B celln. Any of a class of lymphocytes that, when activated by a specific antigen and usually also by helper T cells, proliferate and differentiate into antibody-secreting plasma cells or memory cells. In mammals, B cells mature in the bone marrow. Also called B lymphocyte.[b(ursa-dependent) cell, after the bursa of Fabricius, a gland in birds where the cells mature and the first such tissue found in vertebrates.]American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.B cell n. 1. Also called B lymphocyte. a type of white blood cell that circulates in the blood and lymph and produces antibody upon encountering any antigen that has a molecular arrangement complementary to the antibody. 2. beta cell. [1970?75] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.B cell Any of the lymphocytes that develop into plasma cells in the presence of a specific antigen, such as a bacterium or virus. The plasma cells then produce antibodies that attack or neutralize the antigen. B cells mature in the bone marrow before being released into the blood. See Note at antibody.The American Heritage® Student Science Dictionary, Second Edition. Copyright © 2014 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.