Veterinary Immunology and Serology

B - LYMPHOCYTES

• B-lymphocytes originate from multipotent stem cells of bone marrow but maturation occurs in foetal liver before birth and in the bone marrow or payer’s patches after birth in mammals where as in birds maturation takes place in bursa.
• The principal events during the maturation are rearrangements and expression of Ig (immunoglobulin) genes.
• The mature B cells are not antibody secreting cells but they can differentiate upon antigenic stimulation into antibody secreting plasma cells.
• Plasma cells are large cells (twice the size of a small lymphocytes) with eccentric nucleus and large cytoplasm containing more endoplasmic reticulum; they do not divide and have short life of 2-3 days.
• They are capable of secreting 300 molecules of immunoglobulin per second. Some B cells that do not develop into plasma cells become memory cells. Some of memory cells can survive more than 20 years in absence of antigen. Upon antigenic stimulation they become plasma cells and secrete antibody.

Surface molecules of B-lymphocytes and cluster of differentiation

• Very large numbers of B cell surface molecules have been characterized and it was found difficult to devise a rational nomenclature.
• Functionally distinct classes of lymphocytes express distinct types of cell surface proteins and these have been probed using monoclonal antibodies.
• The cell surface molecules recognized by monoclonal antibodies are called antigen (because antibodies can be raised against them) or markers and they identify or discriminate (‘mark’) between different cell populations.
• These markers can be grouped into several categories; some specifies for cells of a particular lineage or maturational pathway or developed during activation and differentiation.
• Biochemical analyses of cell surface proteins recognized by monoclonal antibodies have been given a uniform nomenclature.
• According to this system a surface marker that identifies a particular lineage or differentiation stage and that has a defined structure which is recognized by a group (‘cluster’) of monoclonal antibodies is called a member of a cluster of differentiation (CD). They have been given a CD designation e.g. CD₁, CD₂ etc.
• Newly recognized molecule is designated as ‘workshop' candidates CD_w and this is given to incompletely characterized molecules. Each CD molecule is characterized by certain function.
• B cell surface molecules which representing different receptors have been designated with different CD numbers. More than 200 CD molecules have been defined so far.

Major B cell surface receptors

• B cells receptor (Ig molecules along with Igα and Igβ)
• Immunoglobulin receptors
• Complement receptors
• Cytokine receptors
• Histocompatibility MHC- ClassII molecules. 
• CD19, CD 20, CD 21 ,CD40 etc.