Answer
Cellular immunity, also known as cell-mediated immunity, involves a series of coordinated stages that are fundamental to mounting an effective immune response against infected or abnormal cells. The three fundamental stages of cellular immunity are:
1. **Antigen Presentation and T Cell Activation**:
In this initial stage, specialized immune cells known as antigen-presenting cells (APCs) encounter foreign antigens, such as those derived from pathogens like viruses or bacteria. APCs process these antigens and present fragments of them on their cell surface using major histocompatibility complex (MHC) molecules. There are two classes of MHC molecules: MHC class I (MHC-I) and MHC class II (MHC-II). MHC-I molecules present antigens to cytotoxic T cells (CD8+ T cells), while MHC-II molecules present antigens to helper T cells (CD4+ T cells).
When a T cell's T-cell receptor (TCR) recognizes an antigen-MHC complex that matches its specificity, along with co-stimulatory signals from the APC, the T cell becomes activated. This activation prompts the T cell to undergo clonal expansion, resulting in the production of a large number of effector T cells with the same antigen specificity.
2. **Effector T Cell Response**:
The activated T cells differentiate into specialized effector T cell subsets based on the nature of the antigen and the local cytokine environment. These effector T cells perform specific functions that contribute to the immune response:
- **Cytotoxic T Cells (CD8+ T Cells)**: Activated cytotoxic T cells recognize infected or abnormal cells displaying the antigen-MHC-I complex. They release cytotoxic granules containing molecules like perforin and granzymes, leading to the destruction of the target cells through apoptosis.
- **Helper T Cells (CD4+ T Cells)**: Activated helper T cells differentiate into various subsets, such as Th1, Th2, Th17, and T follicular helper (Tfh) cells. Each subset produces distinct cytokines that influence the immune response. For example, Th1 cells secrete interferon-gamma to activate cytotoxic T cells and macrophages against intracellular pathogens, while Th2 cells stimulate B cells to produce antibodies.
3. **Memory and Long-Term Protection**:
After the immune response subsides, a pool of memory T cells is established. These memory T cells have the ability to "remember" the specific antigen they encountered. If the same antigen is encountered again, memory T cells can mount a faster and more robust immune response compared to the primary response. This memory phase provides long-term protection against reinfection and is a key component of vaccination strategies.
These three stages—antigen presentation and T cell activation, effector T cell response, and memory and long-term protection—outline the sequential events that occur during cellular immunity, allowing the immune system to detect and eliminate infected or abnormal cells while also establishing immunological memory.
Work Step by Step
Cellular immunity, also known as cell-mediated immunity, involves a series of coordinated stages that are fundamental to mounting an effective immune response against infected or abnormal cells. The three fundamental stages of cellular immunity are:
1. **Antigen Presentation and T Cell Activation**:
In this initial stage, specialized immune cells known as antigen-presenting cells (APCs) encounter foreign antigens, such as those derived from pathogens like viruses or bacteria. APCs process these antigens and present fragments of them on their cell surface using major histocompatibility complex (MHC) molecules. There are two classes of MHC molecules: MHC class I (MHC-I) and MHC class II (MHC-II). MHC-I molecules present antigens to cytotoxic T cells (CD8+ T cells), while MHC-II molecules present antigens to helper T cells (CD4+ T cells).
When a T cell's T-cell receptor (TCR) recognizes an antigen-MHC complex that matches its specificity, along with co-stimulatory signals from the APC, the T cell becomes activated. This activation prompts the T cell to undergo clonal expansion, resulting in the production of a large number of effector T cells with the same antigen specificity.
2. **Effector T Cell Response**:
The activated T cells differentiate into specialized effector T cell subsets based on the nature of the antigen and the local cytokine environment. These effector T cells perform specific functions that contribute to the immune response:
- **Cytotoxic T Cells (CD8+ T Cells)**: Activated cytotoxic T cells recognize infected or abnormal cells displaying the antigen-MHC-I complex. They release cytotoxic granules containing molecules like perforin and granzymes, leading to the destruction of the target cells through apoptosis.
- **Helper T Cells (CD4+ T Cells)**: Activated helper T cells differentiate into various subsets, such as Th1, Th2, Th17, and T follicular helper (Tfh) cells. Each subset produces distinct cytokines that influence the immune response. For example, Th1 cells secrete interferon-gamma to activate cytotoxic T cells and macrophages against intracellular pathogens, while Th2 cells stimulate B cells to produce antibodies.
3. **Memory and Long-Term Protection**:
After the immune response subsides, a pool of memory T cells is established. These memory T cells have the ability to "remember" the specific antigen they encountered. If the same antigen is encountered again, memory T cells can mount a faster and more robust immune response compared to the primary response. This memory phase provides long-term protection against reinfection and is a key component of vaccination strategies.
These three stages—antigen presentation and T cell activation, effector T cell response, and memory and long-term protection—outline the sequential events that occur during cellular immunity, allowing the immune system to detect and eliminate infected or abnormal cells while also establishing immunological memory.
