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The compiement system consists of at least 20 chemically and immunologically distinct serum proteins comprising the complement components, the properdin system and the control proteins. Complement is a complex of nine different fractions called $\mathrm{Cl}$ to $\mathrm{C}9$. The fraction $\mathrm{Cl}$ occurs in serum as a calcium ion dependent complex, which on chelation with EDTA yields three protein subunits called $\mathrm{Clq},\mathrm{r}$ and $\mathrm{s}$. Thus $\mathrm{C}$ is made of 11 different proteins. $\mathrm{C}$ fractions are named $\mathrm{Cl}$ to $\mathrm{C}9$ in the sequence of the cascading reaction, except that $\mathrm{C}4$ comes after $\mathrm{Cl}$, before $C2$. The model traditionally used to explain $\mathrm{C}$ activity in immune cytolysis is the lysis of erythrocyte sensitized by its antibody. The erythrocyte $(\mathrm{E})$ antibody $(\mathrm{A})$ complex is called $\mathrm{EA}$, and when $\mathrm{C}$ components are attached to $\mathrm{EA}$, the product is called $\mathrm{EAC}$, followed by the components that have reacted. When a $\mathrm{C}$ component acquires enzymatic or other demonstrable biological activity, it is indicated by a bar over the component number, e.g., the enzymatically activated $\mathrm{Cl}$ is shown as $\mathrm{Cl}$. Fragments cleaved from $\mathrm{C}$ components during the cascade are indicated by small letters (e.g., $\mathrm{C}3\mathrm{a}$, C3b). Inactivated forms of $\mathrm{C}$ components are indicated by the suffix ' $\mathrm{i}$ ' (e.g., C4i). Complement Fixation Complement is normally present in the body in an inactive form but when its activity is induced by antigen antibody combination or other stimuli, $\mathrm{C}$ components react in a specific sequence as a cascade. Basically, the complement cascade (C cascade) is a series of reactions in which the preceeding components act as enzymes on the succeeding components, cleaving them into dissimilar

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Updated on: Oct 5, 2022