Answer
When an antigen-presenting cell (APC) detects a foreign antigen, it initiates a series of processes to capture, process, and present the antigen to T cells, which play a central role in the immune response. The two major classes of major histocompatibility complex (MHC) proteins, MHC class I and MHC class II, are involved in presenting antigens to different types of T cells, leading to distinct immune responses.
**Process When an APC Detects a Foreign Antigen:**
1. **Antigen Uptake**: The APC engulfs or takes up the foreign antigen. This process can involve phagocytosis (engulfment of larger particles) or pinocytosis (engulfment of smaller particles).
2. **Antigen Processing**: The engulfed antigen is broken down into smaller fragments within special compartments called endosomes or lysosomes. This processing involves enzymatic degradation of the antigen.
3. **MHC Binding**: The processed antigen fragments, also known as antigenic peptides, bind to MHC molecules within the endosomes or lysosomes. MHC molecules act as platforms for displaying these peptides on the surface of the APC.
4. **MHC-Antigen Complex Presentation**: The MHC molecules with bound antigenic peptides migrate to the cell surface, where they present the antigenic peptides to T cells.
**Functional Differences Between MHC-I and MHC-II Proteins:**
1. **MHC Class I (MHC-I)**:
- **Cellular Expression**: MHC-I molecules are present on the surface of almost all nucleated cells in the body.
- **Antigen Source**: MHC-I presents peptides derived from endogenous antigens, such as viral or intracellular microbial proteins synthesized within the infected cell.
- **T Cell Interaction**: MHC-I presents antigens to cytotoxic T cells (CD8+ T cells), which recognize and eliminate infected or abnormal cells.
2. **MHC Class II (MHC-II)**:
- **Cellular Expression**: MHC-II molecules are primarily expressed on the surface of professional antigen-presenting cells (APCs), such as dendritic cells, macrophages, and B cells.
- **Antigen Source**: MHC-II presents peptides derived from exogenous antigens, such as proteins from extracellular pathogens that have been engulfed and processed by the APC.
- **T Cell Interaction**: MHC-II presents antigens to helper T cells (CD4+ T cells), which coordinate immune responses by activating other immune cells and promoting antibody production.
**How T Cells Respond to MHC Protein Classes:**
1. **Cytotoxic T Cells (CD8+ T Cells)**:
- These T cells respond to antigens presented by MHC class I molecules.
- When a cytotoxic T cell's T-cell receptor (TCR) recognizes an antigenic peptide presented by MHC-I on an infected or abnormal cell, the cytotoxic T cell becomes activated.
- Activated cytotoxic T cells release cytotoxic granules containing molecules like perforin and granzymes, which induce apoptosis in the target cell.
2. **Helper T Cells (CD4+ T Cells)**:
- These T cells respond to antigens presented by MHC class II molecules.
- When a helper T cell's TCR recognizes an antigenic peptide presented by MHC-II on an APC, the helper T cell becomes activated.
- Activated helper T cells release cytokines that influence other immune cells, including B cells, cytotoxic T cells, and macrophages, to mount appropriate immune responses.
In summary, APCs play a crucial role in the immune response by capturing, processing, and presenting antigens to T cells using MHC molecules. The functional differences between MHC-I and MHC-II proteins result in distinct interactions with cytotoxic T cells and helper T cells, respectively, leading to coordinated immune responses tailored to the type of antigen encountered.
Work Step by Step
When an antigen-presenting cell (APC) detects a foreign antigen, it initiates a series of processes to capture, process, and present the antigen to T cells, which play a central role in the immune response. The two major classes of major histocompatibility complex (MHC) proteins, MHC class I and MHC class II, are involved in presenting antigens to different types of T cells, leading to distinct immune responses.
**Process When an APC Detects a Foreign Antigen:**
1. **Antigen Uptake**: The APC engulfs or takes up the foreign antigen. This process can involve phagocytosis (engulfment of larger particles) or pinocytosis (engulfment of smaller particles).
2. **Antigen Processing**: The engulfed antigen is broken down into smaller fragments within special compartments called endosomes or lysosomes. This processing involves enzymatic degradation of the antigen.
3. **MHC Binding**: The processed antigen fragments, also known as antigenic peptides, bind to MHC molecules within the endosomes or lysosomes. MHC molecules act as platforms for displaying these peptides on the surface of the APC.
4. **MHC-Antigen Complex Presentation**: The MHC molecules with bound antigenic peptides migrate to the cell surface, where they present the antigenic peptides to T cells.
**Functional Differences Between MHC-I and MHC-II Proteins:**
1. **MHC Class I (MHC-I)**:
- **Cellular Expression**: MHC-I molecules are present on the surface of almost all nucleated cells in the body.
- **Antigen Source**: MHC-I presents peptides derived from endogenous antigens, such as viral or intracellular microbial proteins synthesized within the infected cell.
- **T Cell Interaction**: MHC-I presents antigens to cytotoxic T cells (CD8+ T cells), which recognize and eliminate infected or abnormal cells.
2. **MHC Class II (MHC-II)**:
- **Cellular Expression**: MHC-II molecules are primarily expressed on the surface of professional antigen-presenting cells (APCs), such as dendritic cells, macrophages, and B cells.
- **Antigen Source**: MHC-II presents peptides derived from exogenous antigens, such as proteins from extracellular pathogens that have been engulfed and processed by the APC.
- **T Cell Interaction**: MHC-II presents antigens to helper T cells (CD4+ T cells), which coordinate immune responses by activating other immune cells and promoting antibody production.
**How T Cells Respond to MHC Protein Classes:**
1. **Cytotoxic T Cells (CD8+ T Cells)**:
- These T cells respond to antigens presented by MHC class I molecules.
- When a cytotoxic T cell's T-cell receptor (TCR) recognizes an antigenic peptide presented by MHC-I on an infected or abnormal cell, the cytotoxic T cell becomes activated.
- Activated cytotoxic T cells release cytotoxic granules containing molecules like perforin and granzymes, which induce apoptosis in the target cell.
2. **Helper T Cells (CD4+ T Cells)**:
- These T cells respond to antigens presented by MHC class II molecules.
- When a helper T cell's TCR recognizes an antigenic peptide presented by MHC-II on an APC, the helper T cell becomes activated.
- Activated helper T cells release cytokines that influence other immune cells, including B cells, cytotoxic T cells, and macrophages, to mount appropriate immune responses.
In summary, APCs play a crucial role in the immune response by capturing, processing, and presenting antigens to T cells using MHC molecules. The functional differences between MHC-I and MHC-II proteins result in distinct interactions with cytotoxic T cells and helper T cells, respectively, leading to coordinated immune responses tailored to the type of antigen encountered.
