4-1BB Ligand: A membrane bound member of the TNF superfamily that is expressed on activated B-LYMPHOCYTES; MACROPHAGES; and DENDRITIC CELLS. The ligand is specific for the 4-1BB RECEPTOR and may play a role in inducing the proliferation of activated peripheral blood T-LYMPHOCYTES.Antigens, CD137: A member of the tumor necrosis factor receptor superfamily that is specific for 4-1BB LIGAND. It is found in a variety of immune cell types including activated T-LYMPHOCYTES; NATURAL KILLER CELLS; and DENDRITIC CELLS. Activation of the receptor on T-LYMPHOCYTES plays a role in their expansion, production of cytokines and survival. Signaling by the activated receptor occurs through its association with TNF RECEPTOR-ASSOCIATED FACTORS.Ligands: A molecule that binds to another molecule, used especially to refer to a small molecule that binds specifically to a larger molecule, e.g., an antigen binding to an antibody, a hormone or neurotransmitter binding to a receptor, or a substrate or allosteric effector binding to an enzyme. Ligands are also molecules that donate or accept a pair of electrons to form a coordinate covalent bond with the central metal atom of a coordination complex. (From Dorland, 27th ed)Receptors, Nerve Growth Factor: Cell surface receptors that bind NERVE GROWTH FACTOR; (NGF) and a NGF-related family of neurotrophic factors that includes neurotrophins, BRAIN-DERIVED NEUROTROPHIC FACTOR and CILIARY NEUROTROPHIC FACTOR.Tumor Necrosis Factors: A family of proteins that were originally identified by their ability to cause NECROSIS of NEOPLASMS. Their necrotic effect on cells is mediated through TUMOR NECROSIS FACTOR RECEPTORS which induce APOPTOSIS.Receptors, Tumor Necrosis Factor: Cell surface receptors that bind TUMOR NECROSIS FACTORS and trigger changes which influence the behavior of cells.Rats, Inbred BB: A strain of Rattus norvegicus which is a model for spontaneous insulin-dependent diabetes mellitus (DIABETES MELLITUS, INSULIN-DEPENDENT).Antigens, CD: Differentiation antigens residing on mammalian leukocytes. CD stands for cluster of differentiation, which refers to groups of monoclonal antibodies that show similar reactivity with certain subpopulations of antigens of a particular lineage or differentiation stage. The subpopulations of antigens are also known by the same CD designation.Antigens, CD28: Costimulatory T-LYMPHOCYTE receptors that have specificity for CD80 ANTIGEN and CD86 ANTIGEN. Activation of this receptor results in increased T-cell proliferation, cytokine production and promotion of T-cell survival.Antigens, CD80: A costimulatory ligand expressed by ANTIGEN-PRESENTING CELLS that binds to CTLA-4 ANTIGEN with high specificity and to CD28 ANTIGEN with low specificity. The interaction of CD80 with CD28 ANTIGEN provides a costimulatory signal to T-LYMPHOCYTES, while its interaction with CTLA-4 ANTIGEN may play a role in inducing PERIPHERAL TOLERANCE.Lymphocyte Activation: Morphologic alteration of small B LYMPHOCYTES or T LYMPHOCYTES in culture into large blast-like cells able to synthesize DNA and RNA and to divide mitotically. It is induced by INTERLEUKINS; MITOGENS such as PHYTOHEMAGGLUTININS, and by specific ANTIGENS. It may also occur in vivo as in GRAFT REJECTION.Mice, Inbred C57BLT-Lymphocytes: Lymphocytes responsible for cell-mediated immunity. Two types have been identified - cytotoxic (T-LYMPHOCYTES, CYTOTOXIC) and helper T-lymphocytes (T-LYMPHOCYTES, HELPER-INDUCER). They are formed when lymphocytes circulate through the THYMUS GLAND and differentiate to thymocytes. When exposed to an antigen, they divide rapidly and produce large numbers of new T cells sensitized to that antigen.Tumor Necrosis Factor-alpha: Serum glycoprotein produced by activated MACROPHAGES and other mammalian MONONUCLEAR LEUKOCYTES. It has necrotizing activity against tumor cell lines and increases ability to reject tumor transplants. Also known as TNF-alpha, it is only 30% homologous to TNF-beta (LYMPHOTOXIN), but they share TNF RECEPTORS.CD40 Ligand: A membrane glycoprotein and differentiation antigen expressed on the surface of T-cells that binds to CD40 ANTIGENS on B-LYMPHOCYTES and induces their proliferation. Mutation of the gene for CD40 ligand is a cause of HYPER-IGM IMMUNODEFICIENCY SYNDROME, TYPE 1.