thymoproteasome Specialized form of the proteasome composed of a unique subunit, β 5t, that replaces β 5i (LMP7) and associates with β 1i and β 2i in the catalytic chamber.
thymus A central lymphoid organ, in which T cells develop, situated in the upper part of the middle of the chest, just behind the breastbone.
thymus-dependent antigens (TD) Antigens that elicit responses only in individuals that have T cells.
thymus-independent antigens (TI) Antigens that can elicit antibody production without the involvement of T cells. There are two types of TI antigens: the TI-1 antigens, which have intrinsic B-cell activating activity, and the TI-2 antigens, which activate B cells by having multiple identical epitopes that cross-link the B-cell receptor.
thymus leukemia antigen (TL) Nonclassical MHC class Ib molecule expressed by intestinal epithelial cells and a ligand for CD8α:α.
TI-1 antigens See thymus-independent antigens.
TI-2 antigens See thymus-independent antigens.
tickover The low-level generation of C3b continually occurring in the blood in the absence of infection.
tingible body macrophages Phagocytic cells engulfing apoptotic B cells, which are produced in large numbers in germinal centers at the height of an adaptive immune response.
TIR (for Toll–IL-1 receptor) domain Domain in the cytoplasmic tails of the TLRs and the IL-1 receptor, which interacts with similar domains in intracellular signaling proteins.
tissue-resident memory T cells (TRM)   Memory lymphocytes that do not migrate after taking up residence in barrier tissues, where they are retained long term. They appear to be specialized for rapid effector function after restimulation with antigen or cytokines at sites of pathogen entry.
TLR-1   Cell-surface Toll-like receptor that acts in a heterodimer with TLR-2 to recognize lipoteichoic acid and bacterial lipoproteins.
TLR-2   Cell-surface Toll-like receptor that acts in a heterodimer with either TLR-1 or TLR-6 to recognize lipoteichoic acid and bacterial lipoproteins.
TLR-3   Endosomal Toll-like receptor that recognizes double-stranded viral RNA.
TLR-4   Cell-surface Toll-like receptor that, in conjunction with the accessory proteins MD-2 and CD14, recognizes bacterial lipopolysaccharide and lipoteichoic acid.
TLR-5   Cell-surface Toll-like receptor that recognizes the ﬂagellin protein of bacterial ﬂagella.
TLR-6   Cell-surface Toll-like receptor that acts in a heterodimer with TLR-2 to recognize lipoteichoic acid and bacterial lipoproteins.
TLR-7   Endosomal Toll-like receptor that recognizes single-stranded viral RNA.
TLR-8   Endosomal Toll-like receptor that recognizes single-stranded viral RNA.
TLR-9   Endosomal Toll-like receptor that recognizes DNA containing unmethylated CpG.
TLR-11, TLR-12   Mouse Toll-like receptor that recognizes profilin and profilin-like proteins.
TNF family   Cytokine family, the prototype of which is tumor necrosis factor-α (TNF or TNF-α). It contains both secreted (for example TNF-α and lymphotoxin) and membrane-bound (for example CD40 ligand) members.