Every protein or peptide from the environment can be considered an antigen. If an individual's immune system does not recognize a substance as foreign antigen, the individual is said to be tolerant-, if it does, he is said to be responsive or sensitized. The first step of the immune response consists of this recognition by macrophages and other antigen-presenting cells (APC), followed by phagocytosis. Phagocytosis is the digestion of peptides by enzymes in the endoplasmic reticulum and the presentation of these pep-tides back to the cell's surface through HLA class II (also called DR) antigen on APC and T-cell receptor on T cells together with accessory molecules. These accessory molecules, when present, amplify the immune response, when absent, they reduce it and limit its intensity. If the antigen is presented in the context of class II molecules and T-cell receptors, but without accessory molecules, the stimulated cells undergo apoptosis (cell death). However, if accessory molecules are present, the cells multiply and bring about a recall response (the genesis of memory cells). Subsequently, Th are activated and stimulate effector cells such as T cells, B cells, and NK cells.
The whole pathophysiology of MS may be summarized by the fact that myelin may be recognized as a foreign antigen. A certain number of concepts have been added to this: antigen spreading describes the fact that the immune response from a very specific epitope (a small part of the antigenic site) of one antigen tends, when the stimulus is repeated, to react against other antigens often seen together. That is, response is developed to myelin protein other than MBP, such as MOG. Antigen mimicry describes the fact that proteins that appear very different can have some peptides in common; when an immune response develops, this may generate a cross-reactive immune response.
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