B cell differentiation
Mature B cells that undergo stimulation by an antigen undergo class switching, and differentiate into either plasma or memory cells.
In the paracortex region of lymph nodes binding to MHC II and the presence of Il-4 produced by CD4+ T cells (Th2 type) causes B cells to differentiate. Most B cells become plasma cells, however small number become memory cells .
Plasma cells
Appearance
Plasma cells are oval, around 9µm and have a round prominent nucleus. The cytoplasm is extensive and stains strongly basophilic. It contains large amounts of rough endoplasmic reticulum and the Golgi apparatus is large and appears as a clear crescent-shaped structure near the nucleus. Some plasma cells accumulate considerable quantities of, perhaps abnormal, antibody and this appears as a large eosinophilic blob filling the cytoplasm and displacing the nucleus to one side. These blobs are called "Russell Bodies".
Function
Plasma cells produce immunoglobulins/antibodies (thousands a second). The immunoglobulin binding specificity is identical to the binding specificity of the BCR on the B cell that the cell has differentiated from. This means that when a B cell has a BCR that can effectively bind to an antigen the immunoglobulins produced by the plasma cell can bind to that antigen.
Although they can live for months most plasma cells only live for a few days and do not replicate in this time.
Class switching
Initially plasma cell produce IgM however this is not always the most appropriate Ig to be produced and therefore stimulation by T cells and interleukins causes the plasma cells to undergo class switching to produce different classes of Ig.
- In mucosal B cells plasma cells CD40 interaction (with Th2 CD40L) and Il-10 stimulates class switching to IgA
- Eosinophils produce Il-13 which promotes class switching to IgE
Plasma cells produced in the first immune response to an antigen are mainly of the class IgM whereas those produced from memory cells in the second immune response are mainly of the IgG class.
In pathology
Memory cells
The differentiated cells that remain in the cortex become memory cells and these proliferate and form germinal centres in the lymph node.
Memory cells are long lived and responsible for long term immunity providing the immune system with a memory of previously encountered antigens. When they experience an antigen again they proliferate and differentiate into plasma cells. This response produces up to ten times more plasma cells than the original exposure to the antigen and is why the second immune response to an antigen is both more rapid and much stronger than the first response.