USMLE Step 1

/

Immunology

/

B Cells and T Cells

Previous
Next

B Cells - Activation

4318
Watch Video

Summary

B Cell Activation requires both a B-cell and appropriate signaling from Helper T-cell. First, a B-cell has to present foreign antigens on their MHC II, which binds to T-cell receptors on a Helper T-cell. Second, a costimulatory signal is provided when CD40 ligand from the T-cell binds to CD40 receptor on B-cells. Once B-cells are activated, they undergo class switching to change the type of antibody made, and finally produce and release antibodies in the body to fight the threat. The majority of activated B-cells proliferate in the germinal centers of lymph nodes and transform into memory B-cells. Some activated B-cells become plasma B-cells, which migrate to the bone marrow to make antibodies.

Key Points

  • B Cell Activation
    • B cells endocytose antigen and present it on MHC II
      • Endocytosis via B-cell receptor
      • B-cells are antigen-presenting cells
    • Foreign antigen on MHC II recognized by TCR on CD4+ Helper T-cell (signal 1)
    • CD40 receptor on B cells binds CD40 ligand (CD40L) on T cell (signal 2)
    • Activated B cell
      • Unergoes class switching and affinity maturation
        • Antibody isotype produced depends on cytokine secreted by T-cell
      • Proliferates in lymph node