Components of Innate Immunity

Table of Contents

What is Innate Immunity?

The innate immune system, also known as the nonspecific immune system, is one of the two types of immune strategies found in vertebrates; another type is called the adaptive immune system. The innate immune system is an ancient immune strategy that is the first mechanism for host defence. When a pathogen enters our body, it is first recognised by the innate immune system, and then the adaptive immune system is activated.

Components of the Innate Immune System

The innate immune system comprises four main components that help in the defence action. Let us look at them in detail.

1. Anatomical Barriers of Innate Immunity

The anatomical barrier of innate immunity involves mechanical, chemical and biological barriers that act as the first line of defence against the pathogen.

  • Mechanical barriers of innate immunity: Mechanical barriers or physical barriers block the entry of pathogens into the body physically. Skin is the first mechanical barrier that makes the entry of pathogens difficult because of the epidermis. Hair, an accessory organ on the skin, also prevents the entry of pathogens.

Mucous membranes in the urinary tract, gastrointestinal tract and respiratory tract are also mechanical barriers.

  • Chemical barriers of innate immunity: Chemical barriers block entry of pathogens at body openings and inner body surfaces. Examples of chemical barriers include sweat, breastmilk, mucus, saliva, tears and semen. Vaginal secretion is acidic in nature and is not endured by pathogens. Semen has zinc which is intolerable by pathogens. The digestive juices produced in the stomach also kill foreign invaders.
  • Biological barriers of innate immunity: There are a number of harmless bacteria that live inside our urinary, gastrointestinal and reproductive tracts that make the environment inhospitable for pathogens or harmful bacteria to survive in our body.

2. Cellular Response

The cellular response of the innate immune system involves different kinds of leukocytes that kill pathogens by phagocytosis. These leukocytes circulate in the blood and single-handedly kill the pathogens that invade our bodies.

Phagocytosis is an important cellular process where a cell engulfs larger and harmful particles by extending its plasma membrane. In this process, the phagocytic cell extends its plasma membrane to envelope the pathogen inside it and form a vesicle called a phagosome. The phagosome then fused with the lysosome, together known as the phagolysosome. Acids and digestive enzymes secreted from the lysosome kill the pathogens.

The cells involved in phagocytosis include neutrophils, macrophages and dendritic cells.

3. Blood Proteins

The liver cells or hepatocytes produce a range of proteins that destroy the invading pathogens. These proteins are produced as a result of the complement system. The complement system is a biochemical cascade that complements the ability of antibodies to kill pathogens. The proteins of the complement system work in the following ways:

  • They help in the recruitment of inflammatory cells.
  • They coat the surface of the pathogen, making it an easy target for destruction.
  • It forms a hole in the pathogen cell wall, causing its cytolysis and destruction.
  • It filters the neutralised antibody-antigen complexes out of the body.

4. Inflammatory Response

The inflammatory response is observed when a pathogen is able to break anatomical barriers and enter our body. Inflammation is recognised as redness, swelling, pain and loss of function in the infected area. It is triggered by chemicals such as histamine and cytokines that are secreted by injured cells or immune system cells such as macrophages.

These chemicals recruit tissue-repairing proteins and pathogen-destroying leukocytes to the site of injury for pathogen killing and tissue repair. Some cytokines are anti-viral in nature such that they block the protein synthesis in host cells which is a prerequisite for the virus to survive and divide.

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Frequently Asked Questions


What is the difference between innate and acquired immunity?

Innate immunity is found in an organism since birth, whereas acquired immunity is adapted by an organism during its lifetime.

What are the characteristics of innate immunity?

  • It activates the adaptive immune response by antigen presentation.
  • It activates the complement system for the clearance of pathogenic organisms.
  • It acts as a chemical and physical barrier to invading pathogens.
  • It recruits immune system cells to the site of injury with the help of cytokines.