INTRODUCTION

The immune system is well distributed throughout the whole body, its main sites of activity being in the bone marrow, thymus, lymph nodes, spleen, lung, and intestines. An important exception, however, is the CNS, which is normally isolated from the immune system. This special relationship between the CNS and the immune system (and its consequences) provides a key point in the understanding of the pathogenesis of MS, and its significance is discussed in this chapter. The key elements of the immune system are the lymphocytes and accessory cells, such as macrophages. These cells are armed with a highly complex arsenal of receptors and cellular hormones (cytokines) which enable them to communicate with their cellular environment and to protect the organism from intruders (Fig. 3.1).

First, we shall have a closer look at the receptors which help the cells of the immune system to recognize each other, other cells, and free molecules. The six most important types of receptors are: (1) the antigen receptors of the B-lymphocytes, known as antibodies, or immunoglobulins when they are in soluble form rather than cell bound; (2) the antigen receptors of the T-cells; (3) the Class I histocompatibility antigens; (4) the Class II histocompatibility antigens; (5) the cell surface markers of the T-cell subpopulation CD4; and (6) the cell surface markers of the T-cell subpopulation CD8. These recognition molecules all have a remarkably similar molecular structure (Fig. 3.2).