Immunology · Adaptive immunity
Antigen processing and presentation !
- Process
Cells chop proteins into small pieces and show them on MHC molecules so T cells can check for something foreign.
Updated 2026-11-01
See it in 3D: Neck lymph nodes
What it is
T cells cannot see whole antigens; they recognize short peptides bound to MHC molecules. Proteins made inside the cell (including viral proteins) are cut by the proteasome and loaded onto MHC class I for CD8 T cells, while proteins taken up from outside are cut in endosomes and loaded onto MHC class II for CD4 T cells. Dendritic cells are the key presenters that start responses in lymph nodes, and they can also cross-present external antigen on class I.
Key facts
- Cross-presentation: dendritic cells put exogenous antigen on class I, allowing CD8 responses to viruses that do not infect them and to tumors.
- B cells capture antigen with their surface antibody and present it on class II to get T-cell help.
- Viruses such as herpesviruses and CMV interfere with TAP or class I to avoid CD8 T cells.
Step by step
- Class I pathway: cytosolic proteins are tagged with ubiquitin and cut by the proteasome.
- TAP pumps the peptides into the endoplasmic reticulum, where they bind class I with beta-2 microglobulin.
- Loaded class I goes to the surface and is read by CD8 T cells.
- Class II pathway: the cell takes up external antigen by phagocytosis or endocytosis, and endosomal proteases cut it.
- New class II leaves the ER blocked by the invariant chain; in the endosome only a fragment (CLIP) remains in the groove.
- HLA-DM swaps CLIP for an antigenic peptide; class II goes to the surface and is read by CD4 T cells.
- Dendritic cells loaded with antigen migrate through lymphatics (CCR7) to the lymph node and activate naive T cells.
Clinical notes
- TAP deficiency: low surface class I, chronic respiratory infections and skin lesions.
- MHC class II deficiency (bare lymphocyte syndrome): few CD4 T cells and severe combined immunodeficiency.
- Loss of class I by tumors avoids CD8 T cells but exposes them to NK cells.
Test yourself
Think of the answer, then tap the question to see it.
What does TAP do in antigen presentation?
It transports peptides made by the proteasome from the cytosol into the endoplasmic reticulum, where they load onto MHC class I.
What is the function of the invariant chain?
It blocks the groove of newly made MHC class II so it cannot bind ER peptides and guides it to the endosome.
What is cross-presentation?
Presentation of exogenous antigen on MHC class I, mainly by dendritic cells, which allows CD8 responses against viruses and tumors that do not infect those cells.
Which molecule exchanges CLIP for an antigenic peptide?
HLA-DM, in the endosomal compartment.
See it in 3D
The structures of Durgical's 3D anatomy where it is found.
Related
- MHC/HLA class I and II
- T-cell activation: two signals and superantigens
- Phagocytosis and the respiratory burst
- Adaptive immunity (overview: humoral vs cellular)
- Dendritic cell (professional antigen-presenting cell)
- Monocyte and macrophage
- B cell and plasma cell
- Lymph node: cortex, paracortex, medulla
- CD8⁺ cytotoxic T cell
Educational content on the immune system; not a treatment or individual vaccination guide and no substitute for a health professional's advice. No doses are given.
Sources: 18.2 Major Histocompatibility Complexes and Antigen-Presenting Cells (OpenStax (Microbiology), 2016); 21.3 The Adaptive Immune Response: T lymphocytes and Their Functional Types (OpenStax (Anatomy and Physiology 2e), 2022).
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Representative model, for educational purposes only.