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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."

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    $\begingroup$ The quote from the textbook is incorrect in that $\zeta$ has both transmembrane and extracellular domains, so it is not "purely cytoplasmic". The ECD is small, but not non-existent. $\endgroup$
    – MattDMo
    Aug 19, 2022 at 17:54

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A is most likely the answer they're looking for. However, CD3$\epsilon$ and either CD3$\delta$ or CD3$\gamma$ associate with CD4 or CD8 (depending on cell type) in the TCR complex, bringing in kinases like Lck to phosphorylate the ITAMs on the CD3 chains' intracellular domains, leading to further downstream signaling. So, CD3 does associate/"bind" CD4/CD8, making D correct as well, at least in my opinion.

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