Primary and Secondary Immune Response ☃️❄️

Define primary immune response

Primary immune response refers to the immune response elicited by the body during the first exposure to an antigen. It is mediated by naive B and naive T lymphocytes that are in contact with antigens for the first time.

Describe what happens during the primary immune response

1. The antigen is taken up by an antigen-presenting cell (APC) via phagocytosis, processed by the APC and presented on Class II MHC to naive CD4 T cells.

2. Naive CD4 T cell with receptors recognise and bind to the antigen presented by the APC.

3. This activates naive CD4 T cell to proliferate and differentiate to form many antigen-specific helper T cells and memory cells.

4. Naive B cell with receptors recognise and bind to the same antigen, takes it up via phagocytosis, processes it and presents it to helper T cells.

5. B cell is activated.

6. Cytokines released by helper T cells completes activation of B cell.

7. This activates B cell to proliferate and differentiate to form many antigen-specific plasma cells and memory cells.

8. Viral antigen synthesised inside the cell is presented on Class I MHC to naive CD8 T cells.

9. Naive CD8 T cell with receptors recognise the antigen and bind to the infected cell.

10. CD8 T cell is activated.

11. Cytokines released by helper T cells completes activation of CD8 T cell.

12. This activates CD8 T cell to proliferate and differentiate to form many antigen-specific cytotoxic T cells and memory cells.

Define secondary immune response

Secondary immune response refers to the immune response elicited by the body during the re-exposure to the same antigen. It is mediated by memory B and memory T cells that can quickly proliferate and differentiate into large clones of effector cells and memory cells.

Describe what happens during the secondary immune response

1. The antigen is taken up by an antigen-presenting cell (APC) via phagocytosis, processed by the APC and presented on Class II MHC to memory CD4 T cells.

2. Memory CD4 T cell with receptors recognise and bind to the antigen presented by the APC.

3. This activates memory CD4 T cell to quickly proliferate and differentiate to form large clones of antigen-specific helper T cells and memory cells.

4. Memory B cell with receptors recognise and bind to the same antigen, takes it up via phagocytosis, processes it and presents it to helper T cells.

5. Memory B cell is activated.

6. Cytokines released by helper T cells completes activation of memory B cell.

7. This activates memory B cell to quickly proliferate and differentiation to form large clones of antigen-specific plasma B cells releasing large amounts of IgG antibodies and memory cells.

8. Viral antigen synthesised inside the cell is presented on Class I MHC to memory CD8 T cells.

9. Memory CD8 T cell with receptors recognise the antigen and bind to the infected cell.

10. Memory CD8 T cell is activated.

11. Cytokines released by helper T cells completes activation of memory CD8 T cell.

12. This activates memory CD8 T cell to quickly proliferate and differentiate to form large clones of antigen-specific cytotoxic T cells and memory cells

Describe the differences between the primary and secondary immune response

1. Duration of latent period: primary immune response has a longer latent period while secondary immune response has a shorter latent period.

2. Duration of response: primary immune response occurs for a shorter duration while secondary immune response occurs for a longer duration.

3. Type of antibodies: antibodies produced in primary immune response are mainly IgM and have a lower affinity for antigen while antibodies produced in secondary immune response are mainly IgG and have a higher affinity for antigen.