leukocyte adhesion deficiencies (LADs) A class of immunodeficiency diseases in which the ability of leukocytes to enter sites infected by extracellular pathogens is affected, imparing elimination of infection. There are several different causes, including a deficiency of the common β chain of the leukocyte integrins.
leukocyte adhesion deficiency type 2 Disease causes by defects in the production of sulfated sialyl-LewisX that prevent neutrophils from interacting with Pand E-selectin, eliminating their ability to migrate properly to sites of infection.
leukocyte functional antigens (LFAs) Cell-adhesion molecules on leukocytes that were initially defined using monoclonal antibodies. LFA-1 is a β2 integrin; LFA-2 (now usually called CD2) is a member of the immunoglobulin superfamily, as is LFA-3 (now called CD58). LFA-1 is particularly important in T-cell adhesion to endothelial cells and antigen-presenting cells.
leukocyte receptor complex (LRC) A large cluster of immunoglobulin-like receptor genes that includes the killer cell immunoglobulin-like receptor (KIR) genes.
leukocytosis The presence of increased numbers of leukocytes in the blood. It is commonly seen in acute infection.
leukotrienes   Lipid mediators of inﬂammation that are derived from arachidonic acid. They are produced by macrophages and other cells.
LFA-1 See leukocyte functional antigens.
LGP2 A member of the RLR family, it cooperates with RIG-I and MDA-5 in the recognition of viral RNA.
licensing The activation of a dendritic cell so that it is able to present antigen to naive T cells and activate them.
light chain, L chain The smaller of the two types of polypeptide chains that make up an immunoglobulin molecule. It consists of one V and one C domain, and is disulfide-bonded to the heavy chain. There are two classes, or isotypes, of light chain, known as κ and λ, which are produced from separate genetic loci.
light-chain variable region (VL)   Referring to the V region of the light chain of an immunoglobulin.
light zone See germinal center.
lingual tonsils Paired masses of organized peripheral lymphoid tissue situated at the base of the tongue, in which adaptive immune responses can be initiated. They are part of the mucosal immune system. See also palatine tonsils.
linked recognition The rule that for a helper T cell to be able to activate a B cell, the epitopes recognized by the B cell and the helper T cell have to be derived from the same antigen (that is, they must originally have been physically linked).
linker for activation of T cells A cytoplasmic adaptor protein with several tyrosines that become phosphorylated by the tyrosine kinase ZAP-70. It helps to coordinate downstream signaling events in T-cell activation.
LIP10 A cleaved fragment of invariant chain retaining the transmembrane segments that remains bound to MHC class II proteins and helps target the complex to the endosome.
LIP22 The initial cleaved fragment of invariant chain bound to MHC class II molecules.
lipid bodies Storage organelles rich in neutral lipids within the cytoplasm.
lipocalin-2 An antimicrobial peptide produced in abundance by neutrophils and mucosal epithelial cells that inhibits bacterial and fungal growth by limiting availability of iron.
lipopeptide antigens A diverse set of antigens derived from microbial lipids typically presented by nonclassical MHC class Ib molecules such as CD1 molecules to invariant T-cell populations, including iNKT cells.
lipopolysaccharide (LPS) The surface lipopolysaccharide of Gram-negative bacteria, which stimulates TLR-4 on macrophages and dendritic cells.