Cellular immunodeficiency in protein-losing enteropathy. Predominant reduction of CD3+ and CD4+ lymphocytes. Cellular immunological abnormalities were studied in a patient with protein-losing enteropathy associated with constrictive pericarditis. Analysis of lymphocyte subpopulations in peripheral blood showed lymphopenia with a decrease of CD3+ and CD4+ T cells, whereas CD8+ lymphocytes, B cells and NK cells were within the normal range. Fecal loss of lymphocytes as a cause of lymphopenia was evidenced by a marked excretion of 111-indium-labeled peripheral blood mononuclear cells via stool. Proliferative responses against several mitogens were severely reduced as was in vitro IgG production. Delayed-type hypersensitivity reaction against a variety of antigens was absent. Vaccination with tick-borne encephalitis virus, used for primary immunization, and with the recall antigen tetanus toxoid resulted in a blunted antibody response. After pericardectomy, the severity of enteric protein loss declined, serum immunoglobulin levels returned to the normal range, and total lymphocytes and CD3+ and CD4+ counts increased but remained low even 12 months after surgery. Fecal loss of lymphocytes was found to be reduced after pericardectomy, but was higher than that seen in a disease control patient with active inflammatory bowel disease. In vitro immunoglobulin production returned to normal, DTH could be demonstrated against purified protein derivative and proteus antigen, but mitogen-driven blastogenic response of lymphocytes remained low. Revaccination with tick-borne encephalitis and tetanus toxoid antigens seven months after surgery resulted in a dramatic increase of serum levels of antibodies against both antigens, comparable to that seen in healthy control individuals. 