The main biological function of the immune system is to protect a host from adverse activities of germs and its breakdown results in an enhanced susceptibility to various microorganisms. Neutropenia patients, because of their inefficient phagocytosis, suffer from pyogenic bacteria, enterobacteria and yeasts infections, respectively. Agamaglobulinemic and hypogamaglobulinemic patients are threatened by numerous bacteria, molds, viruses and parasites, respectively; when is there a selective IgG2 deficiency, patients are susceptible to infections caused by encapsulated bacteria. Complement deficient patients suffer from pyogenic (C3) and neisseria infections (C6, 7, 8), respectively, however they can develop autoimmune disorders (C1q, C2, C4), esp. SLE, too. A breakdown of cellular immunity is accompanied by severe viral and parasitic infection, however, as B cell do not usually function properly too, bacterial infections are also not unusual.