Técnicas de biología molecular que usan anticuerpos

Main Ideas

The lesson surveys laboratory techniques that use antibodies to detect proteins, cells, or signs of an immune response. The instructor emphasizes choosing techniques in a sensible order and interpreting results with appropriate controls.

Western Blot

Western blot is a widely used method for detecting a specific protein and comparing its abundance between samples.

Basic workflow:

1. Collect cells and lyse them to release their proteins. Add protease inhibitors to reduce protein degradation.
2. Mix the lysate with sample buffer and heat it to denature the proteins. Reducing conditions also break disulfide bonds.
3. Separate the proteins by SDS-PAGE. SDS gives proteins a broadly similar negative charge, so they separate mainly by size: smaller proteins migrate farther through the polyacrylamide gel.
   • Load a molecular-weight marker in a lane for comparison.
   • Polyacrylamide gels are used for proteins, while agarose gels are used for nucleic acids.
4. Transfer the separated proteins from the gel onto a nitrocellulose membrane.
5. Block the membrane, for example with skim milk, to limit nonspecific antibody binding.
6. Add a primary antibody that recognizes the protein of interest. Wash, then add a labeled secondary antibody.
7. Detect the resulting signal, such as by chemiluminescence. A band indicates that the target protein was detected.

Normalize to a loading control: A stronger target-protein band does not automatically mean that the protein is more abundant per cell. Differences in cell number or the amount of material loaded can also change the signal. Measure both the target band and a consistently expressed loading-control protein, such as actin or tubulin, and compare the target signal relative to that control. Without this normalization, conclusions about changes between treatments or time points may be unreliable.

Western blots can be used to compare treatments or time points, but a loading control is still needed. Results can also be technically variable, so researchers may present Western blots alongside another method, such as immunocytochemistry.

Immunoprecipitation and Pull-Down

This method uses an antibody attached to a bead or similar support to capture a chosen protein from a mixture.

Basic workflow:

1. Combine a cell extract containing many proteins with a support-bound antibody specific to the target.
2. Allow the antibody to capture the target and associated molecules.
3. Centrifuge and remove the unbound material in the supernatant.
4. Release the captured material from the support.
5. Analyze it, for example by SDS-PAGE and Western blot, using antibodies to check for the target and other proteins of interest.

If a second protein is detected in the captured material, this supports the conclusion that the proteins are present in the same complex. It does not, by itself, prove that they bind directly to one another; another protein could connect them. FRET and double-hybrid approaches are examples of techniques used to test more direct interactions.

Choosing Methods in a Logical Sequence

Begin with simpler, less demanding tests and move to stronger or more specialized evidence as needed:

• Use immunocytochemistry to see whether two proteins appear in the same cellular location. Colocalization suggests proximity but does not establish a physical interaction.
• If the result supports further investigation, use pull-down or immunoprecipitation to test whether the proteins occur in the same complex.
• Use a more direct interaction assay, such as FRET or a double-hybrid method, if the question is whether the proteins physically interact.

The goal is to match the method to the question and build evidence progressively, rather than beginning with a technically difficult assay.

ELISA

The instructor describes two general uses of ELISA, illustrated with a COVID example:

• Detecting antibodies in a patient’s serum: Viral antigens are attached to a plate. If the patient has antibodies that recognize them, those antibodies bind. A labeled secondary antibody produces a detectable signal. A positive result indicates antibodies against the viral antigen.
• Detecting viral antigen directly: A capture antibody is attached to the plate. If the virus or its antigen is in the sample, it binds. A second labeled antibody recognizes another part of it and generates a signal.

A negative antibody test does not necessarily mean that someone was never infected. The person may have been tested before antibodies developed or may have a low antibody level. The instructor gives roughly 10–15 days as an example of how long antibodies can take to appear. PCR is mentioned as a separate method for detecting viral genetic material. The instructor also refers to newer, more expensive ELISA formats that can detect antibodies and antigens in the same test.

Other Techniques Mentioned

Immunocytochemistry, flow cytometry, and immunohistochemistry are named as antibody-based techniques covered elsewhere in the course. The instructor distinguishes immunohistochemistry on tissue from immunocytochemistry on cultured cells.

Speakers and Sources

• Instructor: Unnamed lecturer explaining the methods and examples.
• Students: Unnamed students who respond to questions and ask about the newer ELISA format.
• No other speakers or external sources are identified in the subtitles.