[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) › T-cell activation: two signals and superantigens [Immunology](https://durgical.com/en/immunology/) · [Adaptive immunity](https://durgical.com/en/immunology/#a-adaptive) # T-cell activation: two signals and superantigens - Process A naive T cell needs to see its antigen and receive a second «go» signal; bacterial superantigens bypass this and switch on huge numbers of T cells. Updated 2026-11-01 ## Key points - Signal 1: TCR plus MHC-peptide; signal 2: CD28 plus B7 (CD80/86). - Signal 1 without signal 2 causes anergy. - IL-2 drives T-cell proliferation; CD25 is the IL-2 receptor alpha chain. - CTLA-4 and PD-1 are inhibitory checkpoints, targets of cancer immunotherapy. - Superantigens (TSST-1, staphylococcal enterotoxins, streptococcal pyrogenic exotoxins) cross-link MHC II and TCR Vβ outside the groove. ## What it is Activating a naive T cell requires signal 1 (the TCR recognizing peptide on MHC, with CD4 or CD8) and signal 2 (costimulation, mainly CD28 binding B7 on an activated antigen-presenting cell). Cytokines then shape what kind of effector the cell becomes. Without signal 2 the cell becomes anergic, which protects against autoimmunity. Superantigens skip specificity by linking MHC class II to many TCRs at once, causing a cytokine storm. ## Key facts - Superantigens bind outside the peptide groove of MHC class II and to the variable part of the TCR beta chain, activating a large fraction of T cells. - Examples: toxic shock syndrome toxin-1 and enterotoxins of Staphylococcus aureus, and streptococcal pyrogenic exotoxins of Streptococcus pyogenes. - The result is massive release of IL-2, IFN-γ and TNF: fever, rash, hypotension and multiorgan failure. - CD8 T cells kill with perforin and granzymes or with Fas ligand. ## Step by step - Signal 1: the TCR binds the specific peptide on MHC; CD4 binds class II or CD8 binds class I; CD3 transmits the signal. - Signal 2: CD28 on the T cell binds B7 (CD80/CD86) on an activated dendritic cell. - The T cell produces IL-2 and its high-affinity receptor chain CD25, and proliferates (clonal expansion). - Signal 3: cytokines from the antigen-presenting cell direct differentiation (for example IL-12 toward Th1). - Effector cells leave the lymph node; some become memory cells. - Brakes: CTLA-4 (binds B7 more strongly than CD28) and PD-1 shut the response down. - Signal 1 without signal 2 leads to anergy, a state of unresponsiveness. ## Clinical notes - Toxic shock syndrome: high fever, sunburn-like rash that later peels on palms and soles, hypotension and organ failure. - Abatacept (CTLA-4 fused to an antibody fragment) blocks B7 and is used in rheumatoid arthritis. - Checkpoint inhibitors (anti-CTLA-4, anti-PD-1, anti-PD-L1) release the brakes against tumors and can cause immune-related adverse effects. - Calcineurin inhibitors such as cyclosporine and tacrolimus block IL-2 transcription after signal 1. ## Memory aid To start the car you need the key (signal 1) and the clutch (signal 2). A superantigen hot-wires every car in the parking lot at once. ## Test yourself Think of the answer, then tap the question to see it. ## What happens when a naive T cell receives signal 1 without costimulation? It becomes anergic: functionally unresponsive, which is one mechanism of peripheral tolerance. ## Which molecules provide the main costimulatory signal? CD28 on the T cell binding B7-1 (CD80) or B7-2 (CD86) on the antigen-presenting cell. ## How do superantigens activate so many T cells? They bind MHC class II outside the peptide groove and the variable region of the TCR beta chain, cross-linking them regardless of the antigen the TCR recognizes. ## Which bacteria produce the superantigens of toxic shock syndrome? Staphylococcus aureus (TSST-1 and enterotoxins) and Streptococcus pyogenes (streptococcal pyrogenic exotoxins). ## How does CTLA-4 turn off T-cell activation? It binds B7 with higher affinity than CD28, taking away costimulation and delivering an inhibitory signal. [Add to my cards (5)](https://durgical.app/estudiar/?add=/en/immunology/t-cell-activation-two-signals-and-superantigens/) ## Related - [Antigen processing and presentation](https://durgical.com/en/immunology/antigen-processing-and-presentation/) - [MHC/HLA class I and II](https://durgical.com/en/immunology/mhc-hla-class-i-and-ii/) - [Th1, Th2, Th17, Tfh and Treg](https://durgical.com/en/immunology/th1-th2-th17-tfh-and-treg/) - [T-cell development: positive and negative selection](https://durgical.com/en/immunology/t-cell-development-positive-and-negative-selection/) - [CD4⁺ helper T cell](https://durgical.com/en/immunology/cd4-helper-t-cell/) - [CD8⁺ cytotoxic T cell](https://durgical.com/en/immunology/cd8-cytotoxic-t-cell/) - [Interleukin-2](https://durgical.com/en/immunology/interleukin-2/) - [Fc receptors, BCR and TCR](https://durgical.com/en/immunology/fc-receptors-bcr-and-tcr/) - [Immunosuppressants by mechanism (classes)](https://durgical.com/en/immunology/immunosuppressants-by-mechanism/) - [Tumor immunology and checkpoint inhibitors](https://durgical.com/en/immunology/tumor-immunology-and-checkpoint-inhibitors/) - [Immune tolerance and how autoimmunity begins](https://durgical.com/en/immunology/immune-tolerance-and-how-autoimmunity-begins/) ## Related conditions - [Toxic shock syndrome](https://durgical.com/en/conditions/toxic-shock-syndrome/) ## Microorganisms - [Staphylococcus aureus](https://durgical.com/en/microbiology/staphylococcus-aureus/) - [Streptococcus pyogenes](https://durgical.com/en/microbiology/streptococcus-pyogenes/) ## In Pharmacology - [Abatacept](https://durgical.com/en/pharmacology/abatacept/) [← 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: [18.3 T Lymphocytes and Cellular Immunity](https://openstax.org/books/microbiology/pages/18-3-t-lymphocytes-and-cellular-immunity) (OpenStax (Microbiology), 2016); [Toxic shock syndrome](https://medlineplus.gov/ency/article/000653.htm) (MedlinePlus Medical Encyclopedia, U.S. National Library of Medicine, 2024); [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/t-cell-activation-two-signals-and-superantigens/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/immunology/t-cell-activation-two-signals-and-superantigens/