Page 445 - Veterinary Immunology, 10th Edition

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FIG. 15.10 CD40 and CD154 participate in a dialog between T
cells and the three populations of antigen-presenting cells. In each
case, both cell types are stimulated. In the case of B cells, T cell
stimulation permits B cell proliferation and immunoglobulin
production.

Complement Help

Effective co-stimulation of B cells also requires signals from

complement transmitted through CD21/CD19 on the B cell surface.
CD21 is a receptor (CR2) whose ligand is C3d. CD19 is its
accompanying signaling component. If an antigen with C3d
attached binds to CD21, a signal is transmitted through CD19 to the

B cell (Fig. 15.11). Stimulation of a BCR plus CD19/CD21 lowers the
threshold for B cell activation 100-fold. The importance of
complement in stimulating B cells is emphasized by the observation
that mice deficient in the complement components C3, C4, or CR2

cannot mount effective antibody responses.

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