Antigen-Presenting Cells (APCs)
Antigen-presenting cells are cells that process antigens and display antigenic peptides bound to Major Histocompatibility Complex (MHC) molecules on their surface. These peptide–MHC complexes are recognized by T lymphocytes through their T-cell receptors (TCRs).
A key principle of cellular immunity is that T cells cannot recognize free antigen. Instead, T cells can only respond to antigens that have been
processed and presented by APCs using carrier molecules such as MHC molecules and CD1 molecules. CD1 molecules are a family of glycoproteins structurally related to MHC class I and are involved in the presentation of lipid antigens to T cells.
The term APC is often restricted to specialized cells that are capable of priming naïve T cells, meaning T cells that have not previously encountered antigen.
General Features of APCs
- APCs process antigens and present them to T cells
- APCs express MHC class I, allowing interaction with CD8⁺ cytotoxic T cells
- Many APCs also express MHC class II, allowing interaction with CD4⁺ helper T cells
- Almost all nucleated cells express MHC class I, including professional APCs
- Cells expressing MHC class II are often referred to as professional APCs
If a virus infects a macrophage or dendritic cell, viral antigens presented via MHC class I promote destruction of the infected cell by cytotoxic T cells. However, dendritic cells can also ingest viruses via pinocytosis and present viral antigens on MHC class II, thereby activating helper T cells and promoting antibody production.