Complement system

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 Complement system is significant part of innate immune system.it contains 30 proteins in blood plasma and on the surface of the cell which will activate as a chain reaction (one will activate the next)

Following are the major functions of complement system.

Opsonization- label the microbes so that the phagocytic cells will get activated

Chemotaxis-Attract the immune cells to the infected site

Cell lysis- Forms a membrane attack complex (MAC) that can directly kill bacteria

Clearance of Immune complexes- Helps to remove  antibody antigen complex from the body

Pathways

1. Classical pathway

classical pathway will be triggered by the antibodies and it comes under the part of adaptive immunity.

Example

If a person got infected with the measles virus.

The body will start produced IgG antibodies against the virus.

These antibodies bind to the virus.

The classical pathway is activated to destroy the virus.

classical pathway needs antibodies to induce the immune response.

2. Lectin pathway

Lectin pathway which will be triggered by the MBL (mannose-binding lectin) binding to sugars on microbial surfaces. and this pathway will comes under the Innate immunity.

Example

If a person consume food containing Salmonella bacteria.

The bacteria have mannose sugar on their surface.

MBL binds to the mannose.

This activates the lectin pathway and starts attacking the bacteria.

In Lectin pathway, it doesn't  require antibody, it will work quickly against the organisms

3. Alternative pathway

Direct contact with pathogen surface (spontaneous activation). comes under the part of innate immunity.

Example

  •  fungal infection (like Candida).

  • Complement proteins (like C3) spontaneously activate and attach to the fungal surface.

  • This triggers the alternative pathway and starts killing the fungus.

 Alternative pathway is Always on at a low level and ready to respond immediately.

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