Membrane Attack Complex formation and function

Membrane attack complex (MAC) is a macro-molecular structure, typically formed by the association of complement proteins (present on the blood plasma) and situated on the surface of the plasma membrane of the extracellular pathogen and causes the lysis of the pathogenic cell. After the entering of the pathogen into the host body the host complement system (one of the effector immune system of our body) become activated by different pathways and the activated complement proteins associated with each other in a cascade mechanism and form this complex structure.

MAC form a transmembrane channel on the cell membrane of the target cell which results the cell lysis and disrupt the cell and ultimately causes death to the cell.

Component of Membrane Attack Complex: Membrane attack complex is formed by the association of activated complement proteins C5b, C6, C7, C8 and C9.

Membrane attack complex
Membrane attack complex (MAC).

Formation of MAC: The membrane attack complex is formed by the association of complement proteins of activated complement system, which is done by three different pathways.

  1. CLASSICAL PATHWAY: This pathway begins with the formation of antigen -antibody complex or immune complex. This pathway is a major effective pathway of humoral immune response. In this pathway the complement proteins become activate by the antibody bound with the extracellular antigens.
  2. ALTERNATIVE PATHWAY: The alternative pathway is antibody-independent complement activating pathway. This complement activating pathway is triggered by the foreign antigenic substance. In this complement activation pathway four serum protein are involved these are C3, Factor-B, Factor-D and Factor-P.
  3. MANNOSE BINDING LECTIN PATHWAY: In this pathway the main effector molecules are Mannose Binding Lectin (MBL), and MBL-associated serine protein, which activate the complement proteins and trigger the MAC formation.
Formation of MAC by the association of complement proteins
Formation of MAC by the association of Complement Proteins.

Through these three different pathways the complement activation is done, which results the activation of complement proteins in a cascade mechanism and facilitate their association which ultimately result the formation of membrane attack complex (MAC).

Function of MAC: MAC makes a hole in the cell membrane of the pathogenic cell, as a result the intracellular content leaks out and unwanted substance get in. Thus, the cell can not maintain the osmotic stability and lysis occurs by the influx of water and loss of electrolytes.       

Influx of Extracellular fluid inside the pathogenic cell through MAC
Influx of the extracellular fluid inside the pathogenic cell through MAC.

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