Two features of the Aloe family make them somewhat unique in the field of medicinal plants. First, there is no agreement on which ailments they might cure and second, there is no agreement on the components within the aloe plant that may exert beneficial medicinal activity. Anecdotal reports describe a bewildering array of both immuno-stimulating and immunosuppressive effects. Few controlled studies have supported these claims and to this day, much uncertainty exists as to whether aloe extracts actually have significant beneficial activity. The controlled studies that have been undertaken have tended, unfortunately, to use complex mixtures or partially purified components which makes comparative analysis difficult. Nevertheless, they have tended to support a very limited range of effects on the cells of the immune system. Indeed, the recognized effects of aloe extracts appear primarily to affect innate immune mechanisms such as inflammation, rather than acquired immunity. They also tend to be quantitatively minor in nature. Nevertheless, taken as a whole, they may well account for the 'good press' that aloes have received over the years and for the continuing interest in the use of aloe extracts as immunomodulating and anti-inflammatory agents.
Although there are more than 400 Aloe species recognized, one plant, Aloe vera (L.) Burm. f., dominates both the commercial aloe market and the research literature. (This species is sometimes referred to by the extinct name, A. barbadensis Miller) Other Aloe species that have been extensively examined, especially in Asia, include A. arborescens Miller, and A. saponaria (Ait.)Haw. Such limited comparative studies that have been undertaken have shown that the biochemical composition of extracts from different Aloe species varies widely (Viljoen and van Wyk, 1998). As a result, biological responses obtained with material from one species, cannot automatically be ascribed to others. Nor have any studies been undertaken to determine whether factors such as geographical location, soil quality, fertilization or plant genetics influence biological activity. It is widely believed that the treatment and storage of the harvested leaves prior to, and during, processing influences its biological activity, and although processing has been the subject of much rhetoric between competing suppliers, controlled studies have not been undertaken. The products still hover on the edge of scientific respectability (Reynolds and Dweck, 1999).
Aloes are succulent plants with characteristic thick fleshy leaves. The outer rind contains the bitter yellow sap that originates in the bundle sheath cells and is used for its purgative effects. The center of these leaves consists of a clear mesophyll gel. The mucilaginous texture of the mesophyll gel ensures that it is intrinsically soothing. As a result it has been used to treat superficial dermal inflammation resulting from a wide variety of causes. The gel has been especially useful in the treatment of radiation burns ranging from sunburn to X-ray and radium burns (Sato etal, 1990; Roberts and Travis, 1995; Wright, 1936). Despite the anecdotal nature of much of this work, results are generally positive and the gel is widely employed. Indeed, it is rare to find a commercial 'soothing' preparation that does not contain 'Aloe vera' as one of its components (Grindlay and Reynolds, 1986; Reynolds and Dweck, 1999). It may also have some ill-defined antimicrobial properties (Bruce, 1967; Soeda etal, 1966).
Aloe extracts are commonly made from the whole leaf, or alternatively, from the clear mesophyll gel. The composition of these two extracts differs significantly since the thick photosynthetic rind contains many components not present within the mesophyll. Aloe vera leaves are, however, little different from other plants in their basic composition. They consist primarily of a carbohydrate mixture with some proteins and a host of minor components such as steroids, anthraquinones, flavonoids, and chromones (Holdsworth, 1971, 1972; Hutter etal, 1996; Hirata and Suga, 1977; Makino etal., 1974), as well as enzymes such as carboxypeptidases (Fujita etal., 1979), superoxide dismutase (Sabeh etal., 1996) and glutathione peroxidase (Sabeh etal., 1993).
Their cell walls contain celluloses, hemicelluloses and pectins while their major storage carbohydrate is either an acetylated mannan or a glucomannan. There may be a significant number of proteins within the gel. These have not been well characterized. From an immunological viewpoint, the most important of these proteins are the lectins
(Akev, 1999; Koike etal, 1995a; 1995b; Suzuki etal, 1979; Yagi etal, 1985; Yoshimoto etal, 1987). Their activities are discussed elsewhere in this book.
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