Lymphoid organs

They are divided by function:

• Primary lymphoid organs – where lymphocytes (an important kind of white blood cell) are produced, mature and differentiate: the bone marrow and the thymus gland.
• Secondary lymphoid organs – where lymphocytes are stored in large numbers and meet antigens, and where they mature further and become activated: the spleen, tonsils, Peyer's patches, appendix and lymph nodes.

(a) Bone marrow

• A soft tissue inside the flat bones (clavicle, sternum, skull, vertebrae, ribs, shoulder blade, pelvis) and in the heads of long bones (femur, tibia, humerus).
• It produces all types of white blood cells, and all lymphocytes mature there except T lymphocytes (their maturation is completed in the thymus).
• B cells and natural killer cells therefore mature in the bone marrow.

(b) Thymus gland

• Situated in front of the trachea above the heart and behind the sternum.
• It secretes the hormone thymosin, which stimulates lymphoid stem cells to mature into T cells and to differentiate into the different T-cell types inside the gland.
• The thymus is therefore the site of maturation of T lymphocytes.
• A person with too little thymosin has poorly developed T-cell immunity.

(c) Spleen

• A small, dark-red lymphoid organ (roughly the size of the palm) in the upper left part of the abdominal cavity.
• It contains many macrophages, which pick up anything foreign – microbes, foreign particles – and also old (senescent) body cells such as worn-out red blood cells, and break them down into components the body can dispose of or recycle.
• It also contains lymphocytes that release antibodies into the blood against germs and viruses.

(d) Tonsils

• Two lymphoid glands on either side of the rear of the mouth.
• They trap microbes and foreign bodies that enter with food or air and prevent them from going further, protecting the body with their content of white blood cells.

(e) Peyer's patches (and the appendix)

• Aggregations of small lymphoid cells in the mucous membrane lining the lower part of the small intestine.
• They take part in the immune response against pathogens that enter the gut.
• The appendix has a similar immune role.

(f) Lymph nodes

• Small bean-shaped structures (from pinhead size to the size of a small bean) situated along the lymphatic vessels in all parts of the body: under the armpits, at the sides of the neck, in the upper thigh, and near internal organs.
• Each node is connected to several lymph vessels bringing lymph from the tissues.
• The node filters lymph, removing pathogens and debris, and stores lymphocytes.
• Internally it is divided into compartments filled with B lymphocytes, T lymphocytes, macrophages and other white blood cells that destroy germs and cell debris.
• During infection the nodes swell, because lymphocytes multiply inside them.