Chapter 9, Problem 1Q

Summary Introduction

To write:

Describe B-cell co-receptor and its ligand and explain how it facilitates B-cell activation.

Introduction:

B-cell co-receptor is composed of CD21 (complement receptor 2), CD19, and CD81. CR1 acts as a cofactor by facilitating the cleavage of C3b-binding antigen into C3d molecule. CD19 is a signaling component of the co-receptor that binds to CD81 protein and taken up to coordinate B-cell receptor and co-receptor interaction within the membrane.

Explanation of Solution

The B-cell activation involves coordination between the B-cell co-receptor and B-cell receptor, which in turn enhances the sensitivity of the B cell to antigen. Due to the low concentration of antigen, CD21 binds to complement components such as iC3b and C3d, developed by the cleavage of C3b by factor I and further accumulated on the surface of antigens or pathogens.

In signaling pathway, CD19 gives the long cytoplasmic tail and associate with CD81 to assists the CD19 expression on the B-cell surface; thus, facilitates the coordination between B-cell receptor and co-receptor within the B-cell plasma membrane.

Both the B-cell receptor and co-receptor molecules are ligated by antigen and C3d or iC3b; however, Lyn, a protein tyrosine kinase comes in contact with the cytoplasmic tail of CD19. Lyn is associated with the immunoreceptor tyrosine-based activation motifs (ITAMs) of Iga, which facilitates the phosphorylation of adjacent CD19 molecules. The phosphorylated tail of CD19 will transmits signals that counterpart those signals created by the B-cell receptor complex.

Conclusion

The activation of B-cells may depend on the characteristics of the antigen and stimulatory signals given by CD4 and TH2 cells. TH2 cells also release IL4, IL5, and IL6 cytokines; this, on binding to particular receptor these cytokines stimulate proliferation and differentiation of B-cells into plasma cells.