Learning Objective:

Understand how antigen-presenting cells (APCs) activate T and B lymphocytes, the signals required for activation, and how class switching and cytotoxic functions occur.

Antigen-Presenting Cells (APCs)

• Types: B cells, dendritic cells, Langerhans cells, macrophages.
• Function: Ingest/process antigen → present on MHC I or II → migrate to lymph nodes → activate T cells.

T-Cell Activation

Signal 1 – Antigen Recognition:

• CD4⁺ T helper (Th) cells: Exogenous antigen on MHC II recognized by TCR.
• CD8⁺ cytotoxic (Tc) cells: Endogenous or cross-presented antigen on MHC I recognized by TCR.

Signal 2 – Costimulation:

• B7 (CD80/86) on APC binds CD28 on naïve T cells.
• Ensures proliferation and survival.

Effector Functions:

• Activated Th cells secrete cytokines → help B cells, recruit/activate other leukocytes.
• Activated Tc cells kill virus-infected or tumor cells via perforin/granzymes.

Activity

B-Cell Activation and Class Switching

Antigen Recognition:

• BCR binds antigen → endocytosis and presentation on MHC II.
• Recognized by TCR on Th cells.

Costimulation:

• CD40 on B cells binds CD40L on Th cells.
• Cytokines from Th cells guide Ig class switching.

Outcome:

• Initial production of IgM → class switching to IgG, IgA, or IgE.
• Affinity maturation increases antibody specificity.

Activity