Table 1 summarizes the functions of many of the plasma proteins. Acute-phase proteins such as C-reactive protein (CRP, so named because it reacts with the C polysaccharide of pneumococci), α1-antiproteinase, haptoglobin, α1-acid glycoprotein, and fibrinogen are believed to play a role in the body’s response to inflammation. For example, C-reactive protein stimulates the complement pathway (see later), while α1-antitrypsin neutralizes certain of the proteases released during acute inflammation.

Table1. Some Functions of Plasma Proteins

During chronic inflammatory states and in patients with cancer the levels of acute-phase proteins may increase from 1.5 to as much as 1000-fold (in the case of CRP). Hence, CRP is used as a biomarker of tissue injury, infection, and inflammation. Interleukin 1 (IL-1), a polypeptide released from mononuclear phagocytic cells, is the principal stimulator of acute-phase protein synthesis by hepatocytes. However, additional molecules such as IL-6 also participate.

The small proteins that facilitate cell–cell communication between the components of the immune system, such as interferons, ILs, and tumor necrosis factors, are called cytokines. Cytokines can be both autocrine and paracrine in nature. One of the primary targets of IL-1 and IL-6 is nuclear factor kappa-B (NFκB), a transcription factor that regulates the expression of the genes encoding many cytokines, chemokines, growth fac tors, and cell adhesion molecules. NFκB, a heterodimer comprised of a 50- and a 65-kDa polypeptide, normally resides in the cytosol as an inactive complex with a second protein, NFκB inhibitor-α, also known as IκBα. Inflammation, injury, or radiation stimulates the phosphorylation of IκBα, which in turn targets it for ubiquitination and degradation. Once freed from its inhibitory partner, active NFκB translocates to the nucleus where it stimulates transcription of its target genes.

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