Echinacea has quite an interesting history.As with any herbal product, there are possible pharmcologically active ingredients, which in itself a problem due to lack of standardisation - this being an issue with all herbal remedies - what precisely is the active ingredient, and just how much is needed to be clinically efficacious?
Some lab studies have indeed shown small stimulatory effects of certain components of the immune response - but translating results found in a test tube or petri dish to an actual clinical benefit within humans is a mistake.
There have been lots of trials, looking at the clinical efficacy, but the vast majority of these are low quality trials, with little control, randomisation, or patient numbers to give the findings any weight.
Of those trials where there are good controls, good trial protocol, large patient numbers to give statistical weight to the outcomes - the results on duration and severity of the cold have been negative.
Good article here looking at the history, claims, and evidence behind echinacea.
http://www.sciencebas...cea-for-cold-and-flu/