Adipose tissue – component of the immune system

Adipose tissue is not just an “energy storage”, but an active endocrynic organ affecting immunological processes and metabolism of the body [1-4]. It was evidenced that adipocytes – adipose tissue cells – have the capacity of producing and secreting many biologically active substances referred to as adipocytokines (Table 1), among which there are not just cytokines, but also substances of the nature of enzymes, hormones and growth factors [1, 3, 4]. Adipocytokines have act in a pleiotropic manner, having autoand paracrynic effect on adipocytes, although they also have an endocrynic effect on other tissues and organs [1, 5, 6]. They also have the capacity of modulating the immunological response as a result of affecting the immune system cells. Some adipocytokines, such as IL-6 or PAI-1 (plasminogen activator inhibitor 1), directly cause the inflammatory process, while others have an intermediate effect, as in the case of CRP, protein the expression of which was recorded in hepatocytes stimulated with IL-6 originating from adipocytes [5, 7]. Present studies evidenced that adipose tissue is not uniform. Depending on the location in the body, they differ with the capacity to secrete adipocytokines, as well as cellular composition with varied phenotype, as well as the quantity and proportion of adipocytes forming it, blood vessel stromal cells and immune system cells [7, 8]. In the human body, one may differentiate two basic types of this tissue: visceral and subcutaneous adipose tissue [7]. Both types of this tissue are characterised with production of a unique profile of adipocytokines (Table 1). In the tissue visceral, for example, higher concentrations of IL-6 and PAI-1 are observed, which are secreted to the portal vein system, from where, reaching the liver, they impact on its functions. In turn, in the subcutaneous tissue, there is a higher concentration of leptin and adiponectin, which are released to the circulatory system, affecting the cells of the entire body [7]. Furthermore, differences between these types of the tissue refer to the cellular composition, principally as regards immune system cells. It turned out that visceral adipose tissue surrounding the spleen, stomach, pancreas, and colon, also referred to as omentum, is richer with macrophages, lymphocytes B, as well as dendritic cells and NKT cells [8-10]. Leucocytes of this tissue are also organised in structures referred to as milky spots (MSs), distributed on the peritoneum [8] and are responsible for gathering bacteria and harmful substances from the peritoneal cavity, and affect stimulation and inhibition of immunological response. Also in the mesenterium of mammals, including humans, fat-associated lymphoid clusters (FALC) were discovered, distributed along blood vessels [8, 11]. In Abstract Adipose tissue has so far been considered exclusively as “energy storage”. At present, it is an important element of the body, which also takes part in regulation of immunological processes. It was determined that the cells of the tissue – adipocytes – have the capacity of producing adipocytokines. It was also evidenced that adipocytes feature many specific receptors, among which also TLR receptors were described, which affect the activity of the immune system. Furthermore, adipose tissue contains immune system cells, such as macrophages, as well as lymphocytes B, T and their subpopulations.