Herbs that contain mucilage-like compounds called polysaccharides are soothing to mucous membranes, so they’re commonly used to treat sore throats. A study at the Institute for Pharmaceutical Biology in Germany found that the polysaccharides in marshmallow (Althaea officinalis), plantain (Plantago spp.), mallow (Malva moschata) and linden (Tilia ¥europaea) are absorbed by and lightly coat irritated mucous membranes such as the throat. What turned out to provide the best coating and potentially be the most soothing—though they’re not popular as cough remedies—were calendula flowers (Calendula officinalis) and the seaweed bladderwrack (Fucus vesiculosus).
Source: Schmidgall, J., et al. “Evidence for bioadhesive effects of polysaccharides and polysaccharide-containing herbs in an ex vivo bioadhesion assay on buccal membranes.” Planta Medica 2000, 66(1): 48–53.