Immune system overview
The human body has an immune system that protects it from foreign substances and dangerous invaders.
Intruders include the following:
Microorganisms (bacteria, viruses, fungi, etc.)
Parasites (such as helminths)
cancer cell
Transplanted organs and tissues
To protect the body from these invaders, the immune system must be able to distinguish between:
What is part of the body (self)
Others (non-self or foreign material)
Antigens are substances recognized as non-self and stimulate the body's immune reaction, especially when considered dangerous, like causing illness. Bacteria, viruses, other microorganisms, parasites, cancer cells, etc. have antigens in or on their surfaces. It can also be an antigen itself, such as a food molecule or pollen.
Normal immune reactions include the following:
Find potentially harmful foreign antigens
Activates and mobilizes defense
Attack the antigen
Control and end the attack
When the immune system becomes dysfunctional and considers itself as non-self, it attacks the tissues of the body and causes autoimmune diseases such as rheumatoid arthritis, Hashimoto thyroiditis, and systemic lupus erythematosus.
Abnormalities in the immune system cause the following phenomena.
The body causes an immune response to itself (an autoimmune disease ).
The body will not be able to show proper immune response to the invasion of microorganisms ( immunodeficiency disease ).
Normally, the body's immunity reacts to innocuous foreign antigens, which damages normal tissues ( allergic reaction ).
Immune system components
The immune system has many components.
Antibodies (immunoglobulins) are proteins made by B cells that bind tightly to the invader's antigens, mark them for the attack, or directly neutralize the invader.
An antigen is any substance that the immune system can recognize and stimulates the immune response.
B cells (B lymphocytes) are white blood cells that make antibodies against specific antigens, and antibody production is activated by the antigens.
Basophils are white blood cells that release histamine (a substance involved in allergic reactions) and make substances that attract other white blood cells (neutrophils and eosinophils) to the problem site.
A cell is the smallest unit of a living body enclosed in the cell membrane and is composed of the cytoplasm and nucleus.
Chemotaxis refers to the process by which chemicals attract cells to specific sites.
The complement system consists of a group of proteins involved in a series of reactions (called the complement cascade) that protect the body. For example, macrophages recognize foreign cells and predate them by killing bacteria and foreign cells. It protects the body by making it easier and by attracting macrophages and neutrophils to problematic areas.
Cytokines are proteins secreted by immune cells, which act as information mediators of the immune system and help control immune responses.
Dendritic cells are cells derived from white blood cells, usually in tissues, that help T cells recognize foreign antigens.
Eosinophils are a type of white blood cell that kills bacteria and foreign substances that are too large to feed on, help immobilize and kill parasites, and help destroy cancer cells. Eosinophils also are involved in allergies.
Helper T cells are a type of white blood cell that help B cells to produce antibodies against foreign antigens, activate killer T cells, and activate macrophages to more efficiently prey on infected and abnormal cells. Or help.
Histocompatibility (literally meaning tissue compatibility) is determined by human leukocyte antigens (self-marking molecules). It is used to determine whether the transplanted tissue or organ will be accepted by the recipient.
Human leukocyte antigens (HLA) are a group of labeled molecules that allow self and non-self to be distinguished, the combination of which is almost unique to each person and is present on the surface of all cells. This group of labeled molecules is also called the major histocompatibility complex (MHC).
Immune complexes are antigens bound to antibodies.
An immune reaction is the reaction of the system to an antigen.
Immunoglobulin is another name for antibodies.
Interleukins are a type of messenger (cytokine) that is secreted by certain white blood cells and affects other white blood cells.
Killer (cytotoxic) T cells are T cells that bind to and kill infected or cancer cells.
White blood cells include monocytes, neutrophils, eosinophils, basophils, or lymphocytes (B cells and T cells).
Lymphocytes are white blood cells that control acquired (specific) immunity, and those that make antibodies (B cells) and those that distinguish themselves (T cells) and those that kill infected cells and cancer cells (killer T cells) There is.
Macrophages are large cells made up of white blood cells called monocytes. Macrophages prey on bacteria and other foreign cells, and help T cells recognize microorganisms and other foreign substances. It is usually found in the lungs, skin, liver, and other tissues.
Major Histocompatibility Complex (MHC) is a synonym for the human leukocyte antigen.
Mast cells are cells in tissues that release substances such as histamine, which are involved in inflammatory and allergic reactions.
A molecule is a collection of atoms that chemically combine to form a unique substance.
Natural killer cells are a type of white blood cell that has the ability to recognize and kill certain types of infected cells or cancer cells without first learning that they are abnormal.
Neutrophils are white blood cells that prey on and kill bacteria and other foreign cells.
Phagocytes are cells that prey on and kill or destroy microorganisms, other cells, and cell fragments that have entered the body. Phagocytes include neutrophils and macrophages.
Phagocytosis is the process by which cells take up and invade invading microorganisms, other cells, cell fragments, and so on.
Receptors are molecules on the surface or inside cells that can identify specific molecules that fit closely, such as a key and a lock.
Regulatory (suppressor) T cells are white blood cells that end the immune response.
T cells (T lymphocytes) are white blood cells involved in adaptive immunity. There are three types of T cells: helper, killer (cytotoxic), and regulatory.
Various lines of defense
The body has a series of defenses. Defense functions include physical barriers, white blood cells, and molecules such as antibodies and complement proteins.
Physical barrier
The first line of defense against invaders is the mechanical and physical barrier:
Skin
The cornea of the eye
Mucous membranes that line the respiratory tract, gastrointestinal tract, urinary tract, and reproductive organs
If these barriers are intact, many foreign bodies cannot penetrate the body. However, the risk of infection increases if the barrier is damaged, for example, due to skin damage from a burn.
In addition, these barriers are protected by secretions containing enzymes that destroy bacteria. Examples include sweat, tears, mucus in the respiratory tract and digestive tract, vaginal discharge.
White blood cell
The next line of defense involves white blood cells. White blood cells travel in the bloodstream, circulate in the body, enter tissues, detect foreign substances such as microorganisms, and attack them.
This defense is divided into two parts:
Innate immunity
Acquired immunity
Innate immunity works effectively even if the invading microorganisms are unknown to the body. You don't need to learn to recognize foreign objects, and you can immediately react to them. It involves several types of white blood cells.
Phagocytes phagocytose foreign bodies. Phagocytes include macrophages, neutrophils, monocytes, and dendritic cells.
Natural killer cells are ready to recognize and kill cancer cells or cells infected with certain viruses.
Inflammatory (such as cytokines) substances participating in or leukocytes (which release substances (such as histamine) responsible for the allergic reaction basophils and eosinophils, etc.) are also available. Some of these cells can directly destroy the invader.
In adaptive (adaptive or specific) immunity, when white blood cells ( B cells and T cells ), called lymphocytes, encounter foreign bodies, they learn how to attack, and each next foreign body is attacked so that it can attack them more effectively. I remember. When a new foreign body is encountered, it takes time for the adaptive immunity to occur because the lymphocytes have to adapt to the foreign body. However, once immunized, the body can react quickly. B cells and T cells work together to destroy foreign substances. T cells require the help of cells called antigen-presenting cells (such as dendritic cells—the mechanism by which T cells recognize antigens )so that they can recognize foreign substances. Antigen-presenting cells take in foreign substances into the cells and break them down finely.
Molecule
Innate immunity and adaptive immunity interact and influence each other. They may work directly together, or they may work together through molecules that attract or activate other cells in the immune system as part of the process of recruiting cells for defense. There are the following molecules.
Cytokines (information mediators of the immune system)
antibody
Complement protein ( constitutes the complement system )
These substances are not in cells but are dissolved in body fluids such as plasma, which is a liquid component of blood.
Some of these molecules, like some cytokines, cause inflammation.
Inflammation occurs because these molecules attract cells in the immune system to the problematic tissue. To help these cells collect in problem tissues, the body pumps more blood into the tissues. In order to send more blood to the tissue, the blood vessels expand and become more open, allowing more fluid components and blood cells to enter the tissue through the blood vessels. Inflammation thus tends to cause redness, warmth, and swelling. In other words, inflammation occurs because the infection is kept only there so that it does not spread. And with the help of another substance produced by the immune system, inflammation is subsided, and damaged tissue is healed. Inflammation is troublesome, but it is also a sign that the immune system is working properly. However, severe or long-term (chronic) inflammation can be harmful.
organ
In addition to cells dispersed in the body, several organs are involved in the immune system and are classified into primary lymphoid organs and secondary lymphoid organs.
Primary lymphoid organs are the places where white blood cells are produced and proliferated as follows:
In the bone marrow, all types of white blood cells are made, including neutrophils, eosinophils, basophils, monocytes, B cells, and cells that grow into T cells (progenitor T cells).
In the thymus, T cells proliferate and are trained to recognize foreign antigens while ignoring their own. T cells are extremely important in adaptive immunity.
When it becomes necessary to protect the body, white blood cells are mainly mobilized from the bone marrow and enter the bloodstream to be sent to the required site.
Lymphatic system: Helps protect against infection
The lymph system, along with the thymus, bone marrow, spleen, tonsils, appendix, Peyer's patches in the small intestine, is an important part of the immune system.
The lymph system is a network in which lymph nodes are connected by lymph vessels and serves as a flow path for lymph throughout the body.
Lymph is made up of fluids that permeate and exit extravascular tissue through the thin walls of capillaries. This fluid contains oxygen, proteins, and other nutrients that nourish your tissues. Some of this fluid returns to the capillaries and some enter the lymph vessels (at this stage it becomes lymph).
The thin lymphatic vessels merge and become thicker, eventually forming the thoracic duct. The thoracic duct is the largest lymphatic vessel. The thoracic duct connects to the subclavian vein to bring lymph into the blood.
Lymph also plays a role in transporting foreign substances (such as bacteria) in tissues, cancer cells, dead cells, etc. to lymph vessels and lymph nodes for disposal. Lymph contains many white blood cells.
The substance carried by the lymph always passes through at least one lymph node, and foreign substances are removed and destroyed in the lymph node before the lymph returns to the bloodstream. White blood cells collect in the lymph nodes, and these white blood cells react with other white blood cells and antigens to cause an immune reaction to foreign substances. Lymph nodes are a network of cells packed with B cells, T cells, dendritic cells, and macrophages. Harmful microorganisms are filtered through this network, recognized by B and T cells, and attacked.
Lymph nodes are gathered at the part where lymph vessels branch off, such as the scruff, armpit, and groin.
The secondary lymphoid organs include:
spleen
Lymph node
Tonsils
appendix
Peyer's patch in the small intestine
These organs are the places for catching microorganisms and foreign substances, and for the mature cells that carry the immune system to assemble and interact with each other and foreign substances to carry out specific immune reactions.
Lymph nodes are cleverly placed in the body and are interconnected by an extensive network of lymph vessels ( lymphatic system ). The lymphatic system carries microbes and other foreign substances, cancerous and dead cells, from tissues to lymph nodes where they are filtered and destroyed. Then the filtered lymph returns to the bloodstream.
The lymph node is one of the first sites for cancer cells to spread. Therefore, doctors first examine the lymph nodes to determine if cancer has spread. Lymph nodes may swell if they have metastasized. The lymph nodes can also swell due to infection. This is because the immune response to the infection occurs within the lymph nodes. Sometimes the bacteria carried to the lymph nodes are not killed and can cause infection in the lymph nodes (lymphadenitis), which can cause the nodes to swell.
Mechanism of immune reaction
In order for a proper immune response to a foreign body to occur, it must go through the following processes:
recognition
In order for the immune system to destroy a foreign substance, it must first recognize the presence of the foreign substance. That is, the immune system must be able to distinguish between self and nonself (foreign bodies). The immune system can make this distinction because of the labeling molecule on the surface of all cells. For example, microorganisms can be distinguished because the labeling molecules on the surface are different.
The most important molecules that self-label in humans are called by the names below.
Human leukocyte antigen (HLA) or major histocompatibility complex (MHC)
The HLA molecule is called an antigen because it causes an immune reaction in the body of people other than yourself when a kidney or skin is transplanted (normally, it does not react in one's body). Each person has an HLA consisting of almost unique combinations, and the immune system recognizes this unique combination as self. On the other hand, cells with labeled molecules different from those of their own body are recognized as foreign substances, and the immune system attacks them. Such cells include cells from transplanted tissues and cells that have been infected by invading microorganisms or altered by cancer. (If you need an organ transplant, your doctor will try to match the HLA molecules.)
Among white blood cells, B cells (B lymphocytes) directly recognize foreign substances. However, other T cells (T lymphocytes) require the help of cells called antigen-presenting cells, as described below.
Antigen-presenting cells take in foreign substances into the cells and break them down finely.
The antigen fragments from the foreign body are then combined with the HLA molecule of the antigen-presenting cell itself.
This complex of antigen fragment and HLA molecule migrates to the cell surface.
T cells with matching cell surface receptors can fit tightly in a key-to-key fit with a portion of the HLA molecule presenting the antigenic fragment.
Then, T cells are activated and begin to attack foreign substances having the same antigen.
How T cells recognize antigens
T cells are part of the immune surveillance system. When T cells migrate through the bloodstream or the lymph system and reach lymph nodes and other secondary lymphoid organs, they search for foreign substances (antigens). However, T cells cannot recognize and react with an antigen unless the foreign substance is processed by another leukocyte called an antigen-presenting cell and presented as an antigen. Antigen-presenting cells include dendritic cells, macrophages, and B cells. The most powerful of these are dendritic cells.
Activation and mobilization
White blood cells are activated when they recognize a foreign substance. For example, when an antigen-presenting cell presents an antigen fragment bound to HLA to a T cell, the T cell binds to the antigen fragment and is activated. B cells are directly activated by foreign substances. Activated white blood cells prey on and kill foreign bodies or both. Usually, two or more different types of white blood cells are needed to kill a foreign body.
Immune cells, such as macrophages and activated T cells, recruit defense mechanisms by releasing substances that attract other immune cells to the problem site. The foreign body itself may release substances that attract immune cells.
control
The immune response must be controlled so that it does not cause serious harm to the body as seen in autoimmune diseases. Regulatory (suppressor) T cells secrete cytokines (chemical messengers of the immune system) that block the immune response and help control the body's response. These cells prevent the immune response from lasting forever.
Exclusion
In the process of elimination, foreign objects are trapped and removed from the body. When foreign bodies are cleared from the body, most white blood cells self-destruct and are predated. The cells that escape this are called memory cells and stay in the body. This is part of adaptive immunity that helps you remember certain foreign bodies and react more actively the next time you encounter them.
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