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Introduction to the Immune System 4th in a series

Introduction to the Immune System 4th in a series


By Don Pearson, Technical Director BioBrew Ltd

The immune system is divided into two types of responses: (1) innate immunity, and (2) adaptive immunity (Abbas et. al., 2017).

The innate immune system is our first line of defence. It is composed of physical, chemical and biological barriers. The biological barriers include immune cells, and blood proteins that mediate inflammation (e.g., cytokines).

Physical barriers include the cells that line the outer surfaces and cavities of organs and blood vessels, while chemical barriers include the antimicrobial chemicals they produce. These cells are called epithelial (skin) cells and they create what is known as epithelial barriers.

Some cells of the innate immune system patrol our body looking for threats (e.g., microbes and microbial proteins). They devise quick immune responses when they find these threats that aim to destroy the foreign, damaged, or infected cells.[4] These responses are nonspecific, i.e., general, but they can influence subsequent specific adaptive immune responses (Abbas et. al., 2017).

The adaptive immune system is responsible for the more complex immune responses that develop when innate immunity is insufficient to manage a threat.  Adaptive immunity is mediated by cells called lymphocytes. Lymphocytes are a diverse range of cell that act separately and collectively to suppress pathogens

Lymphocyte receptors may detect up to 1 billion different antigens. Lymphocytes survey our body continuously, looking for potential pathogens. When they come across an antigen they recognize, lymphocytes are activated and trigger a specific immune response. They then proliferate, creating a pool of new lymphocytes with the same specificity. Some of these new lymphocytes will remain in our body after the threat is eliminated. They are called memory cells because they act as the memory of our immune system, ensuring that if it ever encounters the same antigen, the response will be faster, greater, and more effective. Through this we become immune to that specific threat (Abbas et. al., 2017).


The innate immune system, our first line of defence, devises a quick nonspecific response to threats.  The adaptive immune system is what remembers past infections (and vaccines), allowing the immune system to respond quickly when we encounter something similar again.

Abbas, A.K., Lichtman A.H., Pillai S. (2017) Cellular and Molecular Immunology E-Book. Elsevier Health Sciences