Immune peptides are short amino-acid chains that either switch on parts of the immune system or calm inflammation. The main examples are thymosin alpha-1, sold abroad as Zadaxin for hepatitis B; KPV, an anti-inflammatory fragment tested only in animals; and ARA-290, which acts on the body’s innate repair receptor.[2][15][17] None is FDA-approved, and the largest recent trial of thymosin alpha-1, in sepsis, found no survival benefit.[9]

This overview covers what these peptides are, how they act on immunity, how strong the evidence really is, where they are approved, and their US legal and sports status in 2026.

What are immune peptides?

The immune system has two broad arms: the fast, general innate response and the slower, targeted adaptive response led by T cells and antibodies. Immune peptides are not one drug class. They are unrelated molecules that nudge one of these arms.

Thymosin alpha-1 is a 28-amino-acid peptide first isolated from a calf thymus extract in 1977.[1] Despite the similar name, it is unrelated to TB-500, a lab-made 7-amino-acid fragment of thymosin beta-4 marketed for tissue repair. The shared name comes from thymosin fraction 5, the calf thymus extract from which both thymosin alpha-1 and thymosin beta-4 were first isolated.[1][26] KPV is the last three amino acids of alpha-MSH, a hormone with anti-inflammatory as well as pigment effects.[13] ARA-290 is an 11-amino-acid peptide modeled on erythropoietin (EPO).[20][17]

They differ from vaccines and approved immune drugs such as interferons in one key way: none has completed the trials FDA requires for approval in the US.

Browse all immune peptides

See all 9 immunity peptides ↓

How immune peptides work

  • Thymosin alpha-1 activates dendritic cells, the immune system’s scouts, through toll-like receptors, which in turn supports T-cell responses, in lab and mouse studies.[5]
  • KPV enters gut cells through the PepT1 transporter and blocks NF-kB, a master switch for inflammation, in mouse and cell studies.[14] Reviews note it keeps alpha-MSH’s anti-inflammatory effect without its skin-darkening effect.[13]
  • ARA-290 activates the innate repair receptor, a pairing of the EPO receptor with the beta-common receptor (CD131). This receptor is thought to dampen inflammation and switch on repair after tissue injury. ARA-290 was designed to do this without stimulating red blood cell production.[17]

So thymosin alpha-1 aims to strengthen immune responses, while KPV and ARA-290 aim to calm inflammation. That difference matters: a peptide that ramps up immunity is not automatically helpful for an inflammatory or autoimmune problem, and vice versa.

Peptides for immune system support: what trials actually show

Thymosin alpha-1

  • Hepatitis B: in a Taiwanese randomized trial of 98 patients, 40.6% of those treated for 26 weeks had a complete response at 18 months versus 9.4% of controls.[6] A US phase 3 trial found 14% versus 4%, which did not reach statistical significance.[7]
  • Sepsis: the 2013 ETASS trial in 361 patients reported 26.0% versus 35.0% mortality, a borderline result.[8] The much larger, double-blind TESTS trial of 1,106 patients found 28-day mortality of 23.4% versus 24.1% on placebo: no benefit.[9] A 2025 meta-analysis of 11 trials found lower mortality overall, but no benefit in the high-quality or multicenter trials.[10]
  • COVID-19: a 2023 meta-analysis of more than 5,000 patients found no effect on mortality.[11]
  • Vaccines: in a small pilot study of dialysis patients, Zadaxin improved the antibody response to a flu vaccine.[12]

KPV

All KPV evidence comes from mice and cells. Taken by mouth, it reduced chemically induced colitis in mice.[14] FDA found no human studies of KPV by any route.[15]

ARA-290

In a placebo-controlled trial in 38 people with sarcoidosis, an inflammatory disease, and small nerve fiber loss, 28 days of ARA-290 injections improved neuropathy symptoms and corneal nerve fiber area.[18] In a 64-person dose-ranging trial, only the 4 mg dose clearly increased corneal nerve fibers, and pain improved in all groups, including placebo.[19] In a separate trial in 48 analyzed people with type 2 diabetes and painful neuropathy, 28 days of ARA-290 improved neuropathy symptom scores, and HbA1c fell by about 0.2 percentage points on ARA-290 while rising slightly on placebo.[25] These were short trials, many run with the developer’s involvement, and no phase 3 trial has followed.

Approvals abroad vs US status

Thymosin alpha-1 is the only peptide in this category approved as a medicine anywhere. Its maker, SciClone, reported approvals in more than 30 countries as of 2016, with China its largest market.[2] FDA’s 2024 review lists approved uses such as chronic hepatitis B in several countries and, in Italy, as a flu vaccine booster for people with weak immune systems, but notes that it could not independently verify every approval.[3]

ARA-290 holds an EU orphan designation for sarcoidosis, granted in 2013, and EMA reports a US orphan designation for neuropathic pain in sarcoidosis. Orphan status supports development; it is not an approval.[20]

Safety, legality and sports rules

  • Thymosin alpha-1: not FDA-approved. In December 2024, FDA’s compounding advisory committee voted 17 to 4 against adding it to the list of substances pharmacies may compound,[4] and it does not appear in any category of FDA’s May 2026 nominations list, so US pharmacies cannot legally compound it.[21] Labels abroad mainly list injection-site discomfort as a side effect.[3]
  • KPV: FDA had placed KPV on its list of bulk substances that may present significant safety risks in compounding; in April 2026, after the nominations were withdrawn, FDA removed it, which is not an approval.[22] In July 2026, FDA recommended against it, but its advisers reportedly voted 8 to 6 in favor; the vote is not binding.[15][16]
  • ARA-290: investigational and not approved in the US or EU. FDA lists cibinetide among compounding nominations made without adequate support.[21]
  • Sport: WADA’s 2026 list bans innate repair receptor agonists at all times, the class ARA-290 belongs to. Thymosin alpha-1 and KPV are not named; KPV, which is approved nowhere, may still fall under WADA’s S0 ban on unapproved substances.[23]
  • Unregulated products: peptides sold online for “research use” are not made to pharmaceutical standards, and a 2026 review warns of contamination and dosing errors.[24] Thymosin alpha-1 labels advise against use in children, in pregnancy or breastfeeding and in people on deliberate immunosuppression such as transplant recipients, and urge caution in autoimmune disease.[3]

KPV and ARA-290 also appear among healing peptides, and thymosin alpha-1 among longevity peptides.