“It works for me like a bucket.
while i am taking it, my symptoms are markedly less.
once i stop, symptoms start appearing.
once that happens, it takes about 10 days of astragalus to start reducing the symptoms again.”
In this post I am going to summarize what we know about this herb from PubMed. Each section below is done by search term
Astragalus Histamine or Allergy
- “This study aimed to observe the effects of astragalus membranaceus injection on the activity of intestinal mucosal mast cells (IMMC) after hemorrhage shock-reperfusion in rats…may reduce the damage to small intestine mucosa by inhibiting the activated IMMC [which releases histamines] after hemorrhagic shock.” 
- “KOB extracts, which is a polyherbal medicine consisting of 5 different herbs (Atractylodes macrocephala, Astragalusmembranaceus, Saposhnikovia divaricata, Ostericum koreanum and Scutellaria baicalensis) had commonly been used for the treatment of various allergic diseases showed an anti-allergic effect by modulating mast cell-mediated allergic responses in allergic rhinitis, recently.” 
- “herbal formula composed of Astragalus membranaceus (AM) and Angelica sinensis (AS) at a ratio of 5 : 1, has been used for the treatment of various skin diseases in traditional medicine…decreased s…the number of mast cells”
- Astragalus membranaceus modulates Th1/2 immune balance and activates PPARγ in a murine asthma model.
- Prevention of allergic airway hyperresponsiveness and remodeling in mice by Astragaliradix antiasthmatic decoction. 
- Astragalus extract attenuates allergic airway inflammation and inhibits nuclear factor κB expression in asthmatic mice. 
- [Clinical study of astragalus‘s preventing the recurrence of asthma in children].
- “Growth of M. mycoides capri, E. coli, and B. subtilis was inhibited by elderberry extract, oregano oil, ethanol extract of oregano leaves, and ethanol extract of goldenseal root. No inhibition was seen with aqueous extract of astragalus” 
- “We show that APS, a polysaccharide isolated from the roots of Astragalus membranaceus, significantly induces nitric oxide (NO) production and inducible NO synthase (iNOS) transcription through the activation of nuclear factor-kappaB/Rel (NF-kappaB/Rel).”
- “Modern scientific studies have shown that several components from Chinese herbal medicines are immunologically active. Astragalusmongholicus polysaccharide APS, a polymer of glucose and arabinose, increases specific antibody formation as well as synthesis of RNA, DNA and protein in mouse spleen. The polysaccharide PES isolated from Acanthopanax senticosus used as a tonic and sedative agent also induces interferon production.” 
- The decoction of radix Astragali inhibits the growth of Microcystis aeruginosa .
- [Memory-improving effect of aqueous extract of Astragalus membranaceus (Fisch.) Bge]. “The study shows that the aqueous extract of Astragulus membranaceus (AMWE) can improve the anisodine-induced impairment on memory acquisition as well as the alcohol-elicited deficit of memory retrieval (number of errors were reduced and latent period was prolonged) in step down, and can also prolong the gasping duration of mice after decapitation at doses of 50g/kg (ig) for 7 days.”
- Protective effects of astragalus extract against intermittent hypoxia-induced hippocampal neurons impairment in rats.
- Protective effect of extract of Astragalus on learning and memory impairments and neurons’ apoptosis induced by glucocorticoids in 12-month-old male mice .
- Promotion of axonal maturation and prevention of memory loss in mice by extracts of Astragalusmongholicus .
- “the present study indicated that the active components ofEpimedium, Astragalus and Radix Puerariae may exert a neuroprotective effect against AD by reducing iron overload in the AD brain” 
There is considerable literature supporting the role of Astragalus in reducing/moderating histamines. It does not appear to have significant antibiotic impact nor microbiome impact. It does have positive effects on the brain / cognitive functions.
If have histamines issues, I would suggest that you give it a try