T Cell Activation Pathways Summary

Exogenous Pathway:
- Associated with MHC class II molecules.
- Involves CD4 T cells.
- Requires professional antigen-presenting cells (APCs).
Endogenous Pathway:
- Associated with MHC class I molecules.
- Involves CD8 T cells.

Full Activation:
- Requires two signals:
  - Signal 1
  - Signal 2
Differentiation:
- Requires three signals:
  - Signal 1
  - Signal 2
  - Signal 3: Cytokines received during the activation process (specifically for CD4 T cells).
- Cytokines released during differentiation shape the immune response.

Cytokines are important in T cell activation.
The role of cytokines and their mechanisms will be discussed in more detail in Wednesday's lecture.

It is helpful to associate:
- Exogenous pathway with MHC class II and CD4 T cells.
- Endogenous pathway with MHC class I and CD8 T cells.

Using pictures and diagrams is encouraged to illustrate key concepts and pathways.
Focus on including key information that differentiates between immune responses.

Exogenous Pathway:
- Antigens are taken up from outside the cell (e.g., bacteria, toxins).
- Processed within endosomes and lysosomes.
- Peptides are loaded onto MHC class II molecules.
- Involves CD4 T cells, which differentiate into helper T cells.
- Requires professional antigen-presenting cells (APCs) such as dendritic cells, macrophages, and B cells. These cells have the ability to present antigens in the context of MHC class II molecules.
Endogenous Pathway:
- Antigens are derived from inside the cell (e.g., viral proteins, tumor antigens).
- Processed by the proteasome in the cytoplasm.
- Peptides are transported into the endoplasmic reticulum (ER) and loaded onto MHC class I molecules.
- Involves CD8 T cells, which become cytotoxic T lymphocytes (CTLs) capable of killing infected cells.

Full Activation:
- Requires two signals:
  - Signal 1: TCR (T cell receptor) binds to the peptide-MHC complex on the APC. This ensures that the T cell is responding to a specific antigen.
  - Signal 2: Co-stimulatory signal, such as CD28 on the T cell binding to B7 (CD80/CD86) on the APC. This signal confirms that the antigen presentation is occurring in the context of an active immune response.
Differentiation:
- Requires three signals:
  - Signal 1: TCR-MHC interaction (as above).
  - Signal 2: Co-stimulatory signal (as above).
  - Signal 3: Cytokines received during the activation process (specifically for CD4 T cells). These cytokines direct the differentiation of CD4 T cells into different subsets (e.g., Th1, Th2, Th17, Treg).
- Cytokines released during differentiation shape the immune response. For example, IL-12 promotes Th1 differentiation, leading to cellular immunity, while IL-4 promotes Th2 differentiation, leading to humoral immunity.

Cytokines are crucial for communication between immune cells and play a vital role in T cell activation, differentiation, and the overall immune response.
The role of cytokines and their mechanisms will be discussed in more detail in Wednesday's lecture, including specific examples of cytokines and their effects on immune cell function.

It is helpful to associate:
- Exogenous pathway with MHC class II and CD4 T cells. This pairing is essential for understanding how the immune system responds to extracellular pathogens.
- Endogenous pathway with MHC class I and CD8 T cells. This pairing is critical for targeting and eliminating cells infected with intracellular pathogens or displaying tumor antigens.

Using pictures and diagrams is encouraged to illustrate key concepts and pathways, making it easier to remember and understand the differences between the exogenous and endogenous pathways.
Focus on including key information that differentiates between immune responses, such as the source of antigens, the types of cells involved, and the outcomes of T cell activation.