125 lysozyme

L

Particular populations of lymphocytes can be distinguished according to their CD antigens. A calm (metabolically “sleeping”) lympho-cyte is activated by an antigen or mitogen and becomes a metabolically activated lympho-blast, which in association with cytokines is then subject to division and differentiation, whereby memory and efficient lymphocytes, which are involved in immune responses, de-velop.

Lymphocytes – circulation The move-ment of lymphocytes in the body takes place between the blood and lymphatic system, lymph nodes, spleen and tissues. Less than 10% of the total volume of lymphocytes is constantly circulating, while the rest are de-posited in tissues and organs. Approximately 70% of circulating lymphocytes have the ca-pability to re-circulate, i.e. they move in a cer-tain cycle during which they leave the sys-temic circulation and return back to lymph nodes, lymphoid follicles and the spleen, from where the cycle may recommence. These cells are mostly long-life mature T lym-phocytes, but also memory B lymphocytes. Approximately 30% of lymphocytes in the vascular space are not subject to re-circula-tion. These are mostly short-lived immature T and B cells that terminate their life in ves-sels, or can be activated and then leave the vascular space. The re-circulation enables lymphocytes to perform immune surveil-lance throughout all of the body, mobilise themselves to localities where immune pro-tection is needed and also reactivate non-functional cells.

Lymphoid organs Organs in which lym-phocytes prevail but other cells are also pres-ent. They are divided into primary and sec-ondary lymphoid organs. The thymus and bone marrow, which is considered to be the equivalent of the bursa of Fabricius in birds, are amongst the primary lymphoid organs. B lymphocytes (bone marrow) or T lympho-cytes (thymus) mature in primary lymphoid organs. Secondary lymphoid organs comprise lymph nodes, the spleen, lymphoid tissue in mucosa, and tonsils. These are the places

where, during the initiation of immune re-sponses, B and T lymphocytes react with an-tigen presenting cells.

Lymphomatoid granulomatosis (LG) A rare multi-systemic disease from the group of vasculitides which is characterised by focal and transmural infiltration of the vascular wall by lymphocytes, plasma cells and histio-cytes.

Clinical symptoms: Dependent on the se-verity of organ impairment. Most frequent are pulmonary symptoms (cough, dyspnoea, chest pain) and skin rashes (erythema, indu-ration). Fever, malaise and weight loss are non-specific symptoms.

Lymphotoxin (LT) One of the tumour ne-crotizing factors (TNF-β). It is produced mainly by TH1 lymphocytes after antigen or mitogen stimulation. LT inhibits the growth of tumours both in vivo and in vitro and blocks tumour transformation of cells in-voked by carcinogens. LT is also involved in protection against viral and saprophytic in-fections. It was one of the first lymphokines to be detected and currently is considered to be a member of the cytotoxin group.

Lysosomes They are organelles localised in the cytoplasm of all cells that contain a nucle-us. They are notably abundant in the cyto-plasm of professional phagocytes (phagolyso-some). Typically they contain hydrolytic (lysosomal) enzymes. Lysosomes of phago-cytes contain different antimicrobial sub-stances, typical neutral proteases and compo-nents of certain receptors (granules of leuko-cytes). Antibodies against lysosomal enzymes of leukocytes may be produced (ANCA). These antibodies are associated with certain vasculitides and other autoimmune diseases.

Lysozyme An enzyme that lyses the β-1,4-glycosidic linkage of polysaccharides. The older name of this enzyme was muramidase. It occurs in egg white, tears, saliva, nasal se-cretions, skin, specific granules of leukocytes and in serum. Lysozyme is able to lyse cell membrane glycoproteins of certain gram-

lysozyme 126

L

positive (mostly non-pathogenic) bacteria and thereby cause their osmotic lysis. It can only destroy other micro-organisms with the

interaction of other antimicrobial factors (complement, secretory IgA, leukocyte’s pro-teases etc.).