T cell activation is a central event in adaptive immunity because it determines whether the body mounts a targeted immune response against infection, cancer, or abnormal cells. A naive T cell does not respond to antigen alone. It must receive a coordinated set of signals from an antigen presenting cell at the immunologic synapse.
Understanding these signals helps explain normal immune defense, vaccine responses, transplant rejection, and many immunotherapies.
The first activation signal is antigen recognition, where the T cell receptor binds a peptide presented by major histocompatibility complex molecules on the APC. The second signal is co stimulation, most importantly CD28 on the T cell binding B7 molecules on the APC, which prevents anergy and supports survival and proliferation. Cytokines then provide a third signal that directs differentiation into effector subtypes such as Th1, Th2, Th17, Tfh, Treg, or cytotoxic CD8 T cells.
Each subtype produces distinct cytokines and carries out specialized immune functions in tissues.
Understanding T Cell Activation
Antigen presenting cells do more than display a fragment of a microbe. They first capture material, break proteins into short pieces, then load selected pieces onto MHC molecules. Dendritic cells are especially effective at this job.
They travel from infected tissue to lymph nodes, where many naive T cells circulate. A T cell may scan many dendritic cells before finding its matching peptide. This search is highly selective.
The T cell receptor recognizes both the peptide and the particular MHC molecule holding it. This is why a T cell response is specific, yet depends on the body’s own MHC proteins.
Co-stimulation acts like a safety check on this recognition process. Healthy body cells can sometimes display normal peptides, and weak recognition of these peptides can occur. A professional antigen presenting cell increases B7 expression mainly when it detects danger, such as microbial molecules or tissue damage.
This means that the T cell receives a stronger activation message when there is evidence of a real threat. If this control fails, self-reactive T cells may become active and contribute to autoimmune disease. If the control is too strict, useful responses against infections or tumours may be limited.
Once activated, a T cell changes rapidly. It makes interleukin 2, a growth signal that helps it divide repeatedly. One original cell can produce a large clone of cells with the same antigen specificity.
The surrounding cytokines influence what those daughter cells become. For example, helper cells that support macrophages are useful against microbes living inside cells. Other helper cells support antibody production, especially in lymph node follicles.
Cytotoxic T cells kill infected or abnormal body cells by releasing proteins that trigger the target cell to die. Some activated cells become memory T cells. They remain after the infection has passed and can respond faster during a later exposure.
Inhibitory receptors are important because an immune response must stop as well as start. CTLA-4 reduces early activation by competing with CD28 for B7 molecules. PD-1 can reduce T cell activity later, particularly in tissues where inflammation is occurring.
These pathways protect normal tissues from excessive damage. Cancer cells can sometimes exploit PD-1 signals to weaken immune attack. Drugs called checkpoint inhibitors block selected inhibitory signals and can help some patients fight cancer.
The same treatment can cause inflammation in healthy organs, which shows why immune brakes are necessary. When studying this topic, keep the timing and location of each signal clear. Activation begins in lymphoid organs, while effector cells often act later in infected tissues.
Key Facts
- Signal 1: TCR + CD3 binds peptide MHC on the APC.
- MHC I presents endogenous peptides to CD8 T cells; MHC II presents exogenous peptides to CD4 T cells.
- Signal 2: CD28 on T cells binds B7-1 (CD80) or B7-2 (CD86) on APCs.
- Without co stimulation, antigen recognition can lead to anergy or functional unresponsiveness.
- IL-2 promotes clonal expansion of activated T cells through autocrine signaling.
- CTLA-4 and PD-1 are inhibitory receptors that reduce T cell activation and help maintain peripheral tolerance.
Vocabulary
- Immunologic synapse
- The organized contact region between a T cell and an antigen presenting cell where signaling molecules cluster during activation.
- Major histocompatibility complex (MHC)
- A set of cell surface proteins that display peptide antigens for recognition by T cells.
- Co stimulation
- A second activating signal, such as CD28 binding B7, that is required along with antigen recognition for full T cell activation.
- Anergy
- A state in which a lymphocyte remains alive but becomes unresponsive to antigen stimulation.
- Effector T cell subtype
- A differentiated T cell population with a specific cytokine pattern and immune function, such as Th1, Th2, Th17, Tfh, Treg, or cytotoxic T cells.
Common Mistakes to Avoid
- Assuming antigen binding alone activates a naive T cell, which is wrong because full activation usually requires both peptide MHC recognition and co stimulation.
- Confusing MHC I with MHC II, which is wrong because MHC I mainly activates CD8 T cells while MHC II mainly activates CD4 T cells.
- Thinking all activated CD4 T cells become the same helper cell type, which is wrong because local cytokines drive different effector fates with different functions.
- Forgetting inhibitory checkpoints such as CTLA-4 and PD-1, which is wrong because activation is balanced by negative regulators that limit tissue damage and autoimmunity.
Practice Questions
- 1 An APC presents a viral peptide on MHC I to a naive T cell and also expresses B7. Which T cell coreceptor is expected on the responding T cell, and what is the main effector function of the activated cell?
- 2 A naive T cell recognizes peptide MHC on an APC but receives no CD28 to B7 co stimulation. What functional outcome is most likely, and why?
- 3 A patient has an infection with an intracellular bacterium. Explain why a Th1 biased response would be more useful than a Th2 biased response in this situation.