Question

Could someone please explain which of the following is correct and why?

The role of the CD3 proteins and ζ chain on the surface of the cell is to:

a) transduce signals to the interior of the T cell

b) bind to antigen associated with MHC molecules

c) bind to MHC molecules

d) bind to CD4 or CD8 molecules

e) facilitate antigen processing of antigens that bind to the surface of T cells

From what I have read I believe a) to be the correct answer but I am unsure if I have understood it correctly. This is based on this paragraph in the book "The Immune System 4th Edt":

"By inducing this organization of molecules in the T-cell membrane, the interactions of MHC ligands with T-cell receptors activate cytoplasmic protein tyrosine kinases, which phosphorylate particular tyrosine residues in the cytoplasmic tails of the CD3 cell-surface proteins and the associated ζ chain (CD247), a purely cytoplasmic protein (see Figure 5.6, p. 118). The tyrosine residues that become phosphorylated are part of short amino-acid sequence motifs called immunoreceptor tyrosine-based activation motifs (ITAMs). Enzymes and other signaling molecules bind to the phosphorylated tyrosine residues and thus also become activated. In this way, the extracellular binding of antigen to the T-cell receptor initiates pathways of intracellular signaling that lead to alterations in gene expression and end with T-cell differentiation."

Answer 1

The role of CD3 proteins and the ζ chain on the surface of the T cell is to transduce signals to the T cell's interior following antigen recognition. They are part of the signaling apparatus associated with the T cell receptor complex.

The correct answer to your question about the role of the CD3 proteins and ζ chain on the surface of the T cell is a) transduce signals to the interior of the T cell. This is because CD3 proteins and the ζ chain are part of the T cell receptor (TCR) complex, and they play a crucial role in signaling processes rather than directly binding to antigens or MHC molecules.

The TCR itself is responsible for the recognition of antigens presented by MHC molecules on antigen-presenting cells. However, the TCR lacks intrinsic signaling capability, which is provided by the CD3 complex and ζ chains. Upon antigen recognition, signaling is transduced via this complex, leading to protein tyrosine kinase activation, phosphorylation of ITAMs (immunoreceptor tyrosine-based activation motifs), and subsequent activation of downstream signaling pathways that culminate in T cell activation and differentiation.

The role of CD4 or CD8 molecules, on the other hand, is to act as coreceptors that modulate the interaction between the TCR and MHC molecules, thereby influencing the specificity and sensitivity of TCR signaling.