B cells randomly mutate their BCRs and thus antibodies. Now this comes after a helper T cell recognizes an antigen a B cell presents on its BCR, which occurs after a B cell has in on itself recognized an antigen and has processed it for presentation to a T cell. Some of these mutations can actually make the BCR worse at picking up antigens and some of these mutations can make it better at picking up antigens and it will chop them into pieces to present to the helper t cell. This entire process is called affinity maturation. A crucial adaptive immune process. Affinity measures how well one antibody binding site binds to one epitope on an antigen. This is different from avidity, which is the binding strength of multiple antibody binding sites on multiple epitodes on an antigen. Low affinity and high avidity can co exist. This is because the fact that one single antibody binding site were to have a high dissociation rate would still mean that the other binding sights are still holding onto the pathogen. This also means that the antibody binding sites that loosened has a good chance of clinging back onto the target. B cells go through two stages of tolerance when being trained for autoreactivity: central tolerance, named such because it takes place in the bone marrow, a central lymphoid organ. Although the initial stages of tolerance in B cells trake place in the bone marrow, the latter stages take place in the peripheral organs if some autoreactive B cells escape the bone marrow.