Metadata about: The immune defence mechanisms by which an organism combats viral infections can be divided into two systems. On the one hand, there are the unspecific, non-adaptive immune reactions, which recognize and eliminate invading foreign pathogens. This so-called natural or innate immune system becomes primarily active after a virus has overcome the external physical protection barriers of the body (skin, mucous membranes). It consists of dendritic cells, granulocytes, monocytes, macrophages and natural killer cells (NK cells). They have proteins that serve as receptors, e.g. Toll-like receptors (TLRs) and complement receptors, for specific structures of pathogens and for the soluble products of the innate immune system (acute-phase proteins, factors of the complement system, cytokines, chemokines and interferons). The effects and functions of cytokines, chemokines and interferons will be discussed separately in Chap. 8. The specific, adaptive immune response is the second line of defence, and is developed only during or after the establishment of an infection. It includes antibody-producing B cells – the humoral immune system – as well as T-helper (T(H)) cells and cytotoxic T lymphocytes, which collectively constitute the cellular defence system. The adaptive immune reactions can selectively recognize certain pathogen types or subtypes, and in the case of a reinfection, they are able to recognize the pathogens again and eliminate them. They are long-lasting and a subset of stimulated lymphocytes transform into memory cells during their development, which confers on the organism an efficient protective immunity against infections with the same pathogen. The systems of the specific and non-specific immune responses are in close contact with each other, particularly via cytokines, chemokines and interferons. An immune response is generally triggered by antigens. These may be the infectious pathogens, individual protein components or sugar structures. The immune system recognizes these as foreign, and thus can distinguish between endogenous and exogenous components. However, the antigens must be of a certain size to trigger different immune responses. Molecules with a molecular mass of less than 3–4 kDa are usually incapable of doing that.

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