Sec61 A multisubunit transmembrane protein pore complex that resides in the membrane of the endoplasmic reticulum and allows peptides to be translocated from the ER lumen into the cytoplasm.
second messengers Small molecules or ions (such as Ca2+) that are produced in response to a signal; they act to amplify the signal and carry it to the next stage within the cell. Second messengers generally act by binding to and modifying the activities of enzymes.
secondary granules Type of granule in neutrophils that stores certain antimicrobial peptides.
secondary immune response The immune response that occurs in response to a second exposure to an antigen. In comparison with the primary response, it starts sooner after exposure, produces greater levels of antibody, and produces class-switched antibodies. It is generated by the reactivation of memory lymphocytes.
secondary immunization A second or booster injection of an antigen, given some time after the initial immunization. It stimulates a secondary immune response.
secondary immunodeficiencies Deficiencies in immune function that are a consequence of infection (e.g., HIV infection), other diseases (e.g., leukemia), malnutrition, etc..
secondary lymphoid follicle A follicle containing a germinal center of proliferating activated B cells during an ongoing adaptive immune response.
secondary lymphoid organs See peripheral lymphoid organs.
secondary lymphoid tissues See peripheral lymphoid organs.
secretory component (SC) Fragment of the polymeric immunoglobulin receptor that remains after cleavage and is attached to secreted IgA after transport across epithelial cells.
secretory IgA (SIgA) Polymeric IgA antibody (mainly dimeric) containing bound J chain and secretory component. It is the predominant form of immunoglobulin in most human secretions.
secretory phospholipase A2 Antimicrobial enzyme present in tears and saliva and also secreted by the Paneth cells of the gut.
segmented filamentous bacteria (SFB) Referring to commensal Gram-positive Firmicute species and members of the Clostridiaceae family that adhere to the intestinal wall of rodents and several other species that induce TH17 and IgA responses.
selectins Family of cell-adhesion molecules on leukocytes and endothelial cells that bind to sugar moieties on specific glycoproteins with mucin-like features.
self antigens The potential antigens on the tissues of an individual, against which an immune response is not usually made except in the case of autoimmunity.
self-tolerance The failure to make an immune response against the body’s own antigens.
sensitization The acute adaptive immune response made by susceptible individuals on first exposure to an allergen. In some of these individuals, subsequent exposure to the allergen will provoke an allergic reaction.
sensitized   In allergy, describes an individual who has made an IgE response on initial encounter with an environmental antigen and who manifests IgE-producing memory B cells. Subsequent allergen exposure can elicit an allergic response.
sepsis   Bacterial infection of the bloodstream. This is a very serious and frequently fatal condition.
septic shock Systemic shock reaction that can follow infection of the bloodstream with endotoxin-producing Gram-negative bacteria. It is caused by the systemic release of TNF-α and other cytokines. Also called endotoxic shock.
sequence motif A pattern of nucleotides or amino acids shared by different genes or proteins that often have related functions.
serine protease inhibitor (serpin) Class of proteins that inhibit various proteases, originally referring to those specific to serine proteases.
seroconversion The phase of an infection when antibodies against the infecting agent are first detectable in the blood.
serotypes   Name given to a strain of bacteria, or other pathogen, that can be distinguished from other strains of the same species by specific antibodies.