Plenty of reasons. If your kidneys are losing protein in your urine (proteinuria), this can affect the production of antibodies to fight infections. If your kidney disease is a result of uncontrolled diabetes, elevated blood sugars can lower the immune response to infections. Finally, if one is on dialysis, great care must be taken to keep the dialysis site sterile as this is often a source of infections.
Decrease it! All chronic body illnesses weaken the immune system. Physical and mental--- makes no different. Weakens the immune response!