Though they have learning ability, natural killer (NK) cells are conventionally designated as innate immunity lymphocytes. The adaptive immunity system designates 2 lymphocyte cell types: B & T.
T cells take their title from the thymus gland in the neck where these cells mature. In birds B cells mature in a lymphoid organ called the bursa of Fabricus; hence their designation (from where they were discovered). Only birds have a bursa. Mammalian B cells arise from the stem cells which give rise to all types of blood cells.
All lymphocytes are born in bone marrow. T lymphocytes travel to the thymus to mature.
During fetal development, mammalian B cells mature in the liver. After birth, some B cells mature in bone marrow, while others travel to mucous membranes, particularly the gut and lungs, to mature.
B cells surveil for infection by looking for foreign organic substances. This requires knowing what is native (versus foreign). Thus arises the problem of self-tolerance: not mistaking beneficent cells for malignant ones. This takes training. In bone marrow, which is sterile, the investigative receptors of B cells are edited by self-antigens to recognize the body’s own cells.
Then there is the issue of not having B cells attack the friendly bacteria that belong to the body’s microbiome. Hence B cells are sent to mucous membranes, where they are trained with antigens from resident microbes, to recognize the little critters as compatriots, not pathogens.
After T and B lymphocytes mature, they migrate to the lymph nodes and spleen, where they take up residence.
The different types of lymphocytes have different life spans. B cells live a week. T cell life span varies considerably, though typically a week or 2. Serving in the line of duty shortens a T cell’s life expectancy.