The public comment period on FDA's revised generic-peptide guidances, covered on this site for what it added to higher-order structure testing, closes today. Alongside higher-order structure, the same guidance package elevated a second attribute to its own checked category: innate immune response testing, more commonly called immunogenicity testing. It sits next to impurity thresholds in FDA's list of updated technical areas, and that placement is the point of this article. The question immunogenicity testing asks is not how pure a peptide is or whether its main peak is the right molecule. It asks whether something in the sample, often a minor impurity rather than the peptide itself, is capable of provoking an immune response that the parent compound would not.
What FDA actually added
On July 28, 2026, FDA published 17 revised draft product-specific guidances covering generic versions of peptide products including semaglutide, liraglutide, tirzepatide, teriparatide, calcitonin salmon, pegcetacoplan, dasiglucagon, and vosoritide. The agency's own summary lists updated recommendations across five technical areas: submission pathways for recombinantly, synthetically, or semi-synthetically produced peptides, innate immune response testing, impurity thresholds, higher order structure assessment, and biological activity assessment. The same FDA statement notes the agency also withdrew its May 2021 guidance for ANDAs covering highly purified synthetic peptide drug products, stating it no longer reflects the agency's current scientific thinking.
Immunogenicity testing itself is not a new FDA concern. A February 2019 guidance, Immunogenicity Testing of Therapeutic Protein Products, Developing and Validating Assays for Anti-Drug Antibody Detection, already set out how sponsors should build and validate assays that detect anti-drug antibodies, the antibodies a patient's own immune system produces against a therapeutic protein during clinical use. What the July 2026 revision changes is where in the peptide-characterization process that concern gets applied: not only to the finished drug in a patient, but to the impurity profile of the generic drug substance itself, before it ever reaches a clinical trial.
Why an impurity, not the parent peptide, is usually the actual question
A purity result and an immunogenicity risk are answering two different questions about the same sample. Purity compares the main peak against everything else the detector saw and expresses that comparison as a single percentage, a measurement this site has covered in detail. It says nothing about what the remaining, smaller fraction of the signal actually is at the level of sequence.
That distinction is exactly where immunogenicity risk lives. A 2025 review in Pharmaceutical Research (De Groot et al., PMID 40126816, DOI 10.1007/s11095-025-03843-1) states the mechanism directly: "introduction of Human Leukocyte Antigen (HLA)-binding sequences in impurities that were not present in the [reference listed drug] could drive new immune responses." A peptide-related impurity, a truncated chain, a deamidated residue, an oxidized residue, or a diastereomer produced during synthesis, can differ from the parent peptide by only one or two residues while still creating or exposing a sequence the immune system's antigen-presenting machinery recognizes as new. The impurity does not need to dominate the sample to matter for this question; it needs to be present and to carry the wrong sequence at the wrong spot.
What the evidence on a specific peptide shows
The clearest published case study is teriparatide. A December 2025 paper in Frontiers in Immunology (Mattei et al., PMID 41445733, DOI 10.3389/fimmu.2025.1730346) applied an orthogonal immunogenicity risk assessment, in silico epitope prediction (EpiMatrix, ClustiMer, and JanusMatrix) paired with class II HLA-DR binding assays and donor T-cell assays, to teriparatide and nine product-related impurities identified in generic teriparatide products. The authors found that impurities disrupting a cross-conserved, naturally tolerogenic sequence near teriparatide's N-terminus showed increased immunogenic potential compared with the reference drug. Eight of the nine tested impurities carried higher predicted or measured immunogenic risk than the reference product, with donor T-cell response rates reaching 48 percent across the impurities tested versus 19 percent for the approved formulation. None of that difference would show up in an HPLC purity trace or an intact-mass identity check; both of those methods would report the impurity as a small peak or a minor mass signal, with no way to flag that its sequence, not its quantity, was the property that mattered.
How immunogenicity risk is actually assessed
The methods behind these findings are deliberately layered rather than a single test. In silico tools such as EpiMatrix and JanusMatrix scan a sequence in short, overlapping frames and predict which fragments are likely to bind human leukocyte antigen (HLA) class II molecules, the step required before a T-cell response can occur, while also checking how closely a given fragment resembles sequences already common in the human proteome, since a highly conserved fragment is less likely to provoke a response than a genuinely novel one. Predictions from that screening step are then tested directly: class II HLA binding assays measure how strongly a candidate sequence actually binds representative HLA alleles, and T-cell assays expose the peptide or impurity to immune cells from a panel of donors and measure the resulting cytokine or proliferative response. This computational-plus-cellular approach is distinct from the anti-drug-antibody assays covered in FDA's 2019 immunogenicity-testing guidance, which monitor a patient's immune response to a drug already in clinical use; the impurity-focused work described here is a characterization step aimed at flagging risk before a product reaches that stage.
None of these methods appears on a routine peptide Certificate of Analysis, research-grade or pharmaceutical. They require specialized reagents, donor cell panels, and licensed prediction software, and they are built for drug-development characterization packages rather than per-lot release testing.
What this means for reading a COA
A COA that reports only purity and identity has not addressed immunogenicity risk, and that is true regardless of how the report is formatted, how detailed it looks, or whether every party in the supply chain is named on it. The gap is a property of which analytical question purity and identity testing were designed to answer, not a property of any particular laboratory's transparency or a supplier's willingness to identify itself. A redacted manufacturer name changes nothing about whether an immunogenicity assessment was run, because that assessment is a separate, specialized undertaking that neither identity confirmation nor an area-percent purity figure was ever built to perform. What FDA's July 2026 guidance package does is name that gap explicitly, alongside higher-order structure, as a distinct attribute regulators now expect a generic peptide manufacturer to address rather than assume away from a clean purity trace.
Further reading
What a peptide purity percentage does and does not measure is covered in peptide purity testing: what the percentage means. The companion attribute FDA added in the same July 2026 guidance package, higher-order structure, is covered in higher-order structure testing: the gap beyond sequence and mass. How a peptide's sequence is confirmed at the fragment level, the check that would first identify what a given impurity's sequence actually is before any immunogenicity assessment could proceed, is covered in peptide mapping under USP <1055> and ICH Q6B.
For the primary sources: FDA, Revised Draft Product-Specific Guidances for Certain Generic Peptide Products (July 28, 2026); FDA, Immunogenicity Testing of Therapeutic Protein Products, Developing and Validating Assays for Anti-Drug Antibody Detection (February 2019); De Groot et al., "Immunogenicity of Generic Peptide Impurities: Current Orthogonal Approaches," Pharmaceutical Research (2025), PMID 40126816, DOI 10.1007/s11095-025-03843-1; and Mattei et al., "Immunogenicity risk assessment of peptide-related impurities identified in generic teriparatide products," Frontiers in Immunology (2025), PMID 41445733, DOI 10.3389/fimmu.2025.1730346.