What is Second line of defense in immunology? Defensive cells, Defensive proteins, Inflammation and Fever

Second Line of Defence is innate form of non specific defense that removes pathogen entered into the body after bypassing first line of defense by means of defensive cells, proteins, fever and inflammation

In the last post, we understand the first line of defense. Here we understand the second line of defense in a simplified, structured manner.

This is the summary chart of second line of defense:

Second line of defense in immunology

Second line of defense involves

1. Defensive cells

2. Defensive proteins

3. Other coordinated responses like inflammation and associated fever

1. Defensive cells

The first three cells macrophages, neutrophils and dendritic cells are called professional phagocytes. The immune cells  has specific cell surface receptors called pattern recognition receptors(PRRs) that recognize pathogen-associated molecular patterns (PAMPs). PAMPs include carbohydrate, polypeptide, and nucleic acid “signatures” that are expressed by viruses, bacteria, and parasites. This helps immune cells for self-non self recognition. Once recognized, the pathogen is phagocytosed by professional phagocytes. You can refer here for more on phagocytosis.

Cells in Second line of defense
NK cells or Natural Killer cells are capable of killing tumor cells and virus-infected cells at a very early stage. These cells use peroforins to create pores on the membrane of pathogens and uses granyzyme; an apoptotic inducer to destroy infected cell.

Mast cells, basophils and eosinophils mediate inflammatory response and effective against parasitic infections. These cells express many common receptors and cytokines, but function at different locations. Mast cells are tissue resident cells and uniquely required for immediate hypersensitivity. 

Basophils are largely circulating cells, but home to areas of allergic inflammation during the late phase response.

Eosinophils are present on the GI tract, but also on allergic inflammatory sites. 

2. Defensive proteins

a. Interferon

Interferons are antiviral glycoproteins released by living cells in response to viral attack and induce a viral resistant state to neighbouring cells.

Interferons are just like an alert signal informs the neighboring cells about the virus infection. Upon the activation of this interferon receptor by interferons secreted by the infected cell, the cell starts synthesizing RNAses and proteases; RNAses will cleave the viral genome and proteases cleaves the viral proteins preventing its replication inside the uninfected cell or the neighboring cell. Thus protecting the neighboring cell from viral infection.

Proteins in Second line of defense
b. Anti Microbial Peptides (AMPs): 
Antimicrobial peptides are cationic molecules that are released primarily by neutrophils, monocytes, and macrophages by secretion or during degranulation.

These proteins can have broad activity to directly kill bacteria, yeasts, fungi, viruses. These proteins primarily act by disrupting the integrity of the cell membrane and causes lysis of pathogen.

Examples  include defensins, cathelicidins, protegrins

c. Iron binding proteins: Antimicrobial peptides with iron binding property. Lactoferrin is an iron-binding protein that is secreted by serous epithelial cells and neutrophils that competes with bacteria for iron, thereby inhibiting bacterial growth.

d. Complement proteins:   The complement system is a group of serum proteins that functions in second line of defense. These proteins can interact with each other and other components of immune system. On activation, follows a cascade activating many proteases downstream finally forms a membrane attacking complex (MAC) that cause pathogen lysis. The other mechanism is microorganisms coated by complement molecules leads to its uptake by phagocytosis by professional phagocytic cells. More on complement system.

This is the summarized video on the topic for better understanding

 3. Inflammation and fever

Tissue damage caused by wound or invading pathogenic organisms induces a complex sequence of events collectively known as inflammation. Inflammation is localized to the site of infection or injury. these are the steps

  • Tissue damage caused by bacterial infection or injury
  • Release of vasodilators and chemotactic factors like histamine. Pyrogens secreted increases temperature causing fever. High temperature inhibits pathogen growth.
  • This lead to increased capillary permeability and blood flow to the area
  • Margination followed by migration of phagocytes to the site of infection
  • The serum proteins along with phagocytes destroy bacteria.
  • Once intruder is destroyed, inflammation settles down.

This is the summary of second line of defense. Thank you

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