A method for identifying a modulator antibody of a phenotype of a eukaryotic cell type, comprising: (a) expressing, in a population of cells of the eukaryotic cell type, a library of either: (i) secreted candidate antibodies or antigen-binding fragments thereof to produce a heterogeneous population of modified, antibody-expressing cells, or antigen-binding-fragment-expressing cells, wherein the cells are cultured in a diffusion restricting culture medium such that the secreted antibodies or antigen-binding fragments thereof are trapped around the cells producing the secreted antibodies or antigen-binding fragments thereof, and wherein the phenotype is mediated via a target molecule, or (ii) membrane-integrated candidate antibodies or antigen-binding fragments thereof to produce a heterogeneous population of modified, antibody-expressing cells or antigen-binding-fragment-expressing cells, wherein the entire membrane-integrated antibody library, or membrane-integrated antigen-binding-fragment library, and a target receptor are co-integrated into the plasma membranes of a population of reporter cells, and wherein the phenotype is mediated via the target receptor, and (b) selecting, in an autocrine manner, a specific antibody-expressing cell with an altered phenotype relative to that of an unmodified control cell of said eukaryotic cell type; thereby identifying the candidate antibody expressed in the specific antibody-expressing cell as a modulator of the eukaryotic cell type. (a) expressing, in a population of cells of the eukaryotic cell type, a library of either: (i) secreted candidate antibodies or antigen-binding fragments thereof to produce a heterogeneous population of modified, antibody-expressing cells, or antigen-binding-fragment-expressing cells, wherein the cells are cultured in a diffusion restricting culture medium such that the secreted antibodies or antigen-binding fragments thereof are trapped around the cells producing the secreted antibodies or antigen-binding fragments thereof, and wherein the phenotype is mediated via a target molecule, or (ii) membrane-integrated candidate antibodies or antigen-binding fragments thereof to produce a heterogeneous population of modified, antibody-expressing cells or antigen-binding-fragment-expressing cells, wherein the entire membrane-integrated antibody library, or membrane-integrated antigen-binding-fragment library, and a target receptor are co-integrated into the plasma membranes of a population of reporter cells, and wherein the phenotype is mediated via the target receptor, and (i) secreted candidate antibodies or antigen-binding fragments thereof to produce a heterogeneous population of modified, antibody-expressing cells, or antigen-binding-fragment-expressing cells, wherein the cells are cultured in a diffusion restricting culture medium such that the secreted antibodies or antigen-binding fragments thereof are trapped around the cells producing the secreted antibodies or antigen-binding fragments thereof, and wherein the phenotype is mediated via a target molecule, or (ii) membrane-integrated candidate antibodies or antigen-binding fragments thereof to produce a heterogeneous population of modified, antibody-expressing cells or antigen-binding-fragment-expressing cells, wherein the entire membrane-integrated antibody library, or membrane-integrated antigen-binding-fragment library, and a target receptor are co-integrated into the plasma membranes of a population of reporter cells, and wherein the phenotype is mediated via the target receptor, and (b) selecting, in an autocrine manner, a specific antibody-expressing cell with an altered phenotype relative to that of an unmodified control cell of said eukaryotic cell type; thereby identifying the candidate antibody expressed in the specific antibody-expressing cell as a modulator of the eukaryotic cell type.