An antigen-presenting cell (APC) is a cell that displays fragments of an antigen on its surface for recognition by T cells. In conventional antigen presentation, these fragments are peptides bound to major histocompatibility complex (MHC) molecules. Specialized APCs also provide signals that determine whether responding T cells become activated or remain unresponsive. They therefore connect antigen recognition with the initiation and regulation of adaptive immunity. The principal professional APCs are dendritic cells, macrophages, and B cells. (ncbi.nlm.nih.gov)
Definition and scope
“Antigen-presenting cell” has both a broad and a narrower usage. Broadly, almost all nucleated cells display peptides through MHC class I, allowing cytotoxic T cells to inspect material originating inside them. In immunology, however, APC commonly refers to a professional APC: a cell equipped to process antigen, express MHC class II, and supply the additional signals needed for effective T-cell activation. Displaying a peptide is therefore not equivalent to initiating a new immune response. (immunology.org)
The T-cell receptor recognizes the combined peptide–MHC structure rather than a free, intact antigen. APCs also express adhesion molecules that help stabilize their contact with T cells. Antigen processing consequently determines which portions of a larger protein become accessible to T-cell recognition; it does not expose every part of the original antigen equally. (ncbi.nlm.nih.gov)
Principal cell types
Dendritic cells are particularly effective at activating naïve T cells—cells that have not previously responded to their specific antigen. In peripheral tissues, they capture material from their surroundings. Microbial and inflammatory signals can promote maturation, increasing their capacity to stimulate T cells. Migratory dendritic cells transport antigen from tissues into lymphoid organs, where encounters with naïve T cells initiate antigen-specific responses. These properties make them an important connection between innate immunity and adaptive immunity. (ncbi.nlm.nih.gov)
Macrophages ingest microorganisms and cellular material through phagocytosis. Besides destroying ingested material, they present derived peptides to helper T cells. Recognition by these T cells can provide signals that enhance macrophage antimicrobial activity. Their antigen-presenting role is closely associated with their functions in tissue defense and the effector phase of an immune response. (ncbi.nlm.nih.gov)
B cells capture particular antigens through their surface antigen receptors, internalize them, and present derived peptides through MHC class II. This selective uptake differs from the broader sampling performed by dendritic cells and macrophages. A helper T cell recognizing the presented peptide can provide signals that support B-cell proliferation and differentiation into plasma cells that secrete antibodies. Thus, B-cell antigen presentation coordinates antigen-specific recognition by two lymphocyte populations. (ncbi.nlm.nih.gov)
Antigen-processing pathways
In the MHC class II pathway, extracellular proteins are internalized and broken down in intracellular compartments. Newly synthesized MHC class II associates with an invariant chain, which protects its peptide-binding groove during transport. A residual fragment, CLIP, is subsequently exchanged for an antigen-derived peptide with assistance from HLA-DM. The peptide–MHC complex reaches the cell surface, where it can be recognized by CD4-positive helper T cells. (immunology.org)
In the MHC class I pathway, intracellular proteins are generally degraded by the proteasome. Resulting peptides are transported into the endoplasmic reticulum, where they are loaded onto MHC class I. Surface complexes are recognized by CD8-positive T cells. Professional APCs possess this pathway as well as the class II pathway. (immunology.org)
The distinction between intracellular and extracellular antigen sources is not absolute. In cross-presentation, material taken up from outside a cell is displayed through MHC class I. This allows APCs to present antigens from other cells without themselves producing the original antigen. Intracellular material can also enter the class II pathway through processes such as autophagy. (immunology.org)
Signals for T-cell activation
Naïve T-cell activation requires more than peptide recognition. T-cell costimulation commonly involves CD80 or CD86 on the APC binding CD28 on the T cell. Antigen recognition supplies specificity, while costimulation helps establish whether the encounter should produce an activating response. Both signals normally come from the same APC. (ncbi.nlm.nih.gov)
Cytokines provide additional information that influences the response. APC activity therefore depends not only on the peptide displayed but also on the cell’s activation state and surrounding signals. This contextual regulation helps the immune system distinguish encounters requiring defense from those that should not trigger destructive responses. (immunology.org)
Tolerance and related distinctions
Antigen presentation can support peripheral immune tolerance rather than immunity. Recognition without adequate costimulation can produce clonal anergy, a state of reduced responsiveness. Under non-inflammatory conditions, dendritic cells can also promote regulatory T cells, contributing to tolerance toward tissue-specific self-antigens. These outcomes show that APCs regulate responses rather than functioning solely as activators. (ncbi.nlm.nih.gov)
Follicular dendritic cells should be distinguished from conventional dendritic cells. They retain and display native antigen for recognition by B cells within lymphoid follicles. This differs from processing proteins into peptides for presentation to T cells through MHC molecules, despite the shared terminology of antigen display. (pubmed.ncbi.nlm.nih.gov)