Questions: 1. Place these items in the correct sequence. eBook Doint References Drag the text blocks below into their correct order. Helper T-cell secretes IL-4. B-cell is activated and begins to proliferate. Antigen is presented on MHC Class II molecule on B-cell surface. TCR and CD4 molecules on the surface of a helper T-cell interact with MHC Class II molecule expressing antigen. Memory B-cells and plasma cells are produced. Free antigen binds to B-cell receptor (BCR). 2. Which of these processes involves a phagocytic cell?

To determine the correct sequence of events in the activation of a B-cell and its interaction with a helper T-cell, we need to understand the immunological process:

1. Free antigen binds to B-cell receptor (BCR).

   • The process begins when a free antigen binds to the B-cell receptor on the surface of a B-cell.

2. Antigen is presented on MHC Class II molecule on B-cell surface.

   • After binding, the B-cell internalizes the antigen, processes it, and presents it on its surface using MHC Class II molecules.

3. TCR and CD4 molecules on the surface of a helper T-cell interact with MHC Class II molecule expressing antigen.

   • A helper T-cell recognizes the antigen-MHC Class II complex through its T-cell receptor (TCR) and CD4 molecule.

4. Helper T-cell secretes IL-4.

   • Upon recognition, the helper T-cell becomes activated and secretes cytokines such as IL-4.

5. B-cell is activated and begins to proliferate.

   • The IL-4 and other signals from the helper T-cell activate the B-cell, causing it to proliferate.

6. Memory B-cells and plasma cells are produced.

   • The activated B-cells differentiate into memory B-cells and plasma cells, which produce antibodies.

So, the correct sequence is:

1. Free antigen binds to B-cell receptor (BCR).
2. Antigen is presented on MHC Class II molecule on B-cell surface.
3. TCR and CD4 molecules on the surface of a helper T-cell interact with MHC Class II molecule expressing antigen.
4. Helper T-cell secretes IL-4.
5. B-cell is activated and begins to proliferate.
6. Memory B-cells and plasma cells are produced.

Phagocytic cells are involved in the process of engulfing and digesting pathogens and presenting antigens. The primary phagocytic cells in the immune system are macrophages, dendritic cells, and neutrophils.

The process that involves a phagocytic cell is:

• Antigen is presented on MHC Class II molecule on B-cell surface.

Explanation:

• While B-cells are not traditionally considered phagocytic cells, they can internalize antigens through receptor-mediated endocytosis and present them on MHC Class II molecules. However, the more classical phagocytic cells like macrophages and dendritic cells are involved in the initial capture and presentation of antigens to T-cells.

In summary:

1. The correct sequence of events is:

   1. Free antigen binds to B-cell receptor (BCR).
   2. Antigen is presented on MHC Class II molecule on B-cell surface.
   3. TCR and CD4 molecules on the surface of a helper T-cell interact with MHC Class II molecule expressing antigen.
   4. Helper T-cell secretes IL-4.
   5. B-cell is activated and begins to proliferate.
   6. Memory B-cells and plasma cells are produced.

2. The process involving a phagocytic cell is the presentation of the antigen on the MHC Class II molecule, typically performed by professional antigen-presenting cells like macrophages and dendritic cells.