Structure and Function of Natural Antibodies

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The presence of immunoglobulins (Ig) in the circulation of normal humans and animals that bind a variety of foreign antigens, such as bacterial components and products, viruses, protozoa, fungi, as well as self antigens, such as nucleic acids, phospholipids, erythrocytes, serum proteins, cellular components, insulin and thy-roglobulin, has been recognized since the beginning of this century [1–9]. Because their emergence is independent of known and/or intentional immunization, these antibodies have been termed “natural antibodies”. In contrast to antigen-induced antibodies, which are mainly IgG and monoreactive, a considerable proportion of natural antibodies are IgM and polyreactive, that is they bind several unrelated antigens with different affinities. Natural polyreactive and monoreactive IgG and IgA antibodies also exist [7]. The vast majority of natural antibodies, whether IgM, IgG or IgA, are produced by CD5+ B cells, the predominant lymphocytes in the neonatal cell repertoire [10,11]. Because of their broad reactivity with a variety of microbial components, diese antibodies may play a major role in the primary line of defense against infections. Owing to their ability to bind self antigens, they may serve as templates for some of the high-affinity autoantibodies that emerge in patients with autoimmune disease, particularly those associated with a significant expansion of CD5+ B cells [12–14].