1. The human immune system
A simplified account of how your immune system works

2. The Humoral Response - Activation Phase
antigen
The immune response starts with an antigen approaching a cell of the immune system, here a macrophage.
macrophage

3. The macrophage engulfs the antigen by phagocytosis.
Antigen engulfed in vesicle
The macrophage engulfs the antigen by phagocytosis.
Lysosome containing enzymes

4. The vesicle containing an antigen fuses with a lysosome
The vesicle containing an antigen fuses with a lysosome. The enzymes in the lysozome break down the antigen into fragments. This is antigen processing.
antigen processing

5. Inside the macrophage, the processed antigens combine with special
class II MHC proteins. These proteins can move to the cell surface membrane.
enzymes and
proteins combining
Class II
MHC proteins

6. The antigen/MHC protein complex is displayed on the immune cell surface membrane. The macrophage is now known as an antigen presenting cell.
Processed antigen/
MHC protein complex
Antigen-presenting cell

7. The next stage involves a helper T cell (also know as a T-helper cell) as well as the macrophage
receptors which bind to specific
antigen/MHC protein complex
T helper cell (TH)
macrophage (antigen-presenting cell)

8. The receptors on the helper T cell enable it to bind to the specific antigen-MHC complex of the antigen presenting cell.
helper T cell
macrophage (antigen-presenting cell)

9. The binding of the helper T cell with the antigen - MHC protein complex triggers the macrophage to release proteins (cytokines) that activate the helper T cell.
Cytokines from macrophage

10. The activated helper T cell now releases its own cytokines
Cytokines from helper T cell

11. The released cytokines stimulate the helper T cell to reproduce and form a clone of cells. Each new cell has the same receptors as the original helper T cell, so they are specific for the original antigen.
Clone of helper T cells

12. The Humoral Response - Effector Phase
Another phase of the immune response begins with a B cell. The B cell has membrane bound globular receptor proteins (called IgM). Some of these are specific for the same antigen presented earlier by the antigen presenting cell.
IgM receptor
antigen
B cell

13. The B cell’s receptor protein (an IgM) binds to the antigen, and the cell engulfs the antigen by endocytosis.
IgM bound to
antigen engulfed
by cell
lysosome

14. Fused vesicles containing antigen and enzymes from lysosome
The vesicle formed inside the B cell fuses with a lysosome. This contains digestive enzymes which break down the antigen.
B cell

15. Fragments of the digested antigen remain after processing within the vesicle.
Processed antigen
Class II
MHC
protein

16. The processed antigen is attached to Class II MHC receptors within the B cell, and is transported to the membrane.

17. Antigen/MHC protein complex
The MHC proteins form a complex with the antigen which is displayed on the surface of the B cell. It has become another type of antigen presenting cell
B cell (antigen presenting cell)

18. A helper T cell from the clone of cells produced earlier specifically recognises the antigen presented by the B cell.
Helper T cell clone
Antigen-presenting B cell

19. The helper T cell cell binds to the antigen/MHC protein complex displayed by the B cell.
This triggers the release of cytokines from the T cell. Once the cytokines are released the helper T cell no longer binds to the B cell.
helper T cell binds to
antigen complex on B cell
Cytokines released by
helper T cell

20. The cytokines released by the helper T cell stimulate the B cell to divide and form a clone of identical cells
B cell clones

21. The B cells continue to divide and form two groups of clones
The B cells continue to divide and form two groups of clones. Some are long – lived MEMORY cells. Most are antibody-secreting PLASMA cells. Plasma cells have extensive endoplasmic reticulum and many ribosomes.
Memory
cell
Plasma cells

22. Plasma cells are essentially antibody factories
Plasma cells are essentially antibody factories. They produce and secrete antibodies identical to those of the surface receptors of the original parent B cell
antibodies

23. Like the IgM surface receptors on the parent B cell, the antibodies can bind to and inactivate the antigens, forming an antibody-antigen complex. This complex makes it easier for other white blood cells to engulf the antigen (phagocytosis).
Antibody-antigen complex