Transfer Factor Side Effects: What Studies Report

This page is for educational purposes only and is not medical advice; consult a licensed physician about any health decision, product or symptom. It summarises what published papers state about the term "transfer factor" and about safety, and it says plainly where human safety data are absent rather than filling the gap with inference.

Three unrelated things are called "transfer factor"

Anyone searching the indexed literature for "transfer factor" encounters at least three distinct usages that share a name and share nothing else. Conflating them is the single largest source of confusion in this topic, because papers that look relevant by title are often measuring caesium in bananas rather than anything a person might consume as a supplement.

Only the first row describes something ingested deliberately. The remainder of this page treats each meaning separately, because "side effects of transfer factor" is only a coherent question for the first.

Transfer factor supplements and safety: what studies report

The most directly relevant paper in the verified set is a 2021 Biomolecules report whose stated purpose was the characterisation and safety profile of transfer factor peptides marketed as a nutritional supplement for immune system regulation (PMID 33947143). The study was framed around describing what the preparation contains and evaluating its safety profile, rather than around testing a therapeutic claim in a patient population (PMID 33947143). Readers who want the specific assays, matrices and endpoints used would need the full text; the title and abstract scope establish that safety assessment was an explicit objective of the work, which is more than can be said for most consumer immune preparations.

A second paper, published in 2017 in the Revista do Colégio Brasileiro de Cirurgiões, examined the use of transfer factor in immunosuppressed surgical patients (PMID 29019573). That work is relevant because immunosuppressed surgical patients are precisely the population in which an immune-modulating preparation raises the most questions, and because it situates transfer factor within a clinical rather than a consumer context (PMID 29019573).

What was not found in the verified literature

Absence of evidence deserves to be stated directly rather than softened. Within the verified papers reviewed for this page:

• No randomised controlled trial reported a tabulated list of adverse events with frequencies for a transfer factor preparation in humans.
• No paper reported long-term safety follow-up over months or years.
• No paper reported drug–supplement interaction testing.
• No paper reported safety data in pregnancy, lactation, or paediatric populations.
• No paper established a maximum tolerated intake, so no intake figure appears anywhere on this page.

That list is a description of the evidence base, not a statement that a product is safe or unsafe. A preparation can have few published adverse-event reports because it has been studied little, because reports were never collected systematically, or because tolerability genuinely was unremarkable — and published abstracts alone rarely distinguish between those possibilities.

Laboratory work is not safety data

A 2019 PeerJ study reported the cytotoxic activity of IMMUNEPOTENT CRP, a bovine dialyzable leukocyte extract, against non-small cell lung cancer cell lines (PMID 31579619). Cell-line cytotoxicity work describes what happens to cultured cells in a dish; researchers in that study worked with lung cancer cell lines rather than with people (PMID 31579619). Two limits follow from that design. First, cytotoxicity in vitro says nothing about what an intact organism absorbs, distributes, metabolises or eliminates. Second — and this is often missed in popular summaries — the fact that a preparation kills cells in culture is not automatically reassuring about selectivity; it is a finding that generates further questions rather than answers about human tolerability.

When a consumer page cites in-vitro cytotoxicity as evidence of benefit while simultaneously citing the same class of preparation as "gentle" or "natural", it is using one study for two incompatible purposes. The published record supports neither framing on its own.

"Transfer factor" in respiratory medicine

In pulmonary function testing, transfer factor of the lung for carbon monoxide (TLCO, called DLCO in much of the United States literature) is a measurement of gas exchange, not a substance. It appears constantly in interstitial lung disease research. A 2021 Cochrane systematic review assessed pulmonary rehabilitation for people with interstitial lung disease (PMID 34559419). In that literature, transfer factor values are a baseline characteristic or outcome measure describing lung physiology; a reduced transfer factor is a test result reported by researchers, not an effect of taking anything (PMID 34559419).

This matters for anyone reading search results: a paper titled around "transfer factor" in a respiratory journal almost certainly concerns gas exchange measurement and has no bearing whatsoever on supplement tolerability.

"Transfer factor" in environmental and food science

The third usage is a ratio. Environmental scientists define a transfer factor as the concentration of an element in a plant, crop or animal tissue divided by its concentration in soil or feed. It is dimensionless, it is used for exposure modelling, and it dominates the indexed literature by volume.

• Researchers estimated soil-to-plant transfer factors across six local government areas of Kwara State, Nigeria (PMID 39362113).
• A 2025 study reported soil-to-banana transfer factors for radionuclides in Lampung, Indonesia (PMID 40048929).
• A 2019 study assessed radionuclide transfer factors and transfer coefficients near phosphate industrial areas of South Tunisia (PMID 31372949).
• A 2024 paper reported dose and transfer factor of potassium-40 in medicinal plants (PMID 39016500).
• Transfer of plutonium-239+240, americium-241, caesium-137 and strontium-90 to the tissues of horses was reported in a 2020 study (PMID 32565415).

