Antibodies cling onto antigens to either sterilize or kill the target cell. Other than that, they are very different in function. Antigens are locks or security gates in a cell, Antibodies are weapons or keys to attack and destroy a cell. They are ultimately two opposite things.
Here is a video explaining the difference:
Comparison chart
 | Antibody | Antigen |
|---|---|---|
| What it is | A chemical that attaches to the antigen in order to sterilize or kill the cell. | A 'hook' in the cell's surface, partially responsible for what goes in and out of the body. |
| Significance | Medium; needed for the destruction of pathogens in the body. | High; exists in almost every cell in the entire world. |
| Importance | High; without it a lot of us would probably be dead by a virus attack. | Very high; needed for controlling what goes in and out of the cell. Very important for it's inner function. |
| Complexity | Very High; Complex chemical that bonds to a very specific Antigen. | Medium; exists due to random mutations in the cell's gene. |
Bacteria 
DNA 
Antibiotics
Comments: Antibody vs Antigen