killer cell lectin-like receptors (KLRs) Large family of receptors present on NK cells, through which the cells’ cytotoxic activity is controlled. The family contains both activating and inhibitory receptors.
kinase suppressor of Ras A scaffold protein in the Raf–MEK1–Erk MAPkinase cascade that binds to all three members following antigen receptor signaling to facilitate their interactions and to accelerate the signaling cascade.
kinin system An enzymatic cascade of plasma proteins that is triggered by tissue damage to produce several inﬂammatory mediators, including the vasoactive peptide bradykinin.
Kostmann’s disease A form of severe congenital neutropenia, an inherited condition in which the neutrophil count is low. In Kostmann’s disease, this is due to a deficiency of the mitochondrial protein HAX1, which leads to apoptosis of developing myeloid cells and persistent neutropenia.
KSR See kinase suppressor of Ras.
Ku A DNA repair protein required for immunoglobulin and T-cell receptor gene rearrangement.
Kupffer cells Phagocytes lining the hepatic sinusoids; they remove debris and dying cells from the blood, but are not known to elicit immune responses.
kynurenine metabolites Various compounds derived from tryptophan through the actions of the enzymes indolamine-2,3-dioxygenase (IDO) or tryptophan-2,3-dioxygenase (TDO) expressed in various immune cells or the liver.
λ chain One of the two classes or isotypes of immunoglobulin light chains.
λ5 See surrogate light chain.
L-selectin Adhesion molecule of the selectin family found on lymphocytes. L-selectin binds to CD34 and GlyCAM-1 on high endothelial venules to initiate the migration of naive lymphocytes into lymphoid tissue.
lamellar bodies   Lipid-rich secretory organelles in keratinocytes and lung pneumocytes that release β-defensins into the extracellular space.
lamina propria A layer of connective tissue underlying a mucosal epithelium. It contains lymphocytes and other immune-system cells.
large pre-B cell Stage of B-cell development immediately after the pro-B cell, in which the cell expresses the pre-B-cell receptor and undergoes several rounds of division.
LAT See linker for activation of T cells.
late-phase reaction Allergic reactions that occurs several hours after initial encounter with an antigen. Thought to be manifestations of recruitment of multiple leukocyte subsets to the site of allergen exposure.
late pro-B cell   Stage in B-cell development in which VH to DJH joining occurs.
latency A state in which a virus infects a cell but does not replicate.
Lck An Src-family tyrosine kinase that associates with the cytoplasmic tails of CD4 and CD8 and phosphorylates the cytoplasmic tails of the T-cell receptor signaling chains, thus helping to activate signaling from the T-cell receptor complex once antigen has bound.
lectin A carbohydrate-binding protein.
lectin pathway Complement activation pathway that is triggered by mannose-binding lectins (MBLs) or ficolins bound to bacteria.
lentiviruses A group of retroviruses that include the human immunodeficiency virus, HIV-1. They cause disease after a long incubation period.
lethal factor An endopeptidase produced by Bacillus anthracis that cleaves NLRP1, inducing cell death within the infected cell, typically a macrophage.
leucine-rich repeat (LRR) Protein motifs that are repeated in series to form, for example, the extracellular portions of Toll-like receptors.
leukocyte A white blood cell. Leukocytes include lymphocytes, polymorphonuclear leukocytes, and monocytes.