Parallel work applies the same ratio logic to non-radioactive contaminants. Researchers measured lead and mercury in collected vegetables and herbs from Markazi province, Iran, and carried out a non-carcinogenic risk assessment (PMID 29407473). Another study reported heavy metal contamination of staple crops in Sibaté, Colombia (PMID 32642577). A 2024 Journal of Hazardous Materials paper examined the mixed effects and co-transfer of cerium oxide nanoparticles and arsenic through a pakchoi–snail food chain (PMID 37852136).

Why this body of work is worth knowing about anyway

These papers are not about supplements, but they describe a mechanism relevant to any plant-derived consumable: contaminants present in soil can appear in harvested material, and the degree to which that happens is exactly what a transfer factor quantifies. The Iranian herb and vegetable survey applied a non-carcinogenic risk framework to measured lead and mercury concentrations (PMID 29407473), and the medicinal plant study addressed potassium-40 content and the associated dose estimate (PMID 39016500). Botanical products therefore carry an ingredient-independent quality dimension — what was in the soil — that is separate from the pharmacology of the intended constituent. Transfer factor supplements derived from colostrum or leukocyte extracts are animal-sourced rather than plant-sourced, so this literature does not apply to them directly; it is included here because the shared terminology otherwise misleads readers into treating soil-ratio papers as safety evidence.

Reading safety claims about transfer factor preparations

Several practical distinctions help when evaluating claims found online:

1. Check which "transfer factor" a citation refers to. A radioecology transfer factor paper cited as evidence of immune benefit is a category error, and it happens frequently.
2. Distinguish characterisation from clinical trial. The 2021 supplement paper reported characterisation and a safety profile (PMID 33947143); that is a different evidence class from a randomised trial with prespecified safety endpoints.
3. Distinguish in vitro from in vivo. Cell-line results with a dialyzable leukocyte extract (PMID 31579619) do not translate directly into human effects.
4. Note the population. Work in immunosuppressed surgical patients addresses a specific clinical setting under supervision (PMID 29019573), which differs from unsupervised consumer use.
5. Treat silence as silence. No published adverse-event tabulation appeared in the verified set; that is an evidence gap, not a clean bill of health.

Regulatory context

Products sold as dietary supplements in the United States are not evaluated or approved by the Food and Drug Administration for the treatment, prevention or cure of disease, and manufacturers are responsible for the safety of what they market. Immune-related marketing claims are a recurring area of regulatory attention. Nothing in the verified literature reviewed here establishes an approved medical use for a transfer factor preparation, and this page does not describe any preparation as suitable or unsuitable for any individual.

Summary of the evidence position

The published record reviewed here contains one paper explicitly devoted to characterisation and safety profile of a transfer factor peptide supplement (PMID 33947143), one addressing use in immunosuppressed surgical patients (PMID 29019573), one in-vitro cytotoxicity study of a dialyzable leukocyte extract (PMID 31579619), and a much larger set of environmental papers that use the same phrase for an entirely different concept. Anyone weighing safety questions about a specific product should discuss it with a licensed clinician who can review the actual formulation, the person's medical history and any concurrent medications. A separate PeptideU course covers the biology, history and terminology of transfer factor preparations in more depth; this page is limited to what the literature states about safety and adverse events.

References

• Characterization and Safety Profile of Transfer Factors Peptides, a Nutritional Supplement for Immune System Regulation (Biomolecules, 2021)
• Use of transfer factor in immunosuppressed surgical patients (Revista do Colegio Brasileiro de Cirurgioes, 2017)
• Cytotoxic activity of IMMUNEPOTENT CRP against non-small cell lung cancer cell lines (PeerJ, 2019)
• Pulmonary rehabilitation for interstitial lung disease (Cochrane Database of Systematic Reviews, 2021)
• Estimation of soil-to-plant transfer factor across six local government areas of Kwara state, Nigeria (Journal of Environmental Radioactivity, 2024)
• Soil-to-banana transfer factor of radionuclides in Lampung, Indonesia (Applied Radiation and Isotopes, 2025)
• Assessment of radionuclide transfer factors and transfer coefficients near phosphate industrial areas of South Tunisia (Environmental Science and Pollution Research International, 2019)
• Dose and transfer factor of 40K in medicinal plants (Radiation Protection Dosimetry, 2024)
• The transfer of 239+240Pu, 241Am, 137Cs and 90Sr to the tissues of horses (Journal of Environmental Radioactivity, 2020)
• Concentration of lead and mercury in collected vegetables and herbs from Markazi province, Iran: a non-carcinogenic risk assessment (Food and Chemical Toxicology, 2018)
• Contamination of staple crops by heavy metals in Sibaté, Colombia (Heliyon, 2020)
• Mixed effects and co-transfer of CeO2 NPs and arsenic in the pakchoi-snail food chain (Journal of Hazardous Materials, 2024)