C-reactive protein (CRP) is a component of the immune system and mediates nonspecific immune responses. It can serve as a marker for inflammatory processes in the organism, although the focus of inflammation cannot be specified or localized. CRP belongs to the so-called acute-phase proteins.
What is a C-reactive protein?
C-reactive protein always occurs in elevated concentrations in the organism during inflammatory reactions. It represents an acute-phase protein that is formed immediately in the event of infection or injury to support the immune system. Elevated CRP levels can be measured even before the onset of the actual inflammation. As soon as the inflammation subsides, the CRP concentration also immediately drops back to the normal value. CRP is a so-called opsonin, which stimulates the complement system. As an opsonin, it marks antigens such as bacteria and fungi or endogenous cells with destroyed cell membranes for attack by macrophages (phagocytes). An opsonin represents a bridging molecule, which makes bacterial and dead cell substance recognizable for the scavenger cells by docking. Within the complement system protein complexes of parts of the antigen with CRP or antibodies are formed. These protein complexes act as docking points for the phagocytes. C-reactive protein consists of 206 amino acids and belongs to the pentraxin family. Pentraxins are composed of five identical protomers. It is produced on demand in the liver as a result of stimulation by interleukin-6 (Il-6).
Function, effects, and roles
C-reactive protein is part of the immune system and performs important tasks in preparing for an immune response. It serves as a nonspecific indicator of inflammatory responses in the body. The normal concentration of CRP in the organism is usually below 10 milligrams per liter. When elevated concentrations are detected in the organism, an inflammatory reaction is taking place somewhere in the body. It reacts particularly strongly to bacterial infections, rheumatic diseases, further autoimmune reactions in the body or severe necrosis in injuries or serious pathological processes. In the course of these processes, a lot of dead organic material is produced, which must be degraded by macrophages. The CRP concentration also increases during viral infections. But here the increase is by far not as strong. CRP unfolds its effectiveness by binding to the phospholipids of the cell membrane. These include the cell membrane of bacteria, fungi and dead endogenous cells, which form a protein complex with CRP. This protein complex is now taken up by macrophages and further degraded. This reaction can be compared to a garbage disposal. Viral infections are hardly indicated because viruses do not have a cell membrane and enter the cell to multiply. Only when destruction of the host cell then occurs do C-reactive proteins form and dock onto the destroyed cell membrane.
Formation, occurrence, properties, and optimal levels
The nonspecific immune response of CRP is much faster than the specific response of the immune system. Therefore, determination of the parameters can provide a quick overview of whether an inflammatory response is present. Thus, even before the increase in the number of leukocytes and the appearance of fever, the values are greatly increased in the suspicion of such diseases as appendicitis, respiratory tract infections, intestinal inflammation or urinary tract infections. In children, even harmless infections can lead to a sharp increase in values. The CRP level decreases in parallel with the inflammation within the body. In contrast, blood sedimentation only normalizes later. Acute and chronic inflammatory processes can be easily differentiated. In acute inflammation, CRP levels rise more sharply than in chronic processes. CRP values also provide good indications when rheumatic processes are suspected. Furthermore, the measurement is also used to detect complicated infections that may occur after operations or problems during pregnancy. The examination can also provide indications for distinguishing between viral and bacterial infections, since the values do not rise as sharply in viral infections. However, determining CRP levels alone is not sufficient in this case. Finally, CRP measurements also provide early indications of the success of anti-inflammatory or antibiotic treatment.The decrease in inflammatory processes or bacterial load correlates concurrently with the decrease in CRP levels.
Diseases and disorders
Disease severity also correlates with measured CRP levels. As mentioned earlier, the concentration of C-reactive protein in the body is less than 10 milligrams per liter in the normal state. At levels between 10 and 50 milligrams per liter, mild and localized inflammation is usually present. Slightly elevated CRP levels are also possible in smokers and competitive athletes. Concentrations above 50 milligrams per liter already indicate severe inflammation. At a concentration of over 100 milligrams per liter, the most severe infectious processes, severe autoimmune diseases or even tumors are to be expected. With the help of constant measurement of CRP values during the disease process, the course of the corresponding disease can be well documented. This also applies to monitoring the course of immune reactions after organ transplantation. In the case of rejection reactions of the body, the CRP values naturally also increase. In the case of known chronic diseases that develop on the basis of autoimmunological processes, such as Krohn’s disease or rheumatic diseases, the current inflammatory status can thus be easily determined. During a disease flare-up, the values are naturally much higher than in an inactive intermediate phase. Permanently high inflammation values can provide indications of the risk of later arteriosclerosis, diabetes mellitus and the metabolic syndrome. However, although the progression of a diagnosed infection, autoimmune disease, or organ transplant rejection can be well followed by measuring CRP levels, a primary diagnosis of the disease at hand cannot be made by this examination.
