3.3. Evaluation of PD-1/PD-L1, CTLA-4/CD86, and CD200R/CD200 Concentrations in the Serum of Patients with Antibody Deficiencies with Near-Normal Immunoglobulin Levels or Hyperimmunoglobulinemia in Relation to Healthy Volunteers. Here, CTLA4 is linked to agammaglobulinemia.