Lamella-bone - the microscopic structure of cortical bone gives it the appearance of concentric or parallel plates ( from Latin, lamella, the diminutive of lamina, meaning a plate or leaf).     Lamina propria - the layer of loose connective tissue underneath the epithelium of mucosa, which provides physical and nutritional support.     Lamina-dura - the name given to the radiographic appearance of a dense layer of bone around the tooth root. It represents the dense cortical bone lining the tooth socket.     Laminin - an adhesive molecule of connective tissue related to fibronectin andtenascin.     Langherhans cells are active in the immune response of the skin and mucous membrane. They act as sentries, detecting the presence of foreign antigens on the surface of the epithElium. They do not contain keratin and are thus sometimes called clear cells.     Lectin - a protein molecule which bindson to a specific sequence of sugars. Bacteria may use lectin attachments to bind onto each other or oral surfaces.     Leucocytes - un pigmented (white) cells of the blood. Those with granular cy lasm are neutrophils, eosinophils and basophils. The agranulocytes are lymphocytes and monocytes.     Leucotriens- concerned with signalling between cells of the immune system and a member of the eicosanoid family of hormones.     Ligand - a protein molecule which binds to another specific protein molecule. The forces of the bond are week and thus protein-ligand bonds depend on close fit of one molecule to the other, so as to capture as many bonding sites as possible. Ligands are specific for a particular protein. They are found on cell surfaces of microorganisms where they assist in cell adhesion. They are also sights on cell membranes onto which protein messengers attach such ascytokines (see also lectins).     Limbic system - a ring of structures around the thalamus which play a major role in pain as well as other types of behaviour. The limbic system includes the hypothalamus, hippocampus, amygdala, septum and cingulum. The limbic system plays an important role in pain at the level of motivation to avoid it. It thus operates at a slightly higher level than the reticular formation with strong connections to the thalamus and cortex.     Lipid - - large molecules containing hydrogen and carbon which are insoluble in water. Simple lipids consist of long chains of fatty acids. Compound lipids contain phosphoric acid, sugars, nitrogenous bases or proteins, and include the phospholipids, glycolipids and lipoproteins. Steroids may also be classified as lipids.     Lubrication - helping two surfaces to slide over each other.     Lycine - one of 20 aminoacids common in proteins. It is a common amino acid of collagen and like proline must be hydroxylated by ascorbic acid in order to allow the formation of bonds which will hold the triple helix together..     Lymphocytes - white cells involved in the immune response. B lymphocytes are so called because they mature in bone while T lymphocytes mature in the thymus. Both cells look alike until they recognise a foreign antigen. The B cell starts to make antibodies while the T lymphocytes accumulate vesicles loaded with cytotoxic agents. On contact with a foreign cell, the lymphocytes changes shape so that all it vesicles are pointed at the enemy. The release of cytotoxic agents need to be carefully controlled. One of the methods by which the enemy cell is killed is by agents which make holes in its cell membrane. Enemy cells maybe bacteria, or the bodies own cells which have ingested viruses or they may be cancer cells, or the cells of transplanted organs.     Lymphokines - a variety of cytokines released by lymphocytes which coordinate the proliferation of T and B lymphocytes. They also regulate the brains contribution to the immune response via the hypothalamus - adrenal cortex axis.     Lysosomes - small membrane bound vesicles in the cy lasm of cells which contain toxic enzymes. When a cell dies, these membranes rupture and the enzymes are released. They break down the cells structure, and the debris is removed. The lysosome also contains cytokines which summon inflammatory cells and stimulate inflammation. The contents of lysosomes can be released by macrophages and neutrophils both to kill bacteria and viruses, and to stimulate inflammation.