Complement

When the recognition function of the humoral immune system has been carried out through the production of speci?c antibodies and their binding to foreign antigens destruction of the invading pathogen is the next step. One major defense pathway is the complement system that is activated through antibodies bound to the invading microorganism and eventually causes it to lyse through punching holes in its plasma membrane.

The complement system consists of about 20 interacting soluble proteins in which extracellular ?uid and circulates in the blood. Immunoglobulin molecules bound to the surface of the microorganisms activate C1, the ?rst element of the complement pathway.  The activation happens by the Fc portion of the bound antibody.  Just bound antibody can activate complement; soluble antibody not bound to an antigen has no such that effect.

The   early   components   of   the   complement   pathway,   involving   C1,   are proteases which activate their substrate through limited cleavage. Activated C1 now activates various molecules of the next elements  through proteolysis,  every of that activates  various  molecules  of the  next  component  through  proteolysis,  and  so on. thus,  the  early  steps  in  complement   activation  consist  of  a  proteolytic cascade   in   that   more   and   more   molecules   are   activated   at  every   step. An Component  C3 is the key component  whose cleavage  leads to the assembly  of membrane  attack complexes  on the plasma membrane  of the microorganisms that  makes  holes  in  the  plasma  membrane  which  lead  to  cell  death.  Several white  blood  cells  also  become  activated  during  this  procedure  and  phagocytose the pathogen.

This  pathway  of  complement   activation,  which  starts  when  antibody  has bound  to antigen  on the microbe  surface,  is called  the classical  pathway.  An alternative pathway of activation also exists which is activated directly by polysaccharides in the cell wall of microorganisms even in the absence of antibody. The alternative  pathway  therefore  defends  the body against attack in the early stages  before  an immune  response  can occur and also augments  the effects  of the classical pathway of complement  activation when the immune response has occurred.