[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Immunology](https://durgical.com/en/immunology/) › [Adaptive immunity](https://durgical.com/en/immunology/#a-adaptive) › Antigen processing and presentation [Immunology](https://durgical.com/en/immunology/) · [Adaptive immunity](https://durgical.com/en/immunology/#a-adaptive) # 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](https://durgical.com/en/anatomy/lymph-nodes-of-neck/) ## Key points - Endogenous antigen: proteasome, TAP, ER, class I, CD8. - Exogenous antigen: endosome, invariant chain, CLIP, HLA-DM, class II, CD4. - Dendritic cells are the most efficient activators of naive T cells. - Cross-presentation by dendritic cells allows CD8 responses to external antigen. ## 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. ## Memory aid Inside goes out through TAP to the I (class I); outside comes in through the endosome to the II (class II). ## 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. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/immunology/antigen-processing-and-presentation/) ## See it in 3D The structures of Durgical's 3D anatomy where it is found. - [Neck lymph nodes](https://durgical.com/en/anatomy/lymph-nodes-of-neck/) ## Related - [MHC/HLA class I and II](https://durgical.com/en/immunology/mhc-hla-class-i-and-ii/) - [T-cell activation: two signals and superantigens](https://durgical.com/en/immunology/t-cell-activation-two-signals-and-superantigens/) - [Phagocytosis and the respiratory burst](https://durgical.com/en/immunology/phagocytosis-and-the-respiratory-burst/) - [Adaptive immunity (overview: humoral vs cellular)](https://durgical.com/en/immunology/adaptive-immunity/) - [Dendritic cell (professional antigen-presenting cell)](https://durgical.com/en/immunology/dendritic-cell/) - [Monocyte and macrophage](https://durgical.com/en/immunology/monocyte-and-macrophage/) - [B cell and plasma cell](https://durgical.com/en/immunology/b-cell-and-plasma-cell/) - [Lymph node: cortex, paracortex, medulla](https://durgical.com/en/immunology/lymph-node-cortex-paracortex-medulla/) - [CD8⁺ cytotoxic T cell](https://durgical.com/en/immunology/cd8-cytotoxic-t-cell/) [← All of immunology](https://durgical.com/en/immunology/) 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](https://openstax.org/books/microbiology/pages/18-2-major-histocompatibility-complexes-and-antigen-presenting-cells) (OpenStax (Microbiology), 2016); [21.3 The Adaptive Immune Response: T lymphocytes and Their Functional Types](https://openstax.org/books/anatomy-and-physiology-2e/pages/21-3-the-adaptive-immune-response-t-lymphocytes-and-their-functional-types) (OpenStax (Anatomy and Physiology 2e), 2022). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/immunology/antigen-processing-and-presentation/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/immunology/antigen-processing-and-presentation/