A lymphocyte's journey: primary and secondary lymphoid organs.

A lymphocyte's route: made in bone marrow, matured in bone marrow/thymus (primary), meets antigen and proliferates in spleen, nodes, MALT (secondary)

Turn a new T cell into a dividing effector clone, visiting organs in a valid order.

Listing the thymus or bone marrow as secondary lymphoid organs.

• The immune system is made of lymphoid organs, tissues, cells and soluble molecules such as antibodies. It recognises foreign antigens, responds to them and remembers them, and it is involved in allergy, auto-immune disease and organ transplantation.
• Lymphoid organs are where lymphocytes originate and/or mature and proliferate.
• Primary lymphoid organs are the bone marrow and thymus, where immature lymphocytes become antigen-sensitive lymphocytes.
• Secondary lymphoid organs are the spleen, lymph nodes, tonsils, Peyer's patches of the small intestine and the appendix. Mature lymphocytes migrate here, meet antigens and proliferate into effector cells.
• Bone marrow is the main lymphoid organ: all blood cells, lymphocytes included, are made there. Bone marrow and thymus both provide micro-environments for T-lymphocytes to develop and mature.
• The thymus is a lobed organ near the heart, beneath the breastbone. It is quite large at birth and keeps shrinking with age, becoming very small by puberty.
• The spleen is a large bean-shaped organ holding mainly lymphocytes and phagocytes. It filters blood by trapping blood-borne micro-organisms and also holds a large reservoir of erythrocytes.
• Lymph nodes are small solid structures along the lymphatic system. They trap micro-organisms and antigens that enter lymph and tissue fluid, and the trapped antigens activate the lymphocytes there.
• Mucosa-associated lymphoid tissue (MALT) lines the respiratory, digestive and urogenital tracts and makes up about 50 per cent of the lymphoid tissue in the human body.

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