Abstract: The present invention relates to antibodies that bind phosphorylated forms of IRAK4, methods of using such antibodies to detect IRAK4 biological activity, and methods for the detection, diagnosis, and prognostication of pathological conditions related to IRAK4 biological activity. ERAK4 is one member of a small family of highly conserved cytoplasmic signal transduction proteins characterized by the presence of an N-terminal “death domain” and a C-terminal serine-threonine kinase domain. IRAK4 functions in cytoplasmic signal transduction pathways by interacting with membrane spanning proteins which play, inter alia, critical roles in vertebrate immune system function.