2.5.4 The T Lymphocyte Response

Key Features of T Cells:

1. Development and Maturation:

• T cells are produced in the bone marrow.
• They migrate to the thymus gland, where they mature and differentiate.

2. Cell-Mediated Immunity:

• T cells respond to antigen-presenting cells (APCs) displaying non-self antigens on their surface.
• This is crucial for fighting intracellular pathogens (e.g., viruses) and abnormal cells (e.g., cancer).

Stages of the T Cell Response:

3. Recognition of Antigens:

• T cells bind to antigens presented on the surface of APCs, such as phagocytes or infected body cells.
• This recognition is mediated by the T cell receptor (TCR), which is specific to the antigen.

4. Activation of T Cells:

• When a T cell binds to a complementary antigen, it becomes activated.
• Activation often requires the presence of cytokines, signalling proteins released by other immune cells.

5. Clonal Expansion:

• The activated T cell divides by mitosis, producing many identical T cells specific to the antigen.
• This process ensures a rapid and targeted immune response.

6. Differentiation into T Cell Subtypes:

• Activated T cells differentiate into specialised subtypes:
• Helper T cells (Tᕼ):
• Release cytokines to stimulate B cells, cytotoxic T cells, and phagocytes.
• Cytotoxic T cells (Tᴄ):
• Kill infected or abnormal cells by releasing perforins, which create pores in the target cell membrane, causing cell death.
• Regulatory T cells (Tʀ):
• Suppress the immune response once the infection is cleared to prevent autoimmunity.
• Memory T cells:
• Persist after the infection to provide a faster, stronger response if the same antigen is encountered again.

Importance of T Cell Response:

7. Directly Targets Infected Cells:

• Particularly effective against viruses and intracellular bacteria.

8. Stimulates Other Immune Cells:

• Helper T cells enhance the activity of B cells and phagocytes.

9. Immunological Memory:

• Memory T cells provide long-term protection, enabling a quicker immune response upon re-infection.

Diseases Linked to T Cell Dysfunction:

• HIV/AIDS: Targets helper T cells, weakening the immune system.
• Autoimmune diseases: Dysfunctional T cells attack the body's own cells.