, large amounts of nonspecific antibodies are
produced.
These polyclonal antibodies are able to recognizes multiple epitopes that provides robust
detection.
And with the aid of this property, it can give better results in IP/ChIP.
There can be seen variability due to batches.
They belongs to IgG subclass.
Monoclonal antibodies are the ones that
Expensive to produce
It requires more technology and technique.
Its time scale is longer if we consider hybridomas.
It can produce large amounts of specific antibodies but when we compare them polyclonal ones,
these are so specific. And this kind of specificity decreases the possibility of cross-reaction with
other proteins.
It can recognize only one epitope of antigen[7].
3-MATERIALS AND METHODS
MATERIALS
1-Spectrophotometer
2-Plate Shaker
3-Spectrophotometer
4-Water bath
5-pNPL
6-Reagent A
7-Lipase
8-OHdG Microtiter Plate-Precoated With 8-OHdG (8×12 wells, Split Type)
9-Primary Antibody Monoclonal Antibody Specific For 8-OHdG
10-Primary Antibody Dilution Buffer Phosphate Buffered Saline
11-Secondary Antibody HRP-Conjugated Antibody
12-Secondary Antibody Dilution Buffer Phosphate Buffered Saline
13-Chromogen 3,3',5,5'-Tetramethylbenzidine
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