Introduction:
The complement system plays a key role in the host defense against pathogens. The deficiency of complement components predisposes the system to recurrent infections and autoimmune diseases. In particular, serum C5 deficiency (C5D) may be serious for human health, because this protein plays a key role in controlling infections, mainly with Neisseria spp.

Aim:
The aim of this article is to present the structure and function of the human C5 gene encoding complement component C5, with particular regard to the molecular characteristics of the mutations causing hereditary complement C5 deficiency.

Material and methods:
This article is based on the available literature. A total of 35 articles were included in the study.

Results and discussion:
Based on the literature review, it was shown that C5 mediates inflammatory processes and bacterial cytolysis. The cause of hereditary C5 deficiency in humans is inefficient or reduced serum C5 biosynthesis, due to mutations in the C5 gene. This quantitative and functional C5 deficiency is associated with recurrent Neisseria spp. infections, the lack of bactericidal activity and an impaired ability of serum to induce chemotaxis. The molecular characterization of previously described C5D-related mutations in the human C5 gene has been performed, and the clinical presentation of some molecularly examined C5D probands has also been discussed.

Conclusions:
The deficiency of C5 protein, which bridges innate and adaptive immunity, is related with 18 different mutations in the C5 gene found in over 30 families of various origins. Screening for complement defects seems particularly important, especially in asymptomatic relatives of probands.