B Lymphocytes

The lymphoid cells (plasma cells and lymphocytes) are responsible for specific immunity. The three types of lymphocyte cells are B cells, T cells and natural killer (NK) cells. Here, let’s learn more about the B cells or B lymphocytes in detail.

Table of Contents

• What are Lymphocytes?
• B Lymphocytes
• B Cells Function
• Activation of B Lymphocytes
• Frequently Asked Questions

What are Lymphocytes?

Lymphocytes are a type of leukocytes that constitute 20 to 40 per cent of the body’s total white blood cells and 99 per cent of the cells in the lymph. On the basis of function and cell-membrane components, they can broadly be subdivided into three types:

1. B cells
2. T cells
3. Natural killer cells or NK cells

Lymphocyte Structure – Lymphocytes are small, round cells found in lymphoid organs, peripheral blood, lymph and many other tissues. An average human body contains about 1000 to 4800 lymphocytes in one microlitre of blood.

Lymphocytes have antigen recognition mechanisms on their surface, enabling each cell to recognise only one antigen. The reaction of an immunocompetent cell to its specific antigen is based on the initiation of the immune response. The nature of the immune response depends on whether the lymphocyte is a B or T cell. Stimulated T cells produce certain activation products (lymphokines) and induce CMI (cell-mediated immunity), while stimulated B cells divide and transform into plasma cells which synthesise immunoglobulins.

B Lymphocytes

The bone marrow is the location of B cell synthesis and development in humans and other animals. With the aid of cytokines, immature B cells derived from lymphoid progenitors multiply and develop inside the bone marrow. The Bursa of Fabricius is the major location for B cell development in birds, comparable to the human bone marrow.

A selection mechanism within the bone marrow removes B cells with self-reactive antibody receptors. Also all lymphocytes develop in the bone marrow. While T cells develop in the thymus, B cells develop in the bone marrow itself.

B Cells Structure and Location

B lymphocytes have immunoglobulin on their surface. They possess either surface IgM alone or in combination with IgA or IgG, depending on the production of a particular immunoglobulin class. Also there are numerous microvilli on the surface of B lymphocytes when these cells are viewed under a scanning microscope.

The concentration of B cells in various locations are as follows.

B Cells Function

B cells generate antibodies to protect the human body from invading antigens. B cell receptors (BCRs) are found on the outer surface of B cells and are used to bind to a particular protein.

When B cells bond with this protein termed antigen, antibodies are released. They adhere to the antigen and prohibit it from damaging the body. The B cells then generate cytokines in order to recruit other immune cells.

This antibody-mediated immune response is termed humoral immunity. Thus, B cells are the components that are associated with this humoral immune response.

Plasma Cells

Antigenically stimulated B lymphocytes undergo blast transformation to become plasmablasts. They are intermediate, short-lived transitional cells which later become plasma cells. Plasma cells are non-proliferating and long-lived cells that secrete antibodies. Also, they are the principal antibody-producing cells.

See more: T Lymphocytes

Activation of B Lymphocytes

When infectious pathogens penetrate the body, fragments of their machinery may be seen on the surface of their cells. These fragments are known as antigens. B lymphocytes become activated when they contact them and identify those antigens. B cell receptors (BCRs) are found on the surface of B cells and bind to certain antigens. The activation process begins when the cell makes a connection with the antigens.

B lymphocytes recognise antigens by the structures present on their surface. The offshoots from a single B cell also create the same antibodies and recall the antigens. This memory implies that B cells create antibodies that counteract the original antigen, defending the immune system against a second invasion. This activation of B cells can be either T cell-dependent or independent.

1. T cell-dependent activation – T cell-dependent antigens are antigens that cannot be directly stimulated by B cells to produce antibodies without the aid of T cells. This promotes cytokine production. Interferons, interleukins, and growth factors are all examples of cytokines. Cytokines have a role in B cell activation, differentiation, and proliferation.
2. T cell-independent activation – T cell-independent antigens are antigens that have the ability to directly stimulate B cells to produce antibodies without the aid of T cells. Polymeric antigens, such as polysaccharides, are T cell-independent antigens.

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