SLP-65 A scaffold protein in B cells that recruits proteins involved in the intracellular signaling pathway from the antigen receptor. Also called BLNK.
SLP-76 A scaffold protein involved in the antigen-receptor signaling pathway in lymphocytes.
small G proteins   Single-subunit G proteins, such as Ras, that act as intracellular signaling molecules downstream of many transmembrane signaling events. Also called small GTPases.
small pre-B cell Stage in B-cell development immediately after the large pre-B cell in which cell proliferation ceases and light-chain gene rearrangement commences.
somatic diversification theories Gerenal hypotheses proposing that the immunoglobulin repertoire was formed from a small number of V genes that diversified in somatic cells. Cf. germline theory.
somatic DNA recombination DNA recombination that takes place in somatic cells (to distinguish it from the recombination that takes place during meiosis and gamete formation).
somatic gene therapy The introduction of functional genes into somatic cells to treat disease.
somatic hypermutation   Mutations in V-region DNA of rearranged immunoglobulin genes that produce variant immunoglobulins, some of which bind antigen with a higher affinity. These mutations affect only somatic cells and are not inherited through germline transmission.
spacer See 12/23 rule.
sphingolipids A class of membrane lipid containing sphingosine (2-amino4-octadecene-1,3-diole), an amino alcohol with unsaturated 18-hydrocarbon chain.
sphingosine 1-phosphate (S1P) A phospholipid with chemotactic activity that controls the egress of T cells from lymph nodes.
sphingosine 1-phosphate receptor (S1P1) A G-protein-coupled receptor activated by sphingosine 1-phosphate, a lipid mediator in the blood that regulates several physiologic processes, including the trafficking of naive lymphocytes from tissues into the blood.
spleen An organ in the upper left side of the peritoneal cavity containing a red pulp, involved in removing senescent blood cells, and a white pulp of lymphoid cells that respond to antigens delivered to the spleen by the blood.
S-protein (vitronectin) Plasma protein that binds incompletely formed MAC complexes, such as C5b67, preventing bystander complement damage to host membranes.
Spt5 A transcription elongation factor required for isotype switching in B cells that functions in associatioin with RNA polymerase to enable recruitment of AID to its targets in the genome.
SR-A I, SR-A II See scavenger receptors.
Src-family protein tyrosine kinases   Receptor-associated protein tyrosine kinases characterized by Src-homology protein domains (SH1, SH2, and SH3). The SH1 domain contains the kinase, the SH2 domain can bind phosphotyrosine residues, and the SH3 domain can interact with proline-rich regions in other proteins. In T cells and B cells they are involved in relaying signals from the antigen receptor.
staphylococcal complement inhibitor (SCIN) Staphylococcal protein that inhibits the activity of the classical and alternative C3 convertases, promoting the evasion of destruction by complement.
staphylococcal enterotoxins (SEs) Secreted toxins produced by some staphylococci, which cause food poisoning and also stimulate many T cells by binding to MHC class II molecules and the Vβ domain of certain T-cell receptors, acting as superantigens.
staphylococcal protein A (Spa) Staphylococcal protein that blocks the binding of the antibody Fc region with C1, thereby preventing complement activation.
staphylokinase Staphylococcal protease that cleaves immunoglobulins bound to its surface, thereby preventing complement activation.