Your immune system is like the military - with two main branches, the innate immune response and the adaptive immune response.
The innate immune response is immediate and non-specific, meaning that although it can distinguish an invader from a human cell, it doesn’t distinguish one invader from another invader.
In contrast, the adaptive immune response is highly specific for each invader, and that’s because the cells of the adaptive immune response have receptors that differentiate friendly bacteria and potentially deadly ones from their unique parts - called antigens.
This adaptive immune response takes days to weeks to become activated, but is also responsible for immunologic memory.
Now, the key cells of the adaptive immune response are the lymphocytes- the B and T cells -which are generated during lymphopoiesis.
Lymphopoiesis has three goals - first, to generate a diverse set of lymphocytes - each with a unique antigen receptor, second, to get rid of lymphocytes that have receptors that are self-reactive meaning that they’ll bind to healthy tissue, and third, to allow lymphocytes that aren’t self-reactive to continue maturing in secondary lymphoid tissue.
Normally, hematopoietic stem cells, within the bone marrow mature into a common lymphoid progenitor cell, which then becomes either a B-cell or a T-cell.
To become a B cell, it has to develop into an immature B-cell in the bone marrow and then complete its maturation into an antibody secreting B cell, called a plasma cell, in the lymph nodes and spleen.
To become a T cell, it has to migrate to the thymus and become a thymocyte, where it completes its development into a mature T cell.
So, “B” for bone marrow and “T” for thymus.