T cells are activated when they bind to a complementary antigen presented on an antigen-presenting cell (APC) or a pathogen. This binding triggers a cascade of events leading to clonal selection and clonal expansion.
Here's a breakdown of the T cell activation process:
1. Antigen Presentation
- Antigen-Presenting Cells (APCs): T cells don't typically recognize free antigens directly. Instead, they rely on APCs such as macrophages, dendritic cells, and B cells to process and present antigens on their surface.
- MHC Molecules: APCs display antigens on Major Histocompatibility Complex (MHC) molecules. There are two main types:
- MHC Class I: Presents antigens derived from inside the cell (e.g., viral proteins). This is important for activating cytotoxic T cells (TC cells). Nearly all nucleated cells express MHC Class I.
- MHC Class II: Presents antigens derived from outside the cell (e.g., bacteria engulfed by phagocytosis). This is important for activating helper T cells (TH cells). Primarily found on APCs.
2. T Cell Receptor (TCR) Binding
- Specificity: Each T cell has a unique T cell receptor (TCR) that is specific to a particular antigen.
- Recognition: When an APC presents an antigen on an MHC molecule, a T cell with a TCR that is complementary to the antigen can bind. This binding is a crucial first step in activation. The TCR binds to the antigen fragment held within the MHC molecule.
- Co-stimulatory Signals: In addition to TCR binding, T cells also usually require co-stimulatory signals to become fully activated. These signals involve interactions between other molecules on the T cell and the APC. This "double-check" helps prevent T cell activation against self-antigens.
3. Clonal Selection
- Antigen Binding = Selection: The T cell whose TCR binds to the antigen displayed by the APC is "selected" for activation. This is known as clonal selection.
4. Clonal Expansion
- Mitotic Division: The selected T cell undergoes rapid cell division by mitosis, producing a large number of identical T cells (clones). This is called clonal expansion.
- Differentiation: These clones then differentiate into different types of effector T cells, depending on the type of T cell that was originally activated:
- Helper T Cells (TH cells): Secrete cytokines that help activate other immune cells, such as B cells and cytotoxic T cells.
- Cytotoxic T Cells (TC cells): Kill infected or cancerous cells by releasing cytotoxic molecules like perforin and granzymes.
- Regulatory T Cells (Treg cells): Suppress the immune response to prevent autoimmunity.
- Memory T Cells: Some of the cloned T cells differentiate into memory T cells, which provide long-lasting immunity. These cells can quickly respond to future encounters with the same antigen.
Summary of T Cell Activation:
| Step | Description |
|---|---|
| 1. Antigen Presentation | APCs present antigens on MHC molecules. |
| 2. TCR Binding | TCR on T cell binds to antigen-MHC complex. |
| 3. Co-stimulation | Additional signals from APCs are needed for full activation. |
| 4. Clonal Selection | T cell with complementary TCR is selected. |
| 5. Clonal Expansion | Selected T cell divides rapidly, creating clones. |
| 6. Differentiation | Clones differentiate into effector and memory T cells. |
