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Biology LibreTexts

F1. Introduction to the Immune System

This chapter is under development. Some of it is collected from other chapters in Biochemistry Online. In this last chapter on binding, we will consider the daunting task faced by the immune system - to recognize all possible "foreign" molecules and react to them, either by targeting them for elimination, or, paradoxically, to recognize them but not react to them (a process called tolerance).  The same can be said of "self-molecules".  The immune system must recognize them but not respond to them, otherwise autoimmune disease might arise in which the body's powerful immune system targets self. 

It is virtually impossible to give an in-depth description of the immune system in a short chapter.  My goal is simply to illustrate how the immune system recognizes such a myriad of molecules and how, through signal transduction processes, it responds by becoming tolerant of the target, or promoting an immune response.   To accomplish this impossible task, I will brief cover the innate and adaptive immune system and their differences, and how some cells (macrophages in particular) in the innate immune system and cells (B and T cells) in the adaptive immune response recognize and response to target molecules and cells.  Emphasis will be given to recognition and sample signal transduction responses.   Much of the info presented here comes from ideas presented in a fantastic book written by Lauren Sompayrac, How the Immune System Works.   (2003, Blackwell Publishing.  ISBN:  0-632-04702-X)

Three lines of defense protect us from the "enemies", foreign substances (bacteria, viruses and their associated proteins, carbohydrates, and lipids) collectively called antigens.

  • physical barriers of cells that line our outside surface and our respiratory, GI tract, and reproductive system
  • the innate immune system (IS) that all animals have.  Composed of scavenger cells like macrophages (MΦ), neutrophils, dendritic cells, and natural killer cells (NK) that can move around the body through the blood and lymph systems and burrow into tissue to meet the enemy where they can engulf and destroy bacteria and "cellular debris".   Macrophages start off as immature circulating monocytes which enter tissues by slipping through blood vessel walls, and in the process they differentiate into macrophages.  There they lie in wait ready for the enemy.
  • the adaptive immune system, which, as its name implies, can change and adapt to new molecular threats. This branch is better at dealing with viruses which do their damage inside of host cells..  The adaptive IS is comprised of B cells that make and secret protein antibodies that recognize specific foreign molecules, and T cells.