The unfortunate thing is that, not many high-quality researches have been conducted in humans to support these effects. Two often-cited studies report that taking seaweed supplements may have the ability to reduce symptoms of the herpes virus and increase levels of immune cells in HIV patients.
However, neither of these studies had a placebo group, which makes it difficult to interpret their results.
A more recent study explored the effects of the consumption of seaweed supplements in HIV-positive women. Those that received 5 grams of spirulina per day developed 27% fewer disease-related symptoms, compared to the those that did not.
However, there was no visible differences in immune cell levels over the 12-week study period. Additional studies are needed before strong conclusions can be made.
In conclusion, seaweed may have some beneficial effects on your immune system. However, more research is needed to understand it better.