B lymphocyte
Meaning of B lymphocyte: The cell that becomes an antibody factory, and the reason a second meeting with the same germ goes so much better than the first.
Definition of B lymphocyte
A B lymphocyte is a white blood cell that produces antibodies, the proteins that latch onto germs and mark them for destruction. Each B cell carries a receptor tuned to one specific target, and when it meets that target it multiplies. Some of the copies become plasma cells that pour out antibody, and others become memory cells that wait for a later encounter.
Where do they come from?
They are made in the bone marrow. As each cell develops, gene segments coding for its receptor are cut and rejoined in a different combination, so the body ends up holding an enormous library of specificities without needing a gene for each one. Cells whose receptor reacts against the body's own tissue are deleted or silenced before release, and the survivors circulate through blood, lymph nodes, tonsils and spleen, waiting for a match. Each is a lymphocyte, one of the two main classes of small white cell.
What happens when one meets its target?
The cell swallows the antigen, displays a fragment of it, and usually needs a helper T lymphocyte to confirm the alarm before it commits. It then multiplies inside a structure called a germinal center in a lymph node. There the receptor gene is mutated slightly and repeatedly, and cells whose antibody binds more tightly are selected to continue, so the antibody produced improves over days. The cell can also switch the class of antibody it makes, from IgM to IgG for the blood, IgA for mucous surfaces or IgE for parasites and allergy, without changing what it recognizes.
How do they differ from T lymphocytes?
| Feature | B lymphocyte | T lymphocyte |
|---|---|---|
| Where it matures | Bone marrow | Thymus |
| What it recognizes | Intact antigen, free in fluid | Fragments displayed by another cell |
| Main job | Makes antibody | Kills infected cells and directs the response |
What goes wrong with them?
Too few working B cells, or too little antibody, produces repeated sinus and chest infections, seen in inherited immune deficiencies and after drugs that deplete them. Misdirected cells make antibodies against the body's own tissue, the basis of several autoimmune diseases. Uncontrolled growth of one clone gives B cell lymphomas, chronic lymphocytic leukemia and, from their plasma cell descendants, myeloma. Each of these is a disorder of one class of white blood cell rather than of the blood as a whole.
Also known as
Used in a sentence
Flow cytometry showed an expanded population of monoclonal B lymphocytes in the peripheral blood.
Good to know
The letter B came from the bursa of Fabricius, the organ in birds where these cells were first traced, and it happens to fit the bone marrow where they mature in humans.
Word origin
Where “B lymphocyte” comes from
Lymphocyte joins the Latin lympha, meaning clear water, with the Greek kytos, meaning vessel or cell.