The result of data analyses showed that the concentration of complement 3 and 4 were higher among smokers compared to the control group (Table 1). For instance, the minimum C3 concentration was 183.73 mg/dl for the smoker group while it was 146.24 mg/dl for the control group. A similar observation was observed for C4 (Table 1). However, when the independent t-test was applied to the mean concentration of C3 and C4 levels in the control and smoker groups were found to be statistically insignificant (Table 1). Our results disagree with the previous study. For instance,  showed that cigarette smoke contains numerous compounds especially methylamine, which causes break down the thioester bonds in C3 leading to alternative pathway activation, this alternative pathway mechanism may explain the increase in the level of C3 in all smokers. Our study agrees with the study by , which they found the mean serum levels of C3 and C4 statistically insignificant compared with control.
Some of the subjects in our study suffered from wheezing and shortness of breath. The increased levels of C3 in some smokers may be associated with allergic and inflammatory reactions caused by smoke that contains a huge number of harmful compounds.
Activation of complement factors may be an important part of inflammatory processes. Smoke does not activate C1, indicating that it does not activate the classical pathway. However, smoke completely consumes serum C4. Since C4, like C3, contains an internal bond , cigarette smoke may directly consume C4 by cleaving its thioester without activating the classical pathway. This may explain the change in C4 concentration in some of the smokers in this study.
Depending on the collected data, the results exclude the effect of complement 3 and/ or complement 4 in developing the pathogenesis of inflammation induced by smoking cigarettes. Instead, a traditional protocol known as total hemolytic assay (CH50) were used in this study to determine the function of complement activity. In short definition, CH50 calculate the ability of test serum to lyse half of the standard solution of sheep red blood cells coated with anti-erythrocyte antibody (terminal complement components and classic activation were calculated). In which, total complement activity indicated abnormal value if any component were defective .
The CH50 test will determine the ability of serum to hemolysis the sheep red blood cells, this test is always used for assessment the classical activation pathway of the complement system at the first step. The result of CH50 test will report the level of dilution of serum that can induce hemolysis of 50% of sheep red blood cells. Any increase in CH50 test results indicates an increase in activity of classical activation pathway for complement system, these increase in activity of complement system activation were not associated with any clinical signs except it probably indicate a process of active inflammatory and immune response. In other hands, undetectable CH50 test result indicates that a complement system suffering from a deficiency in any component of its elements. So, any defect in the production of any elements of the complement system will lead to dysfunction in classical activation pathways of complement.
The Inherited dysfunctions present in may clinical features included recurrent infection and autoimmune disorders. The recurrent infection can relate to respiratory system inflammation (as in sinusitis, bronchitis, and pneumonia), meningitis and sepsis. Streptococcus pneumonia considers the most common bacterial causes of these previous cases .
Autoimmune disorder and recurrent bacterial infection can associate with deficiencies in elements of complements 1, 2, 3, and 4. In the other hand, the deficiency in complement 2 components did not associate with and clinical illness in individuals, while a deficiency in complement 3 components mostly associated with severe recurrent infection, and autoimmune disorders. The components of complement 5 to complement C9 form the membrane attack complex (MAC) that leads to direct destruction of antigen .
The complement system may be playing an important role as a central responder of the immune system which can affect the production of immunoglobulins . CH50 test result depends on all the classical components, so any changes in the concentration of any of the components will lead to changes in the total activity of the complement system. Hemolytic activity of complement can be affected when there are considerable changes in the concentration of any of the components .
One notable cause of the reduction in CH50 is the presence of underlying hepatic disease  since the liver is responsible for producing most of the complement components. Cigarette smoke causes a wide range of harmful effects on organs that are not related to direct association with smoke, as in the liver. The heavy smoking individual will introduce toxins to their circulation, that lead to necroinflammation result in increased in the severity of the lesions occurs in hepatic parenchyma .
Cigarette smoke can cause harmful changes on the liver and its function which included in three categories (1) toxic changes , (2) immunological changes  and (3) oncogenic changes .
A previous study  indicated that compounds in cigarette smoke can cause complement alternative pathway activation through specific modifying in C3 chemical bounds. This suggests that the alternative pathway works more than the classical pathway, which is dependent on antibody-antigen reaction, despite the increased level of immunoglobulins in smokers . However, the classical pathway seemed to be not so active in smokers compared with the control group. Cigarette smoke contains numerous oxidants, which produce free radicals that affect neutrophils and generate oxidative stress in the body. This, in turn, produces highly reactive oxygen and nitrogen species .
Complement activation begins with a series of enzymatic reactions with C1q until it reaches the C5 component. The proteolytic activity begins when it reaches the C9 component. Oxidative stress directly exerts its effects on enzymatic reactions and reduces the hemolytic activity of the classical complement pathway. In addition, other studies have recently demonstrated the consumption of complement hemolytic activity in serum following its incubation with cigarette smoke condensate .
Some studies that examined the total hemolytic activity of complement chose to study the effect of smoke of cigarettes on purified complement 3, this related to two main facts which are (1) high concentration of this complement in blood serum, also (2) C3 consider as a key master to activate classical and alternative pathways . Despite the high concentration of C3 in 10 smokers in the current study, the rate of hemolytic activity was still lower than in the control group, indicating a decrease in activation of the classical pathway. In contrast, we also observed lower and normal levels of C3 in other smokers. The decline in CH50 was possible because the classical pathway needs a high level of C3 component in serum for its activation.