C3.2.8 Activation of B-lymphocytes by helper T-lymphocytes Notes

Activation of B-Lymphocytes by Helper T-Lymphocytes

1. In the adaptive immune system, both B-lymphocytes and helper T-lymphocytes play critical roles in mounting an effective immune response.
2. Activation of B-lymphocytes is a crucial step in the adaptive immune response.
3. B-cells are responsible for producing antibodies, and they only become activated when they are exposed to their specific antigen.
4. However, for full activation, B-cells also require assistance from helper T-lymphocytes (T-helper cells).

The Role of Antigen-Specific B-Cells and Helper T-Cells

1. Antigen-Specific Cells: Both B-cells and helper T-cells are highly specific, each recognizing only one type of antigen.
2. B-Cells: These cells produce antibodies, which are proteins that bind to antigens and neutralize pathogens.
3. Helper T-Cells: These cells do not produce antibodies but play a critical role in activating B-cells.

Step 1: Antigen Recognition by B-cells

1. A macrophage engulfs a pathogen and breaks it down.
2. The macrophage presents fragments of the pathogen (antigens) on its surface using
3. A B-cell has membrane-bound antibodies (immunoglobulins) on its surface, which act as receptors for antigens.
4. When the B-cell encounters its specific antigen, the antigen binds to these antibodies on the B-cell’s surface.
5. This binding alone is not enough for full activation.

Step 2: Helper T-cell Activation

1. A helper T-cell recognizes an antigen presented on the surface of an Antigen-presenting cells (APC, e.g, dendritic cell) via its T-cell receptor (TCR).
2. The helper T-cell becomes activated when its TCR binds to the antigen-MHC (Major Histocompatibility Complex) complex on the APC.
3. The activated helper T-cell then undergoes clonal expansion and differentiation, producing cytokines.

Step 3: B-cell Activation by Helper T-cells

1. The activated helper T-cell interacts directly with the B-cell through a receptor-ligand interaction (via CD40 ligand on T-cells and CD40 receptor on B-cells).
2. The helper T-cell also secretes cytokines, like interleukin, that further stimulate the B-cell.
3. This interaction leads to full activation of the B-cell.

Step 4: Proliferation and Differentiation

1. Once activated, the B-cell proliferates (clonal expansion) and differentiates into plasma cells, which secrete large quantities of antibodies specific to the antigen.
2. Some B-cells differentiate into memory B-cells, which persist in the body to respond more rapidly to future encounters with the same antigen.

Why Is Activation Necessary?

1. Energy Efficiency: Producing antibodies is resource-intensive. Activation ensures B-cells only produce antibodies when needed.
2. Specificity: The two-step activation process ensures that only B-cells specific to the pathogen are activated.

Memory Cells: Preparing for Future Attacks

1. Plasma Cells: Some activated B-cells become plasma cells, producing large quantities of antibodies.
2. Memory B-Cells: Others become memory cells, which remain in the body long after the infection is cleared.

Applications and Implications

1. Vaccination: Vaccines introduce harmless antigens to the body, activating B-cells and helper T-cells without causing illness. This creates memory cells for future protection.
2. HIV and Immunodeficiency: HIV targets helper T-cells, disrupting their ability to activate B-cells. This weakens the immune system, making it vulnerable to infections.