Most algaes are excellent for the immune system including bladderwrack, nori, dulse, wakeme, hiziki, etc. They contain large amounts of hte immune stimulating polysaccharide fucoidan. Atlantic kelp though is low in this polysaccharide. Kelps also help support adrenal function which helps in immune support.
Red algaes though are more recommended for cancer patients, since they are more antitumor. Dulse is one of the easiest to come by, and is sold in most health food stores.