Plasma cells (also called
plasma B cells or
plasmocytes) are cells of the
immune system that secrete large amounts of
antibodies. They differentiate from
B cells upon stimulation by
CD4+ lymphocytes. The B cell acts as an
antigen presenting cell (APC), consuming an offending
pathogen. That pathogen gets taken up by the B cell by receptor mediated endocytosis, and broken down within these endosomes after fusion with
lysosomes releasing proteolytic enzymes onto the pathogen. Once the enzymes break down the pathogen, pieces of the pathogen (which are now known as antigenic peptides) are loaded onto
MHC II molecules, and presented on its extracellular surface. Once on the extracellular surface, the CD4+ T-helper lymphocyte will bind to the MHC II/Antigen molecule and cause activation of the B cell, which includes differentiation into a plasma cell, and subsequent generation of antibody against the consumed pathogen.
Overview
After dividing for around five days, mature B cells differentiate into either plasma B cells or
memory B cells. Plasma B cells originate in the
bone marrow, then travel to the
spleen or
lymph nodes to secrete antibodies (approximately 3,000 per second). During the initial stages of an immune response the lifespan of plasma cells is very short, typically only 3-4 days. However, following the process of affinity maturation, plasma cells can survive for months to years and continue to secrete high levels of antibodies. Memory B cells tend to be longer-lived and can therefore respond quickly upon second exposure to an antigen.
The class of antibody that a plasma cell produces depends on signals, called cytokines, from other immune system cells, such as macrophages and T helper cells. This process is called isotype-switching. For example, plasma cells will likely secrete IgG3 antibodies if they matured in the presence of the cytokine interferon-gamma. Since B cell maturation also involves somatic hypermutation, these antibodies have a very high affinity for their antigen.
Microscopic anatomy
Plasma cells are large lymphocytes with a considerable
nucleus-to-
cytoplasm ratio and a characteristic appearance on
light microscopy. They have
basophilic cytoplasm and an eccentric nucleus with
heterochromatin in a characteristic
cartwheel arrangement. Their cytoplasm also contains a pale zone that on
electron microscopy contains an extensive
Golgi apparatus and
centrioles (
EM picture). Abundant
rough endoplasmic reticulum combined with a well-developed Golgi apparatus makes plasma cells well-suited for secreting immunoglobulins.
Role in disease
Cancer of plasma cells is termed
multiple myeloma. This condition is frequently identified because malignant plasma cells continue producing an antibody, which can be detected as a
paraprotein.
Common variable immunodeficiency is thought to be due to a problem in the differentiation from lymphocytes to plasma cells. The result is a low serum antibody level and risk of infections.
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