# 60 Action of B-lymphocytes

A B-lymphocyte places some of its specific receptor molecules in its cell surface membrane. If it encounters an antigen that binds with this receptor, the B-lymphocyte is activated. It divides repeatedly by mitosis to produce a clone of genetically identical plasma cells.

Some of these synthesise and secrete large quantities of proteins called immmunoglobulins or antibodies. The antibodies have the same binding sites as the specific receptors in the B-lymphocyte's membrane, so they can bind with the antigens. This maydirectly destroy or neutralise the antigens, or it may make it easier for phagocytes to destroy them.

(* MHC - major histocompatibility complex)

Some of the clone of B-lymphocyte cells become memory cells. These remain in the blood for many years. They are able to divide rapidly to produce plasma cells if the same antigen invades the body again. More antibody is therefore secreted more rapidly than when the first invasion happened, and it is likely that the pathogens will be destroyed before they have a chance to reproduce. The person has become immune to this pathogen.

Antibodies

Antibodies are glycoproteins called immunoglobulins that are secreted by plasma cells in response to the presence of antigens.

The variable region of the immunoglobulin molecule is specific to the particular clone of B-lymphocytes that secreted it. It is able to bind with a particular type of antigen molecule.

Immunoglobulins can:

• stick bacteria together, making it impossible for them to divide or making it easier for phagocytes to destroy them;
• neutralise toxins (poisonous chemicals) produced by pathogens;
• prevent bacteria from sticking to body tissues;
• bind to viruses and prevent them infecting cells.

Syllabus 2015   • The immune system (c) describe the modes of action of B-lymphocytes  (e) explain the role of memory cells in long-term immunity; (f) relate the molecular structure of antibodies to their functions;

Syllabus 2016 - 2018 11.1  The immune system b)   describe the modes of action of B-lymphocytes  e)   explain the role of memory cells in long-term immunity f) explain, with reference to myasthenia gravis, that  the immune system sometimes fails to distinguish between self and non-self