The difference between innate and acquired immunity lies in the antigen specificity of lymphocytes. This property is conferred upon lymphocytes by the expression of cell surface receptors that recognise discrete parts of the antigen known as antigenic epitopes . The cell surface receptor of B lymphocytes , derived from B one marrow in mammals or the B ursa of fabricius in chickens, is an immunoglobulin molecule which, when secreted by the B cell, is known as an antibody . Evolution in mammals has led to expansion of the antibody gene pool and a mechanism for generating a vast number of antibodies with a wide range of different specificities. This large repertoire of different antibody specificities arms the immune system to respond to practically all of the potential antigens nature can devise.
Antibodies work in three ways.
During an immune response, a complex lattice of interlinked antigens and antibodies, known as an immune complex , will present an array of constant regions which can activate the various cells mentioned above through ligation of their immunoglobulin receptors. These cells have receptors for antigen and confer specificity on an immune response. 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.