How do antigens and antibodies of the blood develop?

Antigens are proteins that are present on the surface of pathogens and are specific to that pathogen.

Antibodies are 'Y' shaped proteins having a unique binding site shape which locks onto the specific shape of the antigen.

When lymphocytes ( type of white blood cell) recognise a foreign body or antigen, it produces antibodies. These bind to the antigen and destroy it, which is then engulfed and digested by macrophages.
The immune system winds down when the invading microbes have been destroyed.

The human body also produces memory cells specific to an antigen. In case of re infection, these cells quickly produce antibodies and curb the infection.

Blood group genes are multiple alleles. This means the gene for ABO system has multiple expression. When it is recessive blood group O is expressed. When A or B are dominant A and B blood groups are expressed. Same thing is with antibodies for A and B. Blood group O serum is with no antibodies.
If a person is with A blood group is with antibodies against B and vice versa.
A person with AB blood group is with no antibodies in serum.
Blood group is the antigen on the RBC.