Cooperation of immunocompetent cells in the B immune response

Cooperation of the cells in the antibody response: In order for a successful antibody response to occur, the cooperation and interplay of at least 3 types of cells are necessary: ​​APC (antigen presenting cell), Th (T-helper lymphocyte) and B lymphocytes


 * The antigen that penetrates the organism is captured by the B-lymphocytes, which have the corresponding receptor; at the same time, it is absorbed and processed by an antigen-presenting cell (most often a macrophage or a dendritic cell)
 * Fragments of the antigen together with the MHC II molecule are exposed in a highly immunogenic form on the surface of the APC, in this form they are recognized by the Th lymphocytes with the corresponding receptors. This binding represents a signal to activate the Th lymphocyte, but is not sufficient by itself.
 * Others - so-called secondary signals are needed, which consist of binding of several adhesive molecules between the APC and the Th cell - the most important are e.g. CD28 and LFA1 on the Th and their ligands on the APC: B7 and ICAM-1, for further Th activation cytokines produced by APCs – especially IL-1 – contribute
 * The result is division of the Th precursors which create clones of activated Th lymphocytes. These activated Th lymphocytes then deliver the necessary signals to the B lymphocyte
 * Again, at least 2 signals are needed to activate a B lymphocyte: antigen reacting with mIg (membrane immunoglobulin) on B cells and stimulatory signals from Th cells. The very binding of the antigen to the BCR (B cell receptor) often results in the unresponsiveness of the B lymphocyte and sometimes its death; if activated Th lymphocytes are present, they contact B lymphocytes via the CD40 receptor on the B cell and the CD40L ligand on the Th cell; Th lymphocyte also produces IL-2, IL-4 , IL-5 and IL-6. Under the influence of these signals, B cells begin to proliferate
 * During this period, variable genes in B cell clones are subject to increased mutation rates. Mutant forms of mIg with different affinity to the antigen appear on the cell surface, only the B lymphocytes that bind to the antigen the strongest survive, the other ones die; at the same time, DNA segments are further rearranged (class switching) and lymphocytes complete their development into plasma cells. A portion of the B cells become memory cells
 * The primary response to the antigen begins more slowly and is characterized by the presence of antibodies of the IgM class
 * Repeated encounter with the same antigen induces a secondary response that starts much faster, is stronger and lasts for a much longer time; mainly IgG antibodies are present

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