An antigen-presenting cell: a. can be a CD8 ^+ T cell. b....

An antigen-presenting cell:
a. can be a CD8 $^{+}$ T cell.
b. derives from a phagocytic cell and exposes an antigen to a lymphocyte.
c. secretes antibodies.
d. cannot be a B cell.
e. cannot stimulate helper T cells.

Key Concepts

Antigen Presentation

Antigen presentation is the process by which cells process protein antigens and display peptide fragments on their surface bound to major histocompatibility complex (MHC) molecules. This process is critical for alerting T cells to the presence of pathogens and initiating an adaptive immune response.

Professional Antigen-Presenting Cells (APCs)

Professional APCs are specialized cells that efficiently capture, process, and present antigens to T lymphocytes. This group includes dendritic cells, macrophages, and B cells, which all express high levels of MHC class II molecules and costimulatory proteins necessary for effective T cell activation.

MHC Class II Molecules

MHC class II molecules are cell surface proteins predominantly expressed on professional APCs. They are responsible for binding and presenting extracellularly derived antigenic peptides to CD4+ helper T cells, a crucial step in orchestrating the adaptive immune response.

T Cell Subtypes

Different subsets of T cells play distinct roles in the immune system. CD4+ helper T cells recognize antigens presented on MHC class II molecules and help regulate immune responses, while CD8+ cytotoxic T cells recognize antigens presented on MHC class I molecules and are primarily responsible for killing infected or abnormal cells. CD8+ T cells are not typically involved in antigen presentation.

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