What are B cells ?

Lymphocytes are white blood cells that play a key role in the body's disease-fighting immune. They are responsible for carrying out the activities of the immune system.

There are two major classes of lymphocytes
• B cells
• T cells

B cells develop from stem cells in the bone marrow and grow into plasma cells that are the source of antibodies. It is also called b-lymphocytes. The "B" in B- cells refers to the bursa of Fabricius which is an organ unique to birds and was first discovered in birds. It does not stand for bone marrow where all mammalian B cells are produced.

Antibodies are proteins produced by the B-cells in response to an antigen. Its main purpose is to destroy the antigen and each antibody can bind to only one specific antigen. An antigen is a foreign substance that triggers an immune response. There are millions of B cells produced each day and each and every B cell has a different antibody on its surface. Most B cells (more than 90%) die within a few days without encountering an antigen. Only a handful of the cells encounter antigen.

Each antibody is shaped like a Y made up of two identical heavy chains and light chains. The tips of the Y’s arms are a variable region that varies greatly from one antibody to another. It is these unique contours in the antigen-binding site that allow the antibody to recognize a matching antigen, much as a lock matches a key. The stem of the Y links the antibody to other participants in the immune defenses and is called the constant region. Antibodies belong to a family of large protein molecules known as immunoglobulins.

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Your body develops natural immunity only after you have been exposed to a certain organism. B cells are a defense mechanism of your immune system that prevents you from getting sick again from that particular type of virus or bacteria. B cells are a part of Humoral immunity whose antibodies circulate in blood and lymph, which the ancient Greeks called the body's "humors."

The B cells produce plasma cells which in turn produce antibodies designed specifically to fight a particular antigen. Once your body has produced a particular antibody, it can rapidly reproduce more antibodies if needed. Once the antibodies are made, they circulate in the blood and lymph nodes and attach to antigens marking them for destruction.

Once the B cell finds an antigen, the following activities take place
• B cell activation - When the B cell comes in contact with the antigen, it becomes activated. The B cell must come in contact with a T cell (called a helper T cell) to help it multiply into antibody secreting plasma cells. T cells are needed to activate many immune cells including B cells and other T cells.

• B and T cell interaction - The B cell requires T cell help for proliferation as plasma cells and memory cells. T cells produce cytokines which in turn allows the B cells t0 stimulate proliferation and differentiation.

• Differentiation into plasma cells - The differentiation of B cells into plasma cells occurs as the cell continues to divide in the presence of cytokines. This is known as "class switch". B cells are about 6um in diameter while the plasma cells are about 15 um in diameter and contain abundant endoplasmic reticulum in the cytoplasm which is necessary for the production of antibodies.

• Antibody secretion and class switch - Plasma cells are the final stage of development of B cells. They reside in the spleen and lymph nodes and secrete antibodies. Class switch is a process in which a B cell can switch the class of antibody it produces while retaining the same antigen specificity.

• B memory cells - During this process, some of the B cells mature into memory cells. These cells are generated in lymphoid tissue after B cell activation and proliferation and reside in the bone marrow, lymph nodes and spleen. Memory B cells are able to live for a long time and respond quickly upon second exposure to the antigen for which they are specific.

They are called memory cells because they retain the memory of the antigen first encountered.

Article Contributed By: Jaya Suresh


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