Antibody "memory"- how they link up
An antibody is a chemical "lock and key" that connects a pathogen with a WBC, Killer T cell, or spikey protein.
Each antibody is manufactured for a specific pathogen (disease) and remains in the blood stream to help protect you if/when the pathogen infects you again. Antibodies have, for all sakes and purposes, two "ends". One end is a specific shape (say, round) for every antibody that is ever manufactured. This round shape receives the "ball" shape that is on every WBC, Killer T cell, and spikey protein in a particular person. No matter how many are made, the one end- in this case- is always round. No matter how many WBC, Killer T cells, and spikey proteins are made, they always have a ball shape that fits into the antibodies round shape.
When a pathogen infects the body, it has a specific shape on it also- say a pointy arrow. When the antibody is manufactured, one end is the round shape (to attach to the WBC, Killer T cells, and spikey proteins) and the other end is made to receive the pathogen's shape- in this case, a triangle shape to receive the arrow shape.
Should another, different pathogen enter the body with, say, a block shape, then another antibody would be made with a round shape on one end and a square shape on the other to receive the block shape of this new pathogen. It is through this "lock and key" method that the cells of our immune system know which things to destroy and which things to leave alone.
In this way the body builds up a libary of antibodies that know which pathogens to attach to based on the protein shapes that fit into it. Cowpox antibodies have a shape that fits into Cowpox pathogens. However, Edward Jenner reailzed that the antibodies made for Cowpox are so close to the shape for Smallpox that the Cowpox antibodies will attach to Smallpox pathogens. In this way, Jenner discovered the first vaccines.