Abstract: There is provided a composition comprising a polysaccharide obtainable from the microalgae, Prasinococcus capsulatus and strains related to P. capsulatus for use prophylactically and/or therapeutically in the treatment of disorders of the immune system, for example in psoriasis and dermatological conditions, internal immune system disorders, in particular gut inflammatory conditions and respiratory conditions. Further, there is provided derivatives of a polysaccharide obtainable from the microalgae, Prasinococcus capsulatus or an algal strain related to P. capsulatus and the use of such derivatives prophylactically and/or therapeutically in the treatment of disorders of the immune system.