Heterophilic antibodies are antibodies that can non-specifically bind to immunoglobulins from different species, often leading to false-positive or false-negative results in assays like ELISA, RIA, and other immunoassays.

Using normal serum as a heterophilic blocking agent is a common practice in immunoassays to reduce the interference caused by heterophilic antibodies.

Here’s how normal serum works as a heterophilic blocking agent:

Mechanism of Action:

  1. Source of Non-Specific Immunoglobulins: Normal serum contains a broad range of immunoglobulins (IgG, IgM, etc.) that are not specific to the antigens used in the assay. When added to the assay, these immunoglobulins can saturate or block heterophilic antibodies present in the sample.
  2. Blocking Non-Specific Binding: Heterophilic antibodies in a sample might bind non-specifically to assay antibodies (both capture and detection antibodies) or other assay components. By adding normal serum, the non-specific immunoglobulins in the serum compete with the assay antibodies for binding to the heterophilic antibodies. This reduces the likelihood that heterophilic antibodies will bind to the assay antibodies and cause interference.
  3. Reduction of Cross-Reactivity: In the presence of normal serum, the heterophilic antibodies are more likely to bind to the irrelevant immunoglobulins from the serum rather than to the assay-specific antibodies. This decreases the cross-reactivity and potential for erroneous signal generation in the assay.

How It’s Used:

  • Inclusion in Assay Buffer: Normal serum can also be included directly in the assay buffer or blocking buffer, allowing it to continuously neutralize heterophilic antibodies during the assay process.
  • Pre-Incubation: The serum sample might be pre-incubated with normal serum before being introduced into the assay.
Rabbit in Science Lab for AntibodiesRabbit in Science Lab for Antibodies

Limitations:

  • Species Specificity: The effectiveness of normal serum as a blocking agent can depend on the species from which it is derived and how closely it matches the species used in the assay antibodies. For example, human serum may not be effective in assays using murine antibodies.
  • Incomplete Blocking: While normal serum can significantly reduce interference, it might not eliminate all heterophilic antibody effects, particularly in cases where heterophilic antibodies have very high affinity.

In summary, normal serum acts as a heterophilic blocking agent by providing a pool of non-specific immunoglobulins that can bind to and neutralize heterophilic antibodies, thus reducing their potential to interfere with immunoassay results.

Choosing The Serum Species for the Blocking Buffer

Depending on the assay, when choosing which normal serum to use, it is recommended to use serum that is derived from the same host species as the secondary antibody or the serum should come from the same species as the samples being tested to ensure compatibility

Secondary Antibody = Rabbit anti-Mouse IgG-HRP

Blocking Serum = Normal Rabbit Serum

Or

Sample = Dog Serum

Blocking Serum = Normal Dog Serum

*CAUTION: Do not use serum from the same species as the Primary Antibody. This will compete for binding sites with the secondary antibody and reduce the signal.

Serum Blocking buffers are generally comprised of normal serum combined with a nonionic detergent such as Tween-20.

ELISA Kit for Antibody DevelopmentELISA Kit for Antibody Development

Preparation Protocol 5% (v/v) Serum Blocking Buffer with Tween-20

1. Prepare 1L of PBS Buffer: Dissolve the appropriate salts in high quality water and adjust the pH as necessary to approximately 7.2. Add 150mM NaCl to maintain isotonicity.

  • 8.77 g NaCl (Sodium Chloride)
  • 0.2 g KCl (Potassium Chloride)
  • 1.44 g Na₂HPO₄ (Disodium Phosphate)
  • 0.24 g KH₂PO₄ (Monopotassium Phosphate)

2. Add Normal Serum: Mix in the normal serum to the desired final concentration (typically 5% v/v, but can be adjusted according to the assay needs).

  • 50 ml of Normal Serum

3. Optional Tween-20: If desired, add Tween-20 to the solution at a concentration of 0.05% to help reduce hydrophobic interactions that might contribute to non-specific binding.

  • 0.5ml of Tween-20

4. Mix Thoroughly: Ensure that the serum is evenly distributed throughout the buffer.

5. Filter (Optional): For some sensitive applications, you might want to filter the blocking buffer using a 0.22 µm filter to remove any particulates or microbial contaminants.

This use of this blocking buffer is effective in reducing background and minimizes non-specific binding, which will improve the accuracy and reliability of immunoassay results.

 

Get in touch with our team to learn more about ICL's normal serums and how they can be used as heteophilic blocking agents for your next project.