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Immunology ` Immunology—cellular componentS Immunology ` Immunology—cellular componentS SECTIon II 103

T- and B-cell activation APCs: B cells, dendritic cells, Langerhans cells, macrophages.
Two signals are required for T-cell activation, B-cell activation, and class switching.
T-cell activation Dendritic cell (specialized APC) samples Q
and processes antigen, then migrates to the Antigen
draining lymph node. MHC II/I R receptor Naive T cell
T cell
T-cell activation (signal 1): exogenous Dendritic cell
antigen is presented on MHC II and CD4/8
recognized by TCR on Th (CD4+) cell.
Endogenous or cross-presented antigen is CD80/86 CD28
T
presented on MHC I to Tc (CD8+) cell. (B7) S Activated
Proliferation and survival (signal 2): T cell actions
costimulatory signal via interaction of B7
protein (CD80/86) on dendritic cell and
CD28 on naïve T cell.
Activated Th cell produces cytokines. Tc cell
able to recognize and kill virus-infected cell.
B-cell activation and Th-cell activation as above. Antigen R
B cell
class switching B-cell receptor–mediated endocytosis. receptor
Exogenous antigen is presented on MHC II T cell S MHC II
and recognized by TCR on Th cell. Q receptor
Activated
CD40 receptor on B cell binds CD40 ligand CD4+ T cell B cell
(CD40L) on Th cell. CD4
Th cells secrete cytokines that determine Ig
class switching of B cells. CD40L CD40
B cells are activated, undergo class switching T V
Activated
and affinity maturation, and begin B cell actions
producing antibodies. U

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