This paper presents some processes of the antibacterial effect of serum, which mainly results from the activities of complement (C) and lysozyme (muramidase, LZ). The C system consists ofa group of serum proteins and tissue fluids which are activated in a particular order. Complement,operating together with lysozyme, constitutes the main protection from microorganisms entering the body. Pathogenic microorganisms are able to avoid natural protective mechanisms by, among others, molecular mimicry, binding complement control proteins, or secreting proteolytic enzymes.The effectiveness of the cytolytic action of C proteins and LZ also depends on the surface structures of the microorganisms. Imbalance between the activation and deactivation of inflammatory reactions in the presence of pathogens can lead to various pathological states, such as autoimmunological diseases.