3. Physiology of autoantibody

### 3.1. Physiology of antibody

Antibodies are responsible for the humoral type of adaptive immune responses, glycoprotein structure and produced by B lymphocytes.

Antigens can directly bind to antigen receptors of specific B lymphocytes. The type of reversible bond is non-covalent as; electrostatic attraction, hydrogen bonds, Van der Waals-, charge interactions and hydrophobic forces. Membrane–bound antibodies (IgM and IgD type) work as antigen receptors of B lymphocytes (BLR) and can bind to antigens in proteins, lipids, carbohydrates and nucleic acids structures. T lymphocytes can react antigens just in protein structure. For an antigen-presenting cells (APC), there is not any necessity to present antigens to B lymphocytes. Epitopes antigens recognized by T cells are narrow linear peptides from 8 to 20 amino acids [4, 16, 17].

After binding of antigens to the receptors that are membrane- bound antibodies; IgM and IgD type, B lymphocyte become activated. The clonal expansion which means proliferation of antigen specific cells follows the activation of B lymphocytes and they differentiate into antibody-secreting effector cells. The specificity of the naïve B cell membrane-bound antibody receptors is same with the secreted free antibodies. During their differentiation period, some B cells may differentiate to produce antibodies with different heavy chain classes (or isotypes) called as heavy chain class (isotype) switching. After switching, different effector functions can be monitored. Repeated exposure to an antigen leads to the production of antibodies with increasing capacity to bind the antigen; called as affinity maturation [4, 18].

Antibodies responses are classified into two based on the requirement for T cell help; as T-independent or T-dependent
