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1 The immune system in a nutshell

The immune system consists of two functional components: the innate and the adaptive immune system. The innate immune system prevents penetration and spread of many infectious agents by means of a variety of physical, biochemical and cellular barriers (skin, mucosa, lysozymes, complement and phagocytes). Apart from these first lines of defense, the adaptive immune system may be called upon to react against and clear the harmful agent. Furthermore, after the first attack, the adaptive immune system develops a specific immunological memory, leading to a stronger, faster and more effective response upon renewed contact with the same agent. The adaptive immune system consists of a variety of cells and molecules, among which lymphocytes and immunoglobulins are the key elements. There are two types of lymphocytes, T cells and B Cells. T cells play a pivotal role in regulating the immune response and are also responsible for cellular immunity, while B cells are essential in the effector phase of humoral immunity. After exposure to antigen and mostly with the help of T cells, B cells can differentiate into plasma cells which synthesize molecules (antibodies or immunoglobulins) that can react with the antigen(1,2).

Immunoglobulins are a group of closely related glycoproteins composed of 82-96% protein and 4-18% carbohydrate. The basic immunoglobulin molecule has a four-chain structure, comprising two identical heavy (H) chains and two identical light (L) chains, linked together by inter-chain disulfide bonds. Intra-chain disulfide bonds are responsible for the formation of loops, leading to the compact, domain-like structure of the molecule. The aminoterminal portions of the heavy and light chains, characterized by a highly variable amino-acid composition, are referred to a V H and V L, respectively. The constant parts of the light chain are designated as C L, while the constant parts of the heavy chains are further divided into three distinct subunits: CH1, CH2 and CH3 (figure 2). Functionally, the V regions are involved in antigen binding. The C regions interact to hold the molecule together and are involved in several biological activities, the so-called effector functions such as complement binding, placental passage and binding to cell membranes.

Figure 2