The B in B cell comes from an organ found in birds, not from bone marrow
Immunologists often assume B cells are named for bone marrow, where mammals make them. In fact the letter honours the bursa of Fabricius, a lymphoid organ in birds where Chang and Glick first found them. T cells, at least, earn their letter honestly: they mature in the thymus.
Lymphocytes are white blood cells, and they got their name because they are the main cell in lymph. They make up between 18 and 42 percent of the white cells circulating in blood. The three big families are T cells, B cells and natural killer cells. Under the microscope they look like a dark nucleus, roughly the size of a red blood cell at about 7 micrometres, with barely any cytoplasm, and T and B cells are indistinguishable by eye; telling them apart takes flow cytometry, which reads the proteins on their surfaces.
All of them descend from one common lymphoid ancestor produced in the bone marrow. After maturing, they patrol the blood, spleen and lymph nodes, hopping from node to node, unlike the stay-at-home macrophages. T and B cells form the adaptive arm of immunity: each recognises a specific foreign molecule, and once triggered they split into effector cells, which fire antibodies, release toxic granules or rally other cells, and memory cells, which can survive for weeks to several years, far longer than most white cells, ready for the same invader.
Natural killer cells belong to innate immunity. They spot virus-infected and tumour cells by changes in a surface marker called MHC class I and destroy them with toxic granules. Their name reflects that they need no prior activation to kill cells missing that marker. In some cancers, including melanoma and colorectal tumours, lymphocytes invade and attack the growth, occasionally shrinking it.
Counting them reveals a lot. A rise usually points to a viral infection, while HIV works by destroying CD4 helper T cells, and tracking their percentage shows how the infection is progressing. There is even a disputed extra type, the X cell, reported to carry both B- and T-cell receptors.
Source: Lymphocyte
