B Cell : Generation, Development, Maturation and Differentiation :
The development of Effector cells (Plasma cells) and Memory B Cell is divided into three main steps 1) Generation and Maturation of B cell, 2) Activation of Mature B cell 3) Differentiation of Mature B cell. The B-Lymphocytes is produce and Mature in Bone marrow. The immature IgM B-Cell now migrates and mature in IgM and IgD with single antigen specificity. Almost all B cells are Mature in bone marrow throughout the life but only few of these are mature. Most of these cells are circulating as Naive B cell which has short life span about 3 days to 8 weeks.
What are B Cells ?
- B cells is also known as B lymphocytes. The letter B is derived from its development and maturation site bone marrow in humans and in birds bursa of Fabricius.
- In blood 5 to 15% of the circulating B lymphocytes are present. B cells contain large nuclei, Golgi bodies and rough endoplasmic reticulum developed in their cytoplasm.
- B cells produced of membrane bound immunoglobulin (antibody) molecules which serve as receptors for antigen.
- There are generally two types of B Cell : Plasma cell and Memory B cell.
- Naive cell are those B cells which does not previously encountered with antigen. When Naive B lymphocytes first encounters with antigen it makes antigen antibody complex and divide rapidly, Now the progeny of Naive B cell differentiate into effector cells called plasma cells and into memory B cells.
- Generally Memory B cells have a longer life span than naive cells.
- Plasma cells produce the antibody. Plasma cells are end-stage cells and do not divide. Although some long-lived populations can be found in bone marrow, many die within I or 2 weeks.
- Each single plasma cell produce two thousand antibody molecules per second.
B-Cell Maturation :
The generation of mature B cells first occurs during embryonic stages and continues throughout life. Before birth, the yolk sac, fetal liver, and fetal bone marrow are the major sites of B-cell maturation; after birth, generation of mature B cells occurs in the bone marrow. B cell development and maturation begins with hematopoiesis when lymphoid precursors cells diffterentiate into B cell Lineage or Progenitor B cell. Generally B-cell maturation depends on rearrangement of immunoglobulin DNA in the lymphoid stem cell.
B-Cell Activation and Differentiation
The B-cell originate and mature in the bone marrow. The mature naive cell have cell receptors for recognizing specific antigens, these mature B cells leave the bone marrow and they recirculate between the blood, the secondary lymphoid tissues and the lymph.
When these naive these cells encounter their specific antigens they get activated and differentiated into antibody producing plasma cells and memory B cells. There are two ways by which B cell activates. This classification is based on the type of antigen encountered by the B cells.
1) T Cell independent activation
2) T cell dependent activation
1. T cell Independent B cell Activation
- B cell signaling requires clustering or cross linking of B Cell receptors on antigen recognition this clustering of B cell receptors is required to activate the accessory proteins like Ig-α and Ig-β.
- The antigen binding signal is then conveyed to the nucleus This clustering of B cell receptors depends upon the type of antigening encountered.
- For example this antigen is having multiple repeating epitopes that can directly cross link B cell receptors such antigens can activate B cells to produce antibodies without a help T cell.
- The antigens which can trigger B cell activation without T Cell help are called T independent or Thymus independent antigens and B Cell activation without T cell help is known as T independent B cell activation.
- But most of the antigens are proteins in their chemical nature and they form the largest group of antigens but they do not contain multiple repeating units this makes the cross linking of B cell receptor’s difficult.
- So when a B cell encounters with protein antigens T cell required to trigger B cell activation.
- Antigens that trigger B cell activation with the help of T helper cells are known as T dependent or Thymus dependent antigens and B cell activation which requires T cell help is known as T dependent B cell activation.
Functions of B Lymphocytes :
- B Lymphocytes are cells responsible for defending the body from various pathogens. They belong to the group of leukocytes, also called white blood cells,
- They are responsible for humoral immunity.
- B lymphocytes play a key role in pathogen-specific immunity by producing antibodies.
- The B lymphocytes have as their own function, the production of antibodies against a specific antigens.
- Antibodies are proteins called immunoglobulins or immunoglobulins that carry out various activities according to their isotype (IgG, IgM, IgA)
- These antibodies perform various functions such as: opsonins, complement activators, neutralizers of toxic substances, agglutination, neutralization of bacteria etc.
- B lymphocytes when activates they form memory B cells for faster production of antibodies for future encounter with the same Antigen.