Our earth is home to trillions of microorganisms. On a daily basis, we are exposed to countless viruses, bacteria, fungi, and parasites, and yet, while many of these microscopic organisms are disease-causing, we aren’t perpetually bed-ridden and calling in sick from work.
We have our body’s defense system to thank for that.
The What!
In understanding this defense system, we may use the analogy of a medieval city under siege.
There is a wall that serves as a physical barrier to entry. Anatomical and physiological barriers like the skin and mucosal surfaces make it difficult for microbes to enter our body. Once inside, the acidity (from stomach acid) and bodily secretions such as stomach acid and lysozyme make it difficult for microbes to enter our body, let alone survive.
There are soldiers waiting for a breach to occur, ready to attack at will at the site of entry. Our innate immune response functions as a quick and non-specific response to infection. Often involving phagocytosis (“eating” and subsequent destruction of invading microbes), the innate immune response functions to limit the spread of infection. It is often enough to stop the progression of infection altogether.
But sometimes it isn’t enough. We have soldiers (dendritic cells) that function to recognize that further help is required, and send for that additional support. Specialized forces, that have received additional training, put an end to the attack in a highly strategized manner. A last defense of sorts, our adaptive immune response is slower to develop, but is highly specific to the given pathogen that has infected us. It is described as being antibody-mediated (involving the generation of antibodies, which recognize the specific pathogen and prevent further infection) or cell-mediated (involving a robust T cell response, and related killing of infected cells).
The Who!
In our blood, we have red (erythrocytes) and white (leukocytes) blood cells. All of these cells originally came from hematopoietic stem cells, self-renewing cells found in the bone marrow (and umbilical cord blood, early on in life).
While our more commonly talked about red blood cells carry oxygen to different bodily sites and tissues, our white blood cells are responsible for making up our immune system.
There are different types of white blood cells – some are myeloid cells, and many are lymphoid cells.
Myeloid cells are central to our innate immune response, and include monocytes (immature macrophages), granulocytes (such as neutrophils, basophils, and eosinophils), and mast cells.
The more abundant lymphoid cells are largely involved in our adaptive immune response. They include cytotoxic T cells, T helper cells, T regulatory cells, and B cells. As is often the case in science, there are exceptions. Natural killer cells are lymphoid cells important to innate immunity, while dendritic cells are lymphoid cells that are crucial to the linking of the innate response to the development of the adaptive response.
The Where!
Our immune system has parts spread out across our entire body.
Our developing lymphocytes develop and grow-up in our primary lymphoid organs, the bone marrow (within the center portion of our long bones) and thymus (above our heart).
While T cells begin the process of development in a different location (bone marrow) from where they complete maturation (the thymus), all of our other red and white blood cells complete that entire process in the bone marrow.
Once matured, our immune cells have the ability to recognize distinct pathogens and mediate immune-responses. These tasks are carried out in our secondary lymphoid organs, the lymph nodes and spleen.
This article’s content was created based upon the background accrued over the course of the author’s Honours BSc Degree in Microbiology & Immunology, which included Immunology, Virology, and Bacterial Pathogenesis course-work and exposure to topics from Janeway’s Immunobiology.
