enzyme immunoassay
DESCRIPTION
Enzyme Immunoassay. Enzyme Immunoassay. The EIA is a type of nonisotopic immunoassay in which enzymes, coenzymes, fluorigenic substrates, or enzyme inhibitors are used as labels - PowerPoint PPT PresentationTRANSCRIPT
Enzyme Immunoassay
Enzyme ImmunoassayThe EIA is a type of nonisotopic immunoassay in
which enzymes, coenzymes, fluorigenic substrates, or enzyme inhibitors are used as labels
The major prerequisite is that the antigen or antibody must be linked to an enzyme without destroying the immunologic or enzymatic activity of the antigen-antibody complex
• valuable tools for use in clinical labs• can measure biological materials (antibodies or antigens)• inexpensive, rapid, quantitative, specific• sensitive (pg/ml)• expensive equipment not required (but helps)• can be automated
Major labeled immunoassays
RIAEIAChemlumIFA
Key terms
AnalyteImmunoassayLableLigandReactantseperation stepSolid phasesubstrate
BASIC FORMAT
Solid phase = 96 / 384-well microplate
Enzyme labelsHorseradish peroxidase (Horseradish )alkaline phosphatase (calf intestine)Glucose oxidase (Aspergillus niger)B-galactosidase (Ecoli):
- Are not naturally in the patient's sample
- High specific activity- Stability
Enzymes used in immunoassay systemsmust be stableavailable in a highly purified statehave a high turnover rateundergo minimal interference by substances
likely to be in the test solutionand be specific for the substrate
Types of protocols in labeled immunoassay
Competitive bindingNon-competitive bindingSandwich techniqueOne-step assayHomogeneous reactionHeterogeneous reaction
Analyte = antibody Analyte = antigen
Incubate, wash
1. Coat solid phase withantigen when analysing antibody
antibody when analysing antigen
2. Block free binding sites. Incubate. Wash.
Analyte = antibody Analyte = antigen
3. Add sample. Incubate. Wash
Analyte = antibody Analyte = antigen
Enzyme labelled (rabbit) anti-(human) Ig
(Second antibody)anti-(rabbit) Ig-enzyme
EE
Horse radish peroxidase (HRP)Alkaline phosphatase
See Sigma catalogue for list of conjugates and substrates
Orthophenylene diamine Tetramethyl hydrochloride (OPD) benzidine (TMP)
Horse radish peroxidase (HRP) Orange, 490 nm Yellow, 450 nm Spectrophotometer
The most widely used enzyme in EIA is horseradish peroxidase (HRP).
The substrate of HRP is hydrogen peroxide (H2O2) and the product is oxygen
This oxygen produced during the reaction is used to oxidize a reduced, colorless chromagen (usually reduced orthophenylenediamine)
The final product, oxidized orthophenylenediame, has a brown color
Paranitrophenyl phosphate (PNP) Methyl umbelliferol phosphate
Alkaline phosphatase
Yellow, 405 nm Methyl umbelliferone Spectrophotometer
365 nm 445 nm Fluorimeter
5. Add substrate
6. Incubate, stop, measure colour change
Colourless
ENZYME
OD
CONCENTRATION
E
Directly conjugated developingantibody may give weak signal
unlabelled (rabbit) anti-(human) Ig followed by
anti-(rabbit) Ig-enzyme
EE
a anti-rabbit labeled antibody
Biotin-labelled anti-Ig followed by
streptavidin-enzyme
E-S B S-E
ES
streptavidin-enzymesteptavidin-nzyme
streptavidin-enzyme
streptavidin-enzyme
Streptavidin-Enzyme
INDIRECT ELISA TO DETECT SPECIFIC ANTIBODIES
E E E
3. Anti-(human) Ig-enzyme
2. Sample (human) antibody
1.Antigen
Toxoplasma IgG
INDIRECT ELISA TO DETECT SPECIFIC ANTIBODIES
E E E
4. Substrate
3. Anti-(human) Ig-enzyme
2. Sample (human) antibody
1. Antigen
ANTIGEN-CAPTURE ELISA TO DETECT SPECIFIC ANTIBODIES
2. Impure antigen eg tissue homogenate
1. Specific antibody
ANTIGEN-CAPTURE ELISA TO DETECT SPECIFIC ANTIBODIES
3. Wash pure antigen
2. Impure antigen
1. Specific antibody
ANTIGEN-CAPTURE ELISA TO DETECT SPECIFIC ANTIBODIES
E E5. Anti-human Ig-enzyme
4. Sample (human antibody)
3. Wash pure antigen
2. Impure antigen
1. Specific antibody
ANTIGEN-CAPTURE ELISA TO DETECT SPECIFIC ANTIBODIES
6. Substrate5. Anti-human Ig-enzyme
4. Sample (human antibody)
3. Wash pure antigen
2. Impure antigen
1. Specific antibody
ANTIBODY SANDWICH ELISA TO DETECT ANTIGENS
eg. hormones drugs tumour antigens cytokines
1. Anti-analyte
ANTIBODY SANDWICH ELISA TO DETECT ANTIGENS
1. Anti-analyte
2. Sample
3. anti-analyte- biotin followed by streptavidin-enzyme
E-S B S-E
ES
E S E-S B S-E
ANTIBODY SANDWICH ELISA TO DETECT ANTIGENS
1. Anti-analyte
2. Sample
4. Substrate
3. anti-analyte-biotin followed by streptavidin-enzyme
1
2
2
HEALTH
CANCER VIRUSES
MYCOBACTERIAHELMINTHS
ASTHMA, ALLERGYLUPUS
RHEUMATOID ARTHRITISMULTIPLE SCLEROSIS UVEITISDIABETES
IL2IL12IFNTNF
IL4 IL5 IL6 IL10
CMI(AB)
Type 1 Type 2
AB(CMI)
CYTOKINES
MICROARRAYEg. Novagen ProteoPlex
Std 1
Std 3 Std 4
Std 2
Std 6
#2
#4
#6
#8
#10
Std 5
#1
#3
#5
#7
#9
IL1
IL2
IL5
IL7
IL10
IL16
gmCSF
TNF
IL1
IL4
IL6
IL8
IL12
IL18
IFN
TGF
Quadruplicate capture anti-cytokine antibody spotsOverlay with standards, samples. Incubate, washAdd fluorophore-detection antibody. Incubate, washScan
Erroneous Results Caused by antibodies in immunoassay
The application of Abs as an analytical reagent
MoAb novel detection signals
In spite of technologic evolution during past 30 years, still :
False False
Positive Negative
Heterophil antibodiesEndogenous antibodies produced against poorly defined antigens
Can react with several animal species (mouse is most common)
Usually react with Fc
IgG or IgM but can be IgA, IgE
Frequency : up to 40% (0.05% may present clinical
significance)
Produced by enviromental contact, transplacental passage or viral infection
Human anti-animal antibodies (HAAA)Response to parenteral administration of animal MoAb
Following radioimaging, cancer therapy, transplant immunotherapy
High concentration, high avidity, epitope specific
The steps to minimize interferences 1. The use of chimeric Abs (animal Fab + human Fc)
2. Using other methods based upon different MoAb
3. Removal of unusual Ab by PEG
4. Making dilutions usually produce non linear
results
5. Commercial mixture of animal proteins for pre-analysis incubation
Antibody cross reactivity Although MoAb enhances specificity , still remains a source of error
Drug assay vs active, non active metabolites
Cross-reactants 1- positive interferent bias 2- Unexpected negative interferent
Hook effectUnexpected fall in the amount of analyte resulting in the gross under-estimation of the analyte
Particularly in sandwich immunoassays when sample contain extremely high level
Upon further dilution, the result will be out of range
Most commonly occurred in measurement of IgE, hCG, Ferittin, tumor marker, infectious Ag-Ab
Choices of antibodiesPolyclonal
IgG fraction or F (ab') 2 fragments
Affinity purified polyclonal
Monoclonal
How to deal with Hook effect1. Run all samples in duplicate dilution
2. Ensure for adequate washings
3. Know the level of Hook phenomenon according to manufacturer suggestion
4. Good communication with clinicians
Rheumatoid Factor, RFAn IgM that act to IgG Fc70% RA patientsMay bind to rabbit, sheep, goat and mouse IgSpecial technical problem for IgM quantitationFalse positivity is common, but competitive inhibitor may cause false negative
How to evade RF interference1. Using isolated IgM preparation
2. Addition of precipitating anti-IgG
3. Removing the RF using an aggregated IgG4. Using IgM capture assay problem can still occur: all IgM-specific antibody should be
evaluated cautiously.