Lymphocytes express receptors with varying affinity for the antigen in question. The cell with the highest affinity for the most abundant antigen will have growth advantage and will preferentially generate progeny of itself. This process is called clonal expansion and is antigen driven. B lymphocytes produce antibodies and some soluble mediators called cytokines. They arise in the bone marrow in adult mammals.
T lymphocytes arise in bone marrow but mature in the thymus.
They do not produce antibody molecules but have surface receptors structurally related to Ig. T cells see antigen in a different way to B cells. They recognise peptide fragments of antigen complexed with cell surface MHC glycoproteins on neighbouring cells. There are two sub-types of T cell defined on the basis of function, accessory molecule expression and the type of MHC protein presenting antigen to them. NK cells represent a first line of defence to infections, tumour growth and other pathogenic alterations of tissue homoeostasis.