Immune cells act as a soldier who has different skills in maintaining peace and order within their vicinity, our body. Without them, our health will be vulnerable to diseases that can cause moderate to severe damage, and if unlucky it could lead to death. You are probably unaware, but there's a battle in your body every day. Your Armed Forces in the Body are always on Red Alert to defend you. In your Defenses, you have these types of classification of branches in their services and these are Innate and Adaptive. This branch of services can defend and attack without hesitation.
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| A pseudo-colored scanning electron micrograph of an oral squamous cancer cell (white) being attacked by two cytotoxic T cells (red), part of a natural immune response. Image Credit to National Cancer Institute |
1. Innate Immune Cells
Innate Immune Cells are one of the First Lines of Defense along with the Skin, which has the mucus membrane and acts as a shield in our body, to keep us safe from the outside world and minimize threats in our body.
There are several types of this classification. Each of them has unique skills.
- Neutrophils
- Basophils
- Macrophage
- Dendritic cells
- Eosinophils
- Mast cells
- Langerhans cells
- NK cells
- Monocytes
1.1. Neutrophils
A specific kind of white blood cell that aids in the body's defense against infection and is crucial to the immune system. Neutrophils are among the first immune cells to react when bacteria or viruses enter the body. They carry out their duties by phagocytosing infections, producing harmful superoxide and its byproducts, releasing antimicrobial peptides, and constructing neutrophil extracellular traps (NETs). Three to five lobes comprise the characteristic multilobed nucleus of neutrophils, which is joined by thin genetic material threads. The cytoplasm of neutrophils is filled with a large number of purple, microbicidal granules known as azurophilic or main granules.
1.2. Basophils
An immune cell subtype that contains microscopic particles called granules that contain enzymes that are produced during allergic responses and asthma attacks. A granulocyte and a kind of white blood cell, respectively, is a basophil. Basophils, a kind of white blood cell, collaborate closely with your immune system to defend your body against allergies, infections, and parasites. To improve blood flow and prevent blood clots, basophils secrete enzymes. While they start off being rounded, their form frequently alters after they enter tissues. When common basic dyes like hematoxylin are used, it is noted that the nucleus of basophils, which is often bi-lobed or S-shaped, has a purplish-black appearance.
1.3. Macrophage
A white blood cell that surrounds and eliminates pathogens, gets rid of dead cells and activates other immune system cells. Macrophages are immune cells that dwell or are infiltrated inside tissues and are essential for innate immunity, healthy tissue growth, homeostasis, and tissue repair. Macrophage function results from their ontogeny, the habitat in which they live, the kinds of injuries or pathogens they are exposed to, and their local surroundings. According to their location in the body and how they operate, macrophages may go by many names. For instance, macrophages found in the liver sinusoids are known as Kupffer cells, whereas those found in the brain are known as microglia.
1.4. Dendritic Cells
A particular class of immune cells is present in tissues like the skin and stimulates immunological responses by exposing antigens to other immune system cells. A form of phagocyte and antigen-presenting cell is a dendritic cell. Dendritic cells (DCs), often called APCs or antigen-presenting cells (APCs), play a significant role in the adaptive immune system. As antigen presentation is DCs' main job, the cells are frequently referred to as "professional" APCs. Professional antigen-presenting cells known as dendritic cells promote tolerance to self and safe environmental antigens while assisting in the fight against invading infections. They catch viruses and pick up signals from pathogens that affect immune responses.
1.5. Eosinophils
The immune system's specialized cell is called an eosinophil. This kind of pro-inflammatory white blood cell often has a (bilobed) two-lobed nucleus and 200 or more big granules of cytoplasm that contain proteins and enzymes with various (known and unknown) activities. The immune system's primary effector cells are eosinophils. They are active players in numerous immunological responses and have a helpful role in the host's defense against worms and other parasitic diseases. Eosinophils, nevertheless, can potentially cause harm as a result of the allergic disease's inflammatory process. The eosinophil is mostly a cell that lives in tissues. Most eosinophils are located in the stomach, mammary gland, uterus, thymus, bone marrow, and adipose tissues in healthy people.
1.6. Mast Cells
A kind of white blood cell that is present in connective tissues throughout the body, particularly in the lungs and intestines, under the skin, next to blood and lymph veins, and in nerves. Mast cells are important participants in the inflammatory response because a number of antigens, such as allergens, pathogens, and physiological mediators, can trigger them to produce a wide range of inflammatory mediators. Mast cells are multifunctional, tissue-dwelling cells that are generated from bone marrow and are the body's main histamine producers. Mast cells play an integral part in the communication between the neurological, vascular, and immune systems. They sit on the brain side of the blood-brain barrier (BBB) and communicate with astrocytes, microglia, and blood vessels via neuroactive substances that are both preserved from previous generations and newly produced.
1.7. Langerhans cells
The skin's epidermis is covered with a network of cells known as Langerhans cells (LC), a distinct population of tissue-resident macrophages that can move to drain lymph nodes (LN) from the epidermis. Their proximity to the epidermal barrier points to a crucial function as immunological sentinels. The epidermis contains Langerhans cells (LCs), a vast network of immune system sentinels. When they come into contact with foreign substances, these cells analyze their microenvironment to determine the best adaptive immune response (inflammation or tolerance). Following activation in the epidermis, Langerhans cells act as dendritic antigen-presenting cells and move to the lymph nodes. They originate from the bone marrow and are equally dispersed, in a concentration similar to melanocytes, in the suprabasal epidermis.
1.8. Natural Killer (NK)
An immune cell kind that contains granules, or tiny particles, that contain enzymes that can destroy cancerous or virus-infected cells. By preventing the progression of cancers and microbial infections and the resulting tissue damage, natural killer (NK) cells are effector lymphocytes of the innate immune system that regulate various tumor types and microbial infections. As a component of the innate immune system, natural killer (NK) cells are essential for locating and eliminating tumor, virus-infected, and stressed cells. NK cells destroy virally infected cells using three major mechanisms after activation and recruitment to the infection site: cytokine production, the release of cytolytic granules, and death receptor-mediated cytolysis.
1.9. Monocytes
An immune cell that develops into a macrophage or a dendritic cell after traveling via the blood to tissues in the body is generated in the bone marrow. In the peripheral circulation, monocytes work as phagocytic cells and antigen-presenting cells to remove bacteria, antigens, and dead or damaged cells. For an efficient immune response, distinct subsets of monocytes create various cytokines that draw more cells and proteins to the damaged sites. Monocytes are the immune system's "jack of all trades," so to speak. They can really split or specialize into a distinct form of white blood cell, such as a microphage, in specific circumstances and serve various roles.
2. Adaptive Immune
Adaptive Immune Cells are the specialized Armed Forces troops in our Bodies. However, unlike the innate, adaptive takes longer to mature in the thymus, the training ground of our body, but once activated a high chance of survivability rate and a positive that everything falls into control.
There are two types in this classification. Each of them has unique skills and is so powerful against intruders.
- B lymphocytes
- T lymphocytes
2.1. B lymphocytes
B lymphocytes are a weapon manufacturer as they produce antibodies that act as guided missiles and search for targets that fit their receptor and block them.
There are four types of B lymphocytes
- Transitional
- Naive
- Plasma
- Memory
2.1.1. Transitional
Transitional B lymphocytes, also known as TrB cells, are B cells that exist between the immature stage of growth in the bone marrow and the mature stage in the spleen. They are crucial for stopping the development of autoantibodies, which can result in autoimmune disorders. They can control the immune response by generating cytokines that are anti-inflammatory, such as IL-10. They are involved in the production of antibodies and the elimination of self-reactive B cells. There are two transitional stages for the B cells, T1 and T2, which have different surface markers and functional characteristics. T1 and T2 are two transitional stages for the B cells in mice and humans. T1 B cells migrate from the bone marrow to the blood and spleen, where they can mature into T2 B cells. T2 B cells can further differentiate into follicular (FO) B cells, marginal zone (MZ) B cells, or B1 cells (in humans). T1 and T2 B cells have different surface markers and responses to antigen receptor signaling. Transitional B cells are important for maintaining a balanced and diverse B cell repertoire.
2.1.2. Naive
Naive B lymphocytes are a type of white blood cell that is part of the immune system. They are called naive because they have not been exposed to an antigen, which is a foreign substance that triggers an immune response. They circulate through the blood and the lymphatic system and enter secondary lymphoid organs like the spleen, lymph nodes, tonsils, Peyer's patches, and mucosal tissues. They have a functional, non-autoreactive B cell receptor (BCR) that can recognize an antigen. They express the BCR as IgM and IgD molecules and do not express CD27. They have two classes of antibodies on their surface, IgM and IgD, that function as receptors for antigens. When they encounter an antigen that matches their receptors, they become activated and differentiate into either plasma cells or memory B cells. If they do not encounter an antigen, they reenter circulation or die within several days.
2.1.3. Plasma
White blood cells known as plasma cells are known to produce a lot of antibodies. Plasma cells are the ones that secrete large amounts of antibodies into the blood to fight off the infection. Proteins called antibodies attach to foreign substances, or "antigens," and aid in destroying or neutralizing them. B cells, another kind of white blood cell capable of recognizing antigens, are the source of plasma cells. Under a microscope, plasma cells have a distinctive look with a large nucleus and a lot of cytoplasm that is home to several organelles for protein synthesis. The expression of specific surface antigens, such as CD138 and CD27, can help identify plasma cells.
2.1.4. Memory
A subset of B lymphocytes known as memory B cells is a component of the adaptive immune system. They grow inside the secondary lymphoid organs' germinal centers and circulate in the bloodstream in a dormant form, sometimes for decades. Their job is to memorize the characteristics of the antigen that initially infected and activated their parent B cell so that when they come across it again, they can quickly and effectively mount a secondary immune response. Memory B cells are the ones that remember the antigen and can quickly respond if it reappears in the future. They create antibodies that kill infections connected to a certain antigen or the antigen itself. They are crucial for the development of both long-lasting immunity and immunological memory.
2.2. T lymphocytes
T lymphocytes are the ones who confront intruders in a tactical way along with other troops from innate.
There are five types of T lymphocytes
- Effector T lymphocytes
- Helper T lymphocytes
- Cytotoxic T lymphocytes
- Regulatory T lymphocytes
- Memory T lymphocytes
2.2.1. Effector T lymphocytes
Effector T lymphocytes are a type of T cell that actively responds to a stimulus and interacts with host cells to carry out their immune function. They can be distinguished from other lymphocytes by the presence of a T-cell receptor (TCR) on their cell surface. They can be CD4+, CD8+, or Treg cells. Effector T cells can be further classified into CD8+ cytotoxic T cells, which can directly kill virus-infected or cancerous cells, and CD4+ helper T cells, which can activate other immune cells, and Treg cells, that suppress immune responses and prevent autoimmunity. They produce or release different cytokines and effector proteins that interact with target cells displaying specific antigens. Effector T cells can have immune-promoting or negative regulatory roles. They are relatively short-lived and part of the primary immune response.
2.2.2. Helper T lymphocytes
Helper T lