This web page was produced as an assignment for an undergraduate course at Davidson College.



In humans, the immune system has several layers of defenses in place to block, identify, or consume antigen. 

Line 1 - Epithelia
The first line of defense is the epithelia (skin, mucosal membranes) that line the inside and outside of our bodies.  This dense layer of cells prevents many infectious agents from entering the host in the first place.  However, if a pathogen does find its way into the body, the innate immune response is ready for action. 

Line 2 – Phagocytes (Macrophages)
There are phagocytes that lie just beneath the epithelial cells that can engulf and digest any antigen present.  The actions of these phagocytes serve to set off the next step by inducing the inflammatory response.  

Line 3 - Inflammation
Inflammation is helpful as it works to keep the infection localized as the activated phagocytes release chemicals (cytokines and chemokines) that cause vasodilation and white blood cell (leukocyte) attraction.  These leukocytes (neutrophils, monocytes and natural killer cells) move out of the blood vessel and into the infected issue; guided by the chemical gradient set up by the activated phagocyte.  The blood vessel becomes more permeable, allowing plasma proteins and fluid to seep into the surrounding area.  Clotting eventually occurs in the dilated microvessels and the escape route for infection is cut off.  The heat, pain, redness, and swelling characteristic of inflammation are caused by the influx of leukocytes and fluid into the nerve-filled infected tissue. 

Line 4 – Pattern recognition
Cells of the innate immune response have receptors that are (encoded by complete genes inherited through the germline) used to help them identify foreign antigen from self.  However, when referring to antigen response, they actually mediate several different functions.

  1. Phagocyic receptors – stimulate the ingestion of antigen they recognize
  2. Chemotatic receptors – follow chemical gradients to guide cells to site of infection
  3. Signaling receptors – induce effector molecules that contribute to both innate immune responses and adaptive immune responses

Line 5 – Complement cascade
The complement system is an incredibly complicated cascade that requires the combination of many distinct plasma proteins in order to destroy pathogens.  It accomplishes this via one of three pathways:

  1. Classical pathway – antigen:antibody complexes
  2. MB-lectin pathway – lectin binding to pathogen surfaces
  3. Alternative pathway – pathogen surfaces

which all result in activation of the necessary complex and destroy antigen in one of three ways:

  1. Recruitment of inflammatory cells
  2. Opsonization of pathogens
  3. Killing of pathogens

This system also mediates humoral innate immunity of the tissue spaces and the blood.

Line 6 – Call for help!
The innate immune response is mostly used to deter the establishment of infection, however, even if it is unable to do so, it can still set the stage for the adaptive immune response (which is the essential form of host defense in humans).