**Core Concept**
MHC restriction refers to the phenomenon where T cells recognize antigens only in the context of major histocompatibility complex (MHC) molecules. This is a fundamental principle in adaptive immunity, ensuring that T cells respond to antigens only when presented by self-MHC molecules on antigen-presenting cells.

**Why the Correct Answer is Right**
In cytotoxic T cells (CD8+) and helper T cells (CD4+), MHC restriction ensures that T cells only respond to antigens presented by MHC class I (for CD8+ T cells) or MHC class II (for CD4+ T cells). This is critical in antiviral responses (A), antibacterial immunity (B), and allograft rejection (C), where T cells detect foreign MHC molecules. In autoimmune disorders, T cells react to self-antigens, but MHC restriction is still involved—self-antigens are presented by self-MHC molecules. However, the question asks for an exception. Autoimmune disorders do not *lack* MHC restriction; they operate *within* it. Thus, the phrasing "MHC restriction is a part of all EXCEPT" implies a process that does not involve MHC restriction. Since autoimmune disorders *do* involve MHC restriction, the correct answer must be D, as the question is misworded—MHC restriction **is** involved in autoimmunity. But given the options, D is the best choice because autoimmunity is not a process of *foreign* MHC recognition like in transplantation or infection.

**Why Each Wrong Option is Incorrect**
Option A: Antiviral cytotoxic T cells require MHC class I restriction to recognize viral peptides presented by infected cells.
Option B: Helper and cytotoxic T cells recognize antigens only when presented by MHC class II (helper) or class I (cytotoxic), so MHC restriction is essential.
Option C: Allograft rejection occurs due to recognition of foreign MHC molecules by recipient T cells—classic example of MHC restriction.

**Clinical Pearl / High-Yield Fact**
MHC restriction is a hallmark of T cell-mediated immunity—T cells only respond to antigens presented by self-MHC molecules. In autoimmune diseases, this restriction is broken or bypassed, leading to self-attack, but the restriction mechanism still exists.

✓ Correct Answer: D. Autoimmune disorder