You’ve probably gotten a cold or a fever before. You don’t usually just have something to make it go away. But it does anyway. Why? Because inside your body, there is system of specialized cells that work together to vanquish the forces of sickness. This army of cells is called the immune system.
Role of the Immune System:
The immune system protects your body from foreign substances and harmful viruses. Essentially, thee immune system is an army of exterminators bent on eliminating any type of sickness your body receives. The immune system is typically split into two parts: the innate immune system and the adaptive immune system.
Innate Immune System: Surface Barriers:
The innate immune system is the body’s first line of defense against foreign invaders. The first defense mechanisms are surface barriers such as the skin. While the skin is primarily part of the intregumentary system, it also plays a role in preventing foreign agents from entering the body. However, the skin can’t prevent these agents from entering openings in the body. But hope is not lost, as these openings also possess defense mechanisms. In the nose and intestines, a sticky substance called mucus is secreted. This mucus serves to trap and entangle microorganisms that enter the nose. The saliva in the mouth, while not technically alive, contains multiple enzymes (chemicals that break down specific substances) that serve to break down foreign substances. Tears also contain these enzymes in order to protect the eyes, as do mammary fluids produced by females. Additionally, the skin and respiratory tract both secrete fluids that combat foreign substances. The stomach contains powerful fluids that destroy foreign substances. The flushing action of tears and urine helps expel foreign substances. Reproductive fluids also contain substances to battle the invaders.
Innate Immune System: Cellular Barriers
As efficient as the surface barriers of the innate immune system may be, they can’t do everything. To fix this issue, there are leukocytes, better known as white blood cells. There are multiple types of white blood cells: phagocytes (which can include macrophages, neutrophils, and dentritic cells), mast cells, basophils, eosophils, and natural killer cells. Phagocytes are leukocytes that engulf, or eat, foreign pathogens or particles. After being engulfed inside the phagocyte, the foreign substance becomes trapped in a cellular vesicle called a phagosome, which then bonds with a lysosome, which provides fluids to help break down and “digest” the trapped foreign agent. Neutrophils, a type of phagocyte, are commonly found in the bloodstream when in pursuit of a foreign substance to destroy. Macrophages act similarly, but are primarily found around tissue and have a much wider arsenal of tools at their disposal, such as enzymes, complement proteins, and regulatory factors. Dentritic cells are exposed to the outside environment, but otherwise act the same as normal phagocytes. They are named after neuronal dentrites, which are part of the nervous system, which have many spine-like projections that dentritic cells also have. Mast cells reside within tissue, and are primarily involved in inducing inflammation to fight infection. Basophils and eosophils are somewhat like neutrophils and help fight parasites. Finally, natural killer cells, or NK cells, have a purpose that is odd compared to the other leukocytes. Unlike the other leukocytes that attack pathogens directly, NK cells seek out and destroy tumor cells and cells infested with a virus.
Adaptive Immune System:
The innate immune system is already extremely effective in its work, but some pathogens and antigens are simply too clever. This is why we also need the adaptive immune system, which is unique for “remembering” certain foreign agents and how to destroy them. The adaptive immune system contains lymphocytes, specialized white blood cells divided into the types of T-cells (which are again divided into killer T-cells and helper T-cells) and B-cells. All three contain receptors to recognize pathogens and infected cells, but each type can only recognize certain types of foreign agents. Killer T-cells kill virus-infected cells and cells that are damage or dysfunctional. Helper T-cells differ from killer T-cells in thee fact that they don’t destroy foreign agents directly. Instead, helper T-cells control and direct other immune cells (such as phagocytes from the innate immune system) to destroy the foreign agents. B-cells identify foreign agents when the antibodies on their surface comes into contact with the agent. The B-cell then attracts a matching helper T-cell, which gives the B-cell directions on how to produce the antibody needed to combat that particular foreign agent. Afterwards, the B-cell creates millions of duplicate antibodies that recognize the agent, and proceed to destroy it.
Lymphocytes are the primary cellular constituent of lymph, a substance that circulates through the lymphatic system. The lymphatic system is actually part of the larger circulatory system, but plays an important role in the immune system as well. The lymphatic system serves multiple purposes, including the transportation of white blood cells, the removal of interstitial fluids (fluids that reside between cells) from tissues, the removal of fatty acids from the digestive system. Lymph is actually made up of the interstitial fluids collected by the capillaries of the cardiovascular system via osmosis. The lymphatic system then allows the lymph to act out its purpose in the body. The lymphatic system is like an emergency passageway reserved for forces to get to an area where they are needed (in this case, the forces are the white blood cells that are needed at the site of an infection). In addition, the lymphatic system also collects fatty acids from the digestive system. In the digestive system, primarily in the small intestine, are lymph vessels called lacteals. Lacteals take the fatty acids and mix them with lymph to create a milky solution called chyle. The chyle is then brought up to the thoracic duct, where it enters the bloodstream. In addition to the lymph vessels, two other organs exist in the lymphatic system called the spleen and the thymus. The spleen, although playing an important role, can be surgically removed without jeopardizing life. The spleen helps produce antibodies to combat foreign agents. The spleen also removes necrotic (dying) red blood cells from the bloodstream. It additionally creates red blood cells (this function is more effectively performed by the bone marrow after birth). Finally, the spleen stores a reserve of blood cells in the case of hemorrhagic shock. The thymus is also very important. The thymus is where T-cells mature before setting out to perform their respective duties.