8.2 IMMUNITY (Innate immunity) NEET

8.2 IMMUNITY
Everyday we are exposed to large number of infectious agents. However, only a few of these exposures result in disease. Why?
This is due to the fact that the body is able to defend itself from most of these foreign agents (pathogens or antigens). 

  • This overall ability of the host to fight the disease-causing organisms, conferred by the immune system is called immunity. 
Immunity is of two types: (i) Innate immunity and (ii) Acquired immunity. 

8.2.1 Innate Immunity: 

  • Innate immunity is non-specific type of defence, that is present at the time of birth. 
  • This is accomplished by providing different types of barriers to the entry of the foreign agents into our body. 
  • Innate immunity consists of four types of barriers. 
These are — 

 (i) Physical barriers (First line of defense): 

  • Skin on our body is the main barrier which prevents entry of the micro-organisms. 
  • Mucus coating of the epithelium lining the respiratory, gastrointestinal and urogenital tracts also help in trapping microbes (immobilization) entering our body. 
(ii) Physiological barriers: 
  • Acid in the stomach, saliva in the mouth, tears from eyes–all prevent microbial growth. 
  • Saliva and tears of the above contain Ig-A
(iii) Cellular barriers (scavengers): 
  • Certain types of leukocytes (WBC) of our body like polymorpho-nuclear leukocytes (PMNL-neutrophils) and monocytes and natural killer (type of lymphocytes) in the blood as well as macrophages in tissues can phagocytose and destroy microbes. 
  • Wanding macrophages (Histocyes) dispose microbes in connective tissues (blood and lymph).
  • Macrophages also display the antigen of digested microbes to the other lymphocytes  (T-cell) in the lymph node. (Antigen presenting cell)
  • Some microbes such as Mycobacterium tuberculosis, Shigella, Leshmania,Salmonella, manage to survive with in the macrophages due to some special adaptation. 
(iv) Cytokine barriers: 
  • Virus-infected cells secrete proteins called interferons which protect non-infected cells from further viral infection (by binding with virus). 
  • Second line of defense: cellular, physiological and cytokine are the internal or second line of defense.
Interferon types
Function
Interferon type I
·   1. The type-I interferons present in humans are IFN-α, IFN-β, IFN-ε, IFN-κ and IFN-ω.
·   2. Type I interferons are produced when the body recognizes a virus that has invaded it.
·   3. They are produced by fibroblasts and monocytes.
·         type I IFN-α is prohibited by another cytokine known as Interleukin-10.
Interferon type II 
·   1. (IFN-γ in humans):
·   2. This is also known as immune interferon and is activated by Interleukin-12.
·   3. Type II interferons are released by Cytotoxic T cells and T helper cells, type-I specifically.
Interferon type III
·    The importance of Type III IFNs in some types of virus or fungal infections

Interlukins: Interleukins (ILs): 
  • Interlukins are a group of cytokines (secreted proteins and signal molecules) that were first seen to be expressed by white blood cells (leukocytes).
  • All featuring autoimmune diseases or immune deficiency.
  • The majority of interleukins are synthesized by helper CD4 T lymphocytes, as well as through monocytes, macrophages, and endothelial cells.
  • They promote the development and differentiation of T and B lymphocytes, and hematopoietic cells.

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