Leukaemia: The uninhibited, malignant reproduction of white blood cells is known as leukaemia. It is characterized by a very high count of the malignant cells in the blood. Leukaemias can be lymphocytic, myelocytic or monocytic.

Leukocyte is the general name for white blood cells. They include lymphocytes, polymorph nucleus leukocytes and monocytes.

Light chain: The light chain of the immunoglobulins is the shorter of the two chains which make up all immunoglobulins. It consists of a V and a C domain and is bound to the heavy chain by disulfide bridges. There are two classes of light chains which are also called k and I chains.

Lymphatic organs are structure tissues in which high numbers of lymphocytes interact with a non-lymphatic stroma. The primary lymphatic organs in which lymphocytes are formed are the thymus gland and the bone marrow. The most important secondary lymphatic organs in which acquired immune responses are triggered are the lymph nodes, the spleen, the tonsils (tonsillae palatina) and Peyer's Plaques along the small intestine.

Lymphatic system: The lymphatic system consists of the lymph vessel system and the lymphatic organs.

Lymphoblast: Stem cells of the lymphocytes.

Lymphocytes: All acquired immune responses are brought about by lymphocytes. Lymphocytes possess gene segments which can be recombined and code surface receptors for antigens. There are two main classes of lymphocytes: the B lymphocytes bring about the humoral immune response and the T lymphocytes the cellular response. Small lymphocytes possess very little cytoplasm, and their chromatin in the cell nucleus is condensed. Upon contact with an antigen, the cells enlarge into lymphoblasts, divide and differentiate into antigen-specific effector cells.

Lymphokines are cytokines produced by lymphocytes.

Lymph follicles consist of groups of B cells which arrange themselves around a tight-meshed network of follicular dendritic cells. The origin of these cells is unknown. They have long, branched extensions which make close contact with various B cells. Follicular dendritic cells possess non-phagocytic Fc receptors which enable them to secure antibody-antigen complexes on their surface over longer periods of time. These complexes play an important role in the selection of antigen-binding B cells during the antibody reaction.

Lymph nodes are secondary lymphatic organs in which the acquired immune response is triggered. They are found at the intersections of many lymph vessels where antigens come into contact with antigen-displaying cells displaying the antigens to the numerous lymphocytes circulating through the lymph nodes. Some of these lymphocytes recognize the antigen and produce an acquired immune response by their reaction.

Lymph vessels are thin-walled vessels in which the lymph - the extracellular liquid in tissue - is transported through the lymph nodes into the superior vena cava.

Lysozyme is an enzyme which renders pathogens in saliva and tears harmless.