Immunology Notes on Second Line of Defense in Immune system

What is the second line of defence (internal defence) in immune system

In the last post we discussed how difficult is for a microbe to get into the body. Now if once crossed the first hurdle, these microbes are greeted by variety of cells and chemicals. These body response is also non-specific, mounting similar response against all pathogens irrespective of the identity. Even though this second line of defense is non-specific there is a central system in place for collection and distribution of these cells called as lymphatic system.

Body’s internal defence comprises of :

1.    Phagocytes that include macrophages and white blood cells specifically neutrophils and monocytes. Read more

2.    Inflammatory reactions: We often feel redness, heat, swelling and pain at the site of wounds. Actually this is body’s defense mechanism to destroy the pathogen. Tissue damage caused by wound or invading pathogenic organisms induces a complex sequence of events collectively known as inflammation. Read more

Inflammatory Response

3.    Fever is often an associated response of inflammation. Pyrogens (temperature rising chemicals) released from damaged tissues and cells involved in inflammation induce a rise in temperature. It helps in enhancing physiological processes at times destroy infecting pathogens.

4.    Complement system:
   The complement system is a group of 30 serum proteins that functions in controlling inflammation. These proteins can interact with each other and other components of immune system. Read more

     Complement system participate in both innate and acquired immunities.

     Two pathways: alternate and classical pathway

•    Alternate pathway: Sometimes microorganisms or its toxins can directly activate complement system, which is an innate and non-specific reaction. Sooner microorganisms are coated by complement molecules leading to its uptake by phagocytosis. Read more 
•    Classical pathway:Here complement system is activated by antibodies bound to pathogen surface or antibody-antigen complexes, thus involved in specific and acquired immunity. Read more  

5.    Interferons:

•     Interferons are antiviral glycoproteins released by living cells in response to viral attack and induce a viral resistant state to neighbouring cells.
•      Theseare small proteins with MW 20,000-34,000 Daltons
•     -sensitive to proteolytic enzymes such as trypsin

Based on cell of origin interferons are classified into:

•      α interferon produced by virus infected leucocytes
•      β interferon produced by virus infected fibroblasts
•     ϒ interferon produced by virus infected lymphocytes (T cells and NK cells)

Functions:

•      Inducing anti-viral state to surrounding cells
•     Inhibit the multiplication of abnormal cells in the body. So widely used as anti- cancer agents  

6.    Natural killer cells: NK cells play an important role in the innate host defences. These cells are specialised to kill viral infected cells and tumor cells.

    Mode of action: The secrete cytotoxins, perforins and granyzymes which create pores in the plasma membrane of the target cell facilitating water entry into the target cell. Target cells swells and burst.

     These cells are vital in natural resistance to tumors as these cells participate in Fas- Fas ligand mediated apoptosis.

      Why NK cells are called ‘natural killer cells’?

     NK cells are called ‘natural killer cells’ because they are active without prior exposure to virus and are not specific for any viruses. Prior exposure to viruses has no effect on its activity. But activity of NK cells is enhanced in the presence of antibodies. The process is called antibody-dependent cellular cytotoxicity (ADCC). Other potent activators of NK cells are IL-12 and Gamma interferon.

     IL-2 activated NK (LAK cells) cells are used for the treatment of certain cancers.