Cell-mediated Immune Response
1) Presentation of viral antigen on macrophage
Virus enters the body. Macrophage engulfs the virus. The virus is partly digested. A fragment of it is brought to the cell surface to be placed at the MHC-II protein. The complex formed is called antigen-MHC-II complex.
2a) Recognisation of antigen on macrophage by helper-T cell
Helper-T recognises the antigen on the MHC-II protein and binds with the macrophage by CD4 coreceptor. Then this binding stimulates macrophage to release interleukin-1. Then interleukin-1 stimulates helper-T to produce interleukin-2.
2b) Recognisation of antigen on infected cell by cytotoxic-T cell
Meanwhile cytotoxic-T cell recognises viral antigen on the MHC-I protein of the infected cell by CD8 coreceptor.
3) Proliferation & action of cytotoxic-T cell
Interleukin-2 (from step 2a) stimulates the proliferation of cytotoxic-T cell. Some of the cytotoxic-T cells become memory-T cells while some become effector-T cells. Cytotoxic-T cells secrete perforin to create pores on the infected cell. The infected cell is then destroyed by the pores that cause the cell content to leak out, shrinks, or burst.

Humoral Immune Response
Step 1 & 2a are identical.
2b) Presentation of viral antigen on B-cell
Meanwhile the free-floating viral antigens bind to the antibody on the surface of B-cell. B-cell takes in this antigen, digested it, and a fragment of this antigen is placed on MHC-II.
3) Recognisation of antigen on B-cell by helper-T cell
Helper-T recognises the antigen on MHC-II protein on B-cell. The binding stimulates helper-T to produce interleukin-2.
4) Proliferation of B-cells
Interleukin-2 released from helper-T then stimulates B-cells to proliferate. Some of the B-cells become plasma cells while some becomes memory-B. Memory-B will proliferate faster when encountering the same pathogen for the second time.
5) Production & action of antibodies
Plasma cells will produce antibodies. The antibodies are released into the bloodstream, and is brought to the site of infection. They can agglutinate the bacteria, thus many bacteria can be engulfed at the same time.

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