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Cells of the Immune System - B Cells

The immune system is the body’s main defence against infection. In order to function properly, the immune system must be able to detect and protect against infinite agents such as pathogens including viruses and bacteria and unhealthy or infected cells. In order to do this, various cells are required to carry out specific functions. In the article below, an overview of B cells will be discussed including their function and development.

What is a B Cell?

Most of our immune system is dependent on the huge diversity of immunoglobulin receptors called antibodies which have the ability to bind to antigens. A single person can produce 10 to 100 million different immunoglobulin molecules, each having different antigen-binding specificity. The cells that produce these antibodies are called B cells (Eibel et al., 2014). B cells are a type of white blood cell and are an essential component of the adaptive immune system. B cells are produced from haematopoietic stem cells in the bone marrow and migrate to areas of infection where they carry out their effector functions (Chaplin, 2010).

Antibodies

B cells produce antibodies or immunoglobulin molecules. Antibodies can be compared to receptors that survey the body for antigens. They are four-chained V shaped proteins that consist of a heavy chain and a light chain. The heavy chain can be subdivided into five types, IgG, IgA, IgM, IgD, IgE. The light chain is variable and recognises a multitude of different antigens. It undergoes recombination events which aid in produces many different fragments that can be recognise various antigens (Schroeder & Cavacini, 2010).

Once antigens bind to the designated antibodies, they induce a range of effects. These include antibody-dependent cellular cytotoxicity, complement activation or adherence of a molecular pattern from the pathogen on to one of the receptors. B cells can also produce cytokines which can recruit cells to a site of infection whilst also stimulating the differentiation and proliferation of T cells (LeBien & Tedder, 2008).

Figure 1:Photo of B Cell with surface bound antibodies and soluble antibodies. (Taken from Bailey, 2020)

 B Cell Development

 During development, B cells undergo two different types of selection in the bone marrow. This includes positive selection and negative selection. Positive selection determines which cells have an effective B cell receptor and induces cell death on cells that cannot bind effectively to their ligand. Negative selection determines which B cells bind to self-antigens, and thus causes those cells to undergo one of four fates, clonal deletion, receptor editing, anergy or ignorance. This negative selection helps with the process of central tolerance to prevent B cells from binding to self-antigens in the bone marrow (Pieper et al., 2013)

Following this selection process, B cells need to complete development. After selection, B cells migrate to the spleen where they transition from T1 B cells to T2 B cells. T2 B cells are divided into marginal zone B cells and mature B cells. These then become either naïve B cells or mature B cells (Pieper et al., 2013).

B cell activation occurs in the secondary lymphoid organs such as the spleen and the lymph nodes. B cell activation occurs when the B cell receptor binds either a soluble or membrane-bound antigen (Pierce, 2009). Additionally, B cells require accessory signals from professional antigen-presenting cells such as an armed helper T cell or other microbial constituents. An armed helper T cell is one that has been activated by a specific antigen and can therefore cause the activation and differentiation of the B cell. Furthermore, B cells also have a co-receptor on their surface that consists of either CD19/CD21/CD81, and these are responsible for enhancing the B-cell responsiveness to antigen (Pierce, 2009).

Once B cells have carried out their function based on antigen recognition, such as antibody-dependent cellular cytotoxicity, they can become memory B cells. These cells recognise an antigen that has previously caused infection in the host, so when the antigen re-infects the body, the host is equipped with these cells to immediately recognise and kill the invading pathogen (Eibel et al., 2014).

References

Eibel, H., Kraus, H., Sic, H., Kienzler, A.K., and Rizzi, M. (2014) B cell biology: an overview. Curr Allergy Asthma Rep 14: 434.

LeBien, T.W., and Tedder, T.F. (2008) B lymphocytes: how they develop and function. Blood 112: 1570-1580.

Pieper, K., Grimbacher, B., and Eibel, H. (2013) B-cell biology and development. J Allergy Clin Immunol 131: 959-971.

Pierce, S.K. (2009) Understanding B cell activation: from single molecule tracking, through Tolls, to stalking memory in malaria. Immunol Res 43: 85-97.

Schroeder, H.W., Jr., and Cavacini, L. (2010) Structure and function of immunoglobulins. J Allergy Clin Immunol 125: S41-52.

27th Mar 2020 Sarah Donovan MSc

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