Lines of Defense


The immune system can be divided into three basic lines of defense against pathogenic infection:

  • The first line of defense against infection are the surface barriers that prevent the entry of pathogens into the body
  • The second line of defense are the non-specific phagocytes and other internal mechanisms that comprise innate immunity
  • The third line of defense are the specific lymphocytes that produce antibodies as part of the adaptive immune response


The Immune System: Three Lines of Defense

lines of defence


First Line of Defense

  • The primary defence against infectious disease are the surface barriers that prevent pathogens from entering the body
  • These surface barriers include intact skin (protect external boundaries) and mucous membranes (protect internal boundaries)
  • Both the skin and mucous membranes release chemical secretions which restrict the growth of microbes on their surfaces
  • If pathogens cannot enter the host body, they cannot disrupt normal physiological functions and cause disease


Second Line of Defense

  • The second line of defence against infection are the non-specific cellular and molecular responses of the innate immune system
  • These defences do not differentiate between different types of pathogen and respond the same way upon every infection
  • Phagocytic leukocytes migrate to infection sites and engulf foreign bodies (dendritic cells then present antigens to lymphocytes)
  • Inflammatory responses increase capillary permeability at infected sites, recruiting leukocytes but leading to localised swelling
  • Antimicrobial proteins (such as cytokines and complement proteins) regulate immune activity within the body
  • Fever increases body temperatures to activate heat-shock proteins and suppress microbial growth and propagation


Third Line of Defense

  • The final line of defence against infection are the lymphocytes that produce antibodies to specific antigenic fragments
  • Each B cell produces a specific antibody, and the body has millions of different B cells capable of detecting distinct antigens
  • Helper T cells regulate B cell activation, ensuring that antibodies are only mass-produced at the appropriate times
  • Both B and T cells will differentiate to form memory cells after activation, conferring long-term immunity to a particular pathogen