Plasma cell

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 an antigen and are the principal effector cells involved in humoral immunity.

Overview

After dividing for around five days, mature B cells differentiate into either plasma B cells or memory B cells. Most plasma B cells travel to the spleen or bone marrow to secrete antibodies (approximately 10,000 per second). During the initial stages of an immune response the lifespan of plasma cells is very short, typicall only a few days to weeks. However, following the process of affinity maturation, plasma cells can survive for months to years and continue to secrete high levels antibody. 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 specificity for their antigen.

Microscopic anatomy

Plasma cells are large lymphocytes with a large cytoplasm-to-nucleus 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. 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.