The present invention provides and includes monoclonal antibodies (mAbs) preferentially selective for HER2 antigens, hybridoma lines that secrete these HER2 antibodies or antibody fragments, and the use of such antibodies and antibody fragments to detect HER2 antigens, particularly those expressed by cancer cells. The present invention also includes antibodies that are specific for or show preferential binding to a soluble or secreted form of HER2. The present invention also includes an antibody or antibody fragment that is capable of reducing the activity of HER2 in at least one form, including a soluble form or a secreted form. The present invention further includes chimeric antibodies, processes for producing monoclonal and chimeric antibodies or monoclonal or chimeric antibodies, and their therapeutic uses, particularly in the detection of cancer most preferentially in human breast, stomach, and colon. The present invention further includes methods and kits for the immunodetection and immunotherapy of cells for samples which express HER2 antigens.