The main purpose of this thesis was to evaluate the potential of two Malian medicinal plants and one Chinese cultivated medicinal plant in the treatment of immune related diseases. This thesis also aimed to promote the sustainable use of medicinal plant resources. Seven purified pectic polysaccharides fractions were isolated from Parkia biglobosa bark. All of the fractions exhibited potent complement fixation activity, and fractions PBEII-I, PEBII-III and PBEII-IV also showed potent macrophage stimulating activity. The common structural features of these seven fractions are rhamnogalacturonan I (RG-I) backbone highly branched with arabinogalactan type I and/or type II (AG-I and/or AG-II) side chains. The homogalacturonan region may not present in fractions PBEII-III, PBEII-IV and PB100I-I due to the high ratio of rhamnose (Rha) to galacturonic acid (GalA). The higher yield and biological activities of fractions obtained from the 50% ethanol-water extract suggests that this extract could be more related to the medicinal activity than the 50o C and 100o C water extracts. 27 different crude extracts were obtained by boiling water extraction (BWE) and accelerated solvent extraction (ASE) from root bark, stem bark and leaves of Terminalia macroptera. None of the extracts are toxic against brine shrimp larvae in the test concentration. Significant correlations were found among enzyme inhibition (α-glucosidase, 15-lipoxygenase, xanthine oxidase), DPPH scavenging activity and total phenolic content, thus a screening of phenolic content in T. macroptera extracts will probably indicate the presence of compounds with enzyme inhibitory and antioxidant activities. Based on the results from principle component analysis, the ASE ethanol extracts of root bark and stem bark and the low molecular weight fraction of 50% ethanol-water extract of leaves showed the highest total biological activities, which indicated ASE has higher extraction efficiency than BWE. The results indicate that part of activities like antioxidant activity and enzyme inhibition activities are present in the high molecular weight part of our crude extracts. The observed enzyme inhibition activities, radical-scavenging properties and complement fixation activities may explain some of the traditional uses of T. macroptera, such as against diabetes and wound healing. Fifteen purified pectic polysaccharide fractions were obtained from nine crude extracts of T. macroptera (root bark, stem bark and leaves) by using BWE and ASE. The root bark, leaves and stem bark are all good sources for fractions containing bioactive polysaccharides. But due to sustainability, it is prefer to use leaves rather than the other two plant parts, and then the dosage v by weight must be higher when using leaves. The results also indicated that BWE provide higher yields of crude extracts with comparable complement fixation activities to the crude extracts isolated with ASE. For the purpose of obtaining purified polysaccharide fractions, ASE was more efficient, as the method provided higher yields and higher complement fixation activity. The common structural features of these fifteen polysaccharide fractions are 1,4-linked galacturonan, interrupted by RG-I regions with AG-I and/or AG-II side chains. The most active polysaccharide fraction 100WTRBH-I-I, has different structural feature from other fractions. It has a long RG-I region with galactan side chains, not arabinogalactans. The structural differences present among these fifteen fractions are Mw, chemical compositions and position of side chains. The three most abundant fractions, 50WTRBH-II-I, 50WTSBH-II-I and 50WTLH-II-I were subjected to pectinase degradation; the results indicated that the activity of these three fractions was expressed mainly by their ramified regions. Two purified polysaccharide fractions, 50WCP-II-I and 100WCP-II-I, were isolated from 50o C and 100o C water ASE extracts of cultivated roots of Codonopsis pilosula Nannf. var. modesta L.T.Shen. The structure studies of native and sub-fractions showed the 50WCP-II-I is a pectic polysaccharide fraction, with long homogalacturonan regions (some of the GalpA were methyl esterified), interrupted by short RG-I regions, the side chains (AG-I and AG-II) of RG-I region are attached on position 4 of Rha. The structural feature of fraction 100WCP-II-I is different from that of 50WCP-II-I, the AG-I side chains are attached on position 2 of GalA, and AG-II side chains are attached on position 4 of Rha in the latter. We have compared the complement fixation activity of the different pectic polysaccharides obtained, and it became clear that, parameters as Mw, ramified regions (RG-I or branched galacturonan), side chains (arabinogalactan and/or galactan) and phenolic compounds, are important for the expression of complement fixation activity.