Definition: Enzyme-linked Immunosorbent Assay (ELISA) is an immunological assay used to detect and quantify proteins, antibodies, or antigens in a sample.
Method: Utilizes an indirect detection method for antibody or antigen detection.
Advantages of ELISA
Sensitive: Capable of detecting minute quantities of targets, specific to the target of interest.
Quantitative: Allows for the measurement of test results, providing numerical data.
Ease of Performance: Straightforward protocol and procedures.
Cost-effective: Relatively inexpensive compared to other methods.
High Throughput: Can test many samples simultaneously.
Preparation Flexibility: Plates can be prepared in advance, facilitating quick testing when necessary.
Types of ELISA
Direct ELISA
Procedure:
Antibody Placement: The antibody is placed in the well first.
Sample Addition: Sample is added next, detecting the antigen present in the sample.
Enzyme-linked Anti-antibody: The anti-antigen with an enzyme linked to it is then added.
Indirect ELISA
Procedure:
Antigen Placement: The antigen is placed in the well first.
Sample Addition: Sample is added, detecting the antibody present in the sample.
Enzyme-linked Anti-human Antibody: The anti-human antibody with the enzyme linked to it is added afterward.
Indirect ELISA Details
Purpose
Detection of Antibodies Against HIV: This method is commonly used to check for the presence of antibodies against the HIV virus in patients.
Key Components
Chromogen: A substrate that changes color when cleaved by the enzyme; indicates the reaction of the enzyme.
Patient Scenarios
Patient A: Ex-boyfriend diagnosed with HIV – test for antibodies against HIV.
Patient B: Current boyfriend; not sexually active; engaged in deep kissing – assess potential exposure to HIV.
Patient C: EMT who treated an HIV-positive victim and later found his glove was torn – risk assessment.
Patient D: Recently broke up questioning partner's monogamy; potential exposure to HIV.
Patient E: Six-month-old infant born to an HIV-positive mother – tested positive at birth, undergoing re-testing.
Patient F: Has a history of intravenous drug use; may have shared needles – evaluate for antibodies against HIV.
Procedure Steps
Coating Wells: Each well in the plate is coated with a known antigen.
Washing: Excess antigen is washed off; blocking protein is added to cover any exposed wells, preventing non-specific binding.
Addition of Samples: Controls and patient samples are introduced into designated wells; samples are always processed in duplicate or triplicate.
Binding Process: If the sample contains antibodies against the antigen, it will bind; any unbound antibodies are washed away.
Application of Anti-human Antibody: Anti-human antibody enzyme conjugate is added; unbound antibody is removed through washes.
Substrate Addition: Enzyme substrate is introduced to each well, and a color change indicates the enzyme has cleaved the substrate, confirming the presence of antibodies.
Critical Questions Regarding Indirect ELISA
How does direct ELISA work?
Direct ELISA uses antibodies to detect antigens directly in the sample.
How does indirect ELISA work?
Indirect ELISA involves detecting antibodies in the sample using antigens.
What type of ELISA was performed in lab?
Indirect ELISA was performed.
What was being detected?
Detection of antibodies against HIV in patient samples.
What is a chromogen?
A chromogen is a substrate that changes color upon reaction with an enzyme, indicating the presence of antibodies.
Function of the enzyme-linked antibody:
The enzyme-linked antibody serves to amplify the signal for detection in the assay by generating a measurable signal (color change).
Why did the infant test positive immediately after birth but negative at six months?
Infants can acquire maternal antibodies through placental transfer, which may show up as positive initially, but these antibodies can diminish over time, resulting in negative tests as the infant develops independent immune responses without maternal antibodies.