Cell of the day: B Cells

So I’ve decided that as well as doing reviews of recent discoveries and papers, I’m also going to do features on cells/genes/molecules that I find interesting. I’ll update these pages with more information over time, and link them to other articles on this blog. As always, words in pink will be added to the glossary

My first cell of the day is the B cell, a vital part of the adaptive immune system that helps in the body’s defense against pathogens.


What is a B cell?

A B cell is a type of white blood cell (a lymphocyte), that develops from haematopoietic stem cells and matures in the bone marrow. B cells are covered in B cell receptors (BCRs) that are attached to the cell surface and face outwards. BCRs detect and bind to antigens on pathogens* (antigens are parts of the pathogen that get detected by the immune system- e.g. RIFIN on Plasmodium). Each BCR has a unique antigen binding site, that is specific to one antigen only. There are thousands of unique B cells in the immune system, which circulate to detect invading pathogens. B cells that bind self-antigens are removed early on during maturation, as this would create an immune reaction to the host’s own tissues.

When the B cell detects its unique antigen, it travels to the peripheral lymphoid organs (the lymph nodes & spleen). The B cell is activated and transforms into a plasma cell, which secretes a mobile form of the BCR- the antibody. These antibodies are soluble copies of the bound BCRs on the B cell, and are released to search out the invading pathogen.

What do antibodies do?

When the antibodies bind to their antigen on the pathogen, several different outcomes can occur, including:

  • Agglutination: The antibodies bind to the pathogen and prevent it from functioning
  • Opsonisation: The antibodies bind and signal to other immune system molecules to destroy the pathogen, such a phagocytes (e.g. macrophages)
  • Activation of the complement cascade. The complement cascade is part of the innate immune system, which in turn activates inflammation and recruits phagocytic cells.

APCs

B cells are often stimulated by T helper (Th) cells, that induce binding to antigens. B cells are antigen-presenting cells (APCs) and do not need help in detecting antigens, however they can have antigens presented to them by other APCs, including macrophages and dendritic cells.

Immunological Memory

Immunological memory is sustained by B cells, which are stored at low levels after activation. These B cells can be re-instated quickly if a repeat infection occurs, and provide robustness to the immune system.

 


* See this post on antibodies for a malaria vaccine

Further reading

  • A good place to start for anyone interested in immunology is Janeway’s Immunobiology, a very comprehensive guide.

Image from Wikipedia commons

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