I think you described it pretty well. my version of the same: During their development Lymphocytes have 'gene cassettes' that get shuffled randomly so that the

I think you described it pretty well. my version of the same: During their development Lymphocytes have 'gene cassettes' that get shuffled randomly so that the specificity of the receptor (antibody for cells, T cell receptor for T cells) is random for each individual lymphocyte cell during development. The ones that bind "Self" stuff are killed off during their development and the rest of them other just hang out waiting for something that binds their receptor. (There is a step here where T cells must bind to MHC within an affinity window to survive) If they find it the reproduce rapidly making many more cells with that specify. once amazing thing is that during that cell replication at the sites of antibody specificity the DNA copying gets sloppy, allowing new mutations to 'fine tune' the specificity of the antibody to the antigen (called somatic hypermutation). The cells that bind the tightest to the foreign antigen, reproduce the most robustly providing an 'evolution' of increasing affinity to the antigen. This stuff is only the tip of the iceberg - immunology it is amazing.