CD8 cells recognize antigens present on:

Question

Accepted Answer

The correct option is 3: Hepatocytes

CD8 T lymphocytes, also known as Cytotoxic T Lymphocytes (CTLs), are the primary effector cells of cell-mediated immunity, and their function is highly dependent on the mechanism of antigen presentation. They are specifically tasked with recognizing antigens presented on Major Histocompatibility Complex Class I (MHC Class I) molecules. MHC Class I is expressed on the surface of virtually all nucleated cells in the body, which includes liver cells, or hepatocytes. When a cell, such as a hepatocyte, becomes infected by an intracellular pathogen like a virus (e.g., Hepatitis B or C), it processes fragments of the viral proteins internally and displays them on its surface bound to MHC I molecules. This presentation serves as an immediate alarm signal, allowing circulating CD8 T cells with the corresponding T-cell receptor to bind and confirm the presence of the infection. Upon successful recognition, the CD8 T cell initiates its cytotoxic action, leading to the targeted destruction (apoptosis) of the infected hepatocyte to prevent the pathogen from spreading. Therefore, hepatocytes, particularly in the context of viral infection, are recognized and targeted by CD8 T cells.

The incorrect option is 1: Interferon

Interferon is an immune signaling molecule, or cytokine, not a type of cell that presents antigens to T lymphocytes. Its role is to act as an alarm signal and a defense coordinator, primarily by inducing an antiviral state in neighboring cells to inhibit viral replication. While interferons can indirectly influence the activity of T cells and even increase the expression of MHC molecules on cells, they are soluble proteins and lack the cellular structure required to bind and present antigens to CD8 T cells in the manner MHC molecules do. The CD8 T cell's physical recognition process requires direct cell-to-cell contact with a membrane-bound MHC-antigen complex.

The incorrect option is 2: Plasmocytes

Plasmocytes, or plasma cells, are highly specialized B-cell derivatives whose main purpose is the mass production and secretion of antibodies to fight extracellular pathogens. Like almost all cells, they are nucleated and express MHC Class I, meaning CD8 T cells *could* technically recognize them if they were virally infected. However, they are not the typical or exclusive targets described in the question's context; moreover, the primary T cell interaction for B-cell lineage happens with CD4+ Helper T cells, not CD8+ T cells, during the initial activation and differentiation stage via MHC Class II molecules. The question asks what CD8 cells *recognize antigens present on*, and while a plasmocyte *can* present via MHC I when infected, they are functionally distinct from the widespread target cells (like hepatocytes) that CD8 T cells constantly patrol.

The incorrect option is 4: Erythrocytes

Erythrocytes, or red blood cells (RBCs), are definitively incorrect because mature mammalian red blood cells are **anucleated**—they lack a nucleus and most other cellular organelles. Crucially, because they lack a nucleus, they also do not express any Major Histocompatibility Complex (MHC) molecules, including MHC Class I. Without the mandatory MHC Class I scaffold to present an antigen fragment, CD8 T cells have no molecular mechanism to recognize or interact with red blood cells. Consequently, red blood cells are effectively invisible to CD8 T cells, ensuring that the body's essential oxygen carriers are not erroneously targeted and destroyed by the cellular immune response.