Monday, 5 November 2012

B-Lymphocytes

  • The letter "B" designate from its site of maturation, in the "Bursa of fabricius" in the birds; and "Bone marrow" in the mammalian species, including humans & mice.
  • B-Lymphocytes carries out "Humoral-immune response" by producing antibodies.B-cell
  • B-Lymphocytes is stimulated by "Th -lymphocytes".
  • When the antigen stimulate, the B-cell divides repeatedly & differentiate over a 4 to 5 day period, generating a population of Plasma cells & Memory cells.
  • Plasma cells lack membrane-bound antibody, synthesize and secrete one of the five classes of antibody.
  • Each of the approximately 1.5X105 molecules of antibody on the membrane of a single B- cell has an identical binding site for antigen.

Development of B-Lymphocytes:

B-cell differentiation as assessed by membrane immunoglobulin expression can be divided into antigen-independent and antigen-dependent phases. Surrounding microenvironment influences of the bone marrow i.e. drives antigen-independent differentiation of B-cells: B-cells develop "Clonal diversity" in the absence of antibody receptors for antigens.

B-cell development moves through several stages marked by the rearrangement of immunoglobulin genes, "bone marrow stromal cells" in two ways influence all of these stages:

1. By direct contact of the stromal cells with B-cell precursors in early stages of B-cell development and

2. by stromal cell-derived growth factors in later stages of B-cell development.

Progenetor B-cells use their surface molecules to bind to stromal cell "hyaluronic acid". This activity promotes "C-kit ligand" (stem cell factor) binding to C-kit encoded "Tyrosine kinase" membrane receptors.

B-lymphocyte differentiate from stem cells to pre-progenitor B-cells undergoing heavy V-region gene rearrangements, and then to progenitor B-cell that contain "Cytoplasmic -n-Chains".in the next stage , pre-B-cells undergo light chain V-J-arrangement and express low levels of membrane n-heavy chains associated with surrogate light chains. These surrogate light chains associate with each other to form a "light chain like structure". At this time, the immature B-cells express both n-heavy chains and light chains as surface Ig.M molecules. Finally, the B-cells also start to express Ig.D in addition to Ig.M. These cells are called "Mature B-cells", and exit to the periphery.

Focus point: Once stimulated antigen, the B-cells lose Ig.D immunoglobuiins A portion of the cells becomes a memory cell. B-cells then progress into plasma cells that secrete the class of Immunoglobulin lost represented on their surfaces.

1 comment:

Thank you