Second Line of Defense in the Immune System

When we have an injury such as a cut or burn microbes gat an opportunity to penetrate our body. Thus this succeed in crossing the first line of defense that is skin and now microbes encounter second line of defense which comprises of our body's internal defenses. Second line of defense is a part of Innate immunity.

  Second line of defense compresses of Antimicrobial substances, Phagocytes, Natural killer(NK) cells and Inflammation and Fever.

Antimicrobial substances

• Our body produces antimicrobial substances which discourage microbial growth.
• These antimicrobial substances are present in various body fluids.
• Main type of antimicrobial substances are
- Antimicrobial proteins (AMPs).
- Iron binding proteins
- Interferon (IFNs)
- Complement

Antimicrobial proteins (AMPs)

• Antimicrobial protein are short peptides.That is they are 20 to 50 amino acids long.
• They have a broad-spectrum antibiotic activity. For example,
- Defensins and Cathelicidins these are produced by neutrophils and macrophages.
- Thrombocidin which are produced by platelets
- Dermicidin which produced by sweat glands.

Iron-binding proteins

• Our body contains iron binding proteins which transport and store iron. For example,
- Transferrin is a iron binding protein which is found in blood and tissue fluid.
- Lactoferrin found in milk, saliva and mucus.
- Ferritin which is found in liver, spleen, and bone marrow.
- Hemoglobin which is found in our red blood cells (RBCs).

How Iron binding protein works

• Most microbial pathogen specially bacteria require adequate amount of iron (Fe) for the vegetative growth and reproduction.
• These iron binding proteins they bound the iron molecules and thus they keep the concentration of the free iron low in our body.
• This deprives most pathogens of the available iron and inhibit their growth.

Interferon (IFNs)

• Interferons are named because of their ability to interfere with viral replication in uninfected cells.
• Besides their role in viral replication, they also act as cytokines.


• Component or Complement system is a group of proteins in blood plasma.
• These proteins are normally inactive but to a cascade of events they become activated and result in lysis of microbial or foreign cells.


Phagocytes are defensive cells which help in eliminating microbes from the body.
Two main types of Phagocytes are :
  1. Neutrophils
  2. Macrophages.

• The process by which these phagocytes perform the killing of pathogen is known as phagocytosis.

Natural killer (NK) cells 

They are Non-phagocytic cells and they play role in defense against both intracellular and extracellular pathogens.


• Inflammation is a local defensive response in case of tissue injury.
• Inflammation work towards eliminating microbes, toxins at the site of injury, prevent their spread to other tissues and tissue repair.


• Fever is an abnormally high temperature.
• Elevated body temperature inhibits the growth of microbes and speeds up body reactions that aid repair.
• Fever occur during infection and inflammation.

When the first line of defense fells second line of defense comes into play which is formed by antimicrobial substances, phagocytes, natural killer cells, and processes like inflammation and fever.