Immunoglobulins (Ig) or antibodies are glycoproteins produced by plasma cells. B cells are instructed by specific immunogens, for example, bacterial proteins, to differentiate into plasma cells. Plasma cells are protein-making cells participating in humoral immune responses against bacteria, viruses, fungi, parasites, cellular antigens, chemicals, and synthetic substances. Immunoglobulins constitute about 20% of the protein in plasma.
The immunogen or antigen reacts with a B-cell receptor (BCR) on the cell surface of B lymphocytes. A signal is produced that directs the activation of transcription factors to stimulate the synthesis of antibodies, which are highly specific for the immunogen that stimulated the B cell. Furthermore, one clone of a B cell makes an immunoglobulin (specificity). The immune system remembers the antigens that caused a previous reaction (memory) due to the development of memory B cells. These are intermediate, differentiated B cells that can quickly become plasma cells. Circulating antibodies recognize antigens in tissue fluids and serum.
The following are five types of immunoglobulins in humans:
IgM
IgG
IgA
IgE
IgD
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