What IS the body's Defence System?

The body's defence system is made up of different barriers and the immune system. Some physical barriers within your body include intact skin, mucus and mucosal surfaces (mouth, nose, etc.), and the natural removal of substances through actions such as defecation, and vomiting. Chemical barriers help destroy pathogens or inhibit their growth. Examples would include the slightly acidic pH of your skin and enzymes that are found in places such as saliva, tears, and other secretions. A third barrier that isn't usually mentioned is the microbiome. On your skin, in the large intestine, and other areas scientists are finding a community of bacterial growth that helps your body through secretion of nutrients and their own defences that inhibit the growth of other, deadlier, pathogens.

The immune system itself is a complex group of organs and cells that help defend our body from external and internal threats. Immune cells develop and mature in key areas of the body before moving all over the rest of the body and concentrating in other areas. They also take advantage of the circulatory and lymphatic systems to travel anywhere in the body, because an infection can happen anywhere. There are two main groups of organs in the immune system for development. The generative organs are the bone marrow and the thymus. These are where immune cells develop and are equipped to identify pathogens. The peripheral organs are the lymph nodes. Lymph passes through or drains into the nodes on its way to the heart. This allows more surveillance of the body for any foreign pathogens by immune cells. Certain immune cells will also travel with the lymph into the nodes to activate the naive immune cells that live there.

As a quick side-note, the lymph, lymph vessels, lymph nodes, and bone marrow contribute other mechanisms within the human body other than just defence and immunity mechanisms. Lymph and lymph vessels are also important in the transport of large lipids into the blood stream as they cannot be absorbed directly to the blood stream from the small intestine. There is also a lot of interesting research being done on how the presence of lymph vessels modify and help develop other areas of the body that don't seem to be related at all. Bone marrow is also important for bone strength and rigidity. Therefore, as with a lot of organs, there is a lot of connectivity and interactions between the different systems that cannot, and should not, be ignored.

For now, though, we will focus on the defences including the immune system aspects of these organs and cells. Most immune cells, and all red blood cells, start and finish their development in the bone marrow. They start as semi-stem cells and become more specialized as they continue their development. From here, they are ready to identify any foreign pathogens and react with a response based on how the pathogen was detected and other environmental stimulants. The T-cells are the only immune cell known to develop within the thymus instead of the bone marrow.

In addition to the two main areas of immune cell development, there are also two main branches of the immune system. Innate Immunity is the second line of defence if something gets past the physical, chemical, and microbiological barriers that make up the first line. These cells include macrophages, eosinophils, basophils, neutrophils, mast cells, and dendritic cells. They will mount a response to generic patterns that groups of pathogens express (flagella, type of genetic material, etc.). The second branch is Adaptive Immunity. This branch includes the B-cells and T-cells that are able to mount a response towards a specific pathogen. These cells go through a selection process during development to make sure that they have a variety of patterns available that are responsive towards foreign, non-host, patterns. These patterns are randomly generated, only restricted by the host's DNA and recombination properties. they emerge from the thymus or bone marrow fully developed, but naive or non-activated. Antigen-presenting cells (APCs) are cells that connect these two branches and both are utilized to form a complete immune response with memory on how to respond if the pathogen is detected again.

The Immune System is a double-edged sword. It is necessary to respond to foreign invaders by attacking and silencing them, but it is also necessary to make sure the response doesn't run rampant and damage the body in the process. Therefore, the body's defence system is needed to stop infections from happening and destroying the body, but to also help heal the body when damage is dealt towards it.

Therefore, the body's defence system is a complex organization of physical and chemical barriers, generic, and specific immune responses that all play a role in keeping us healthy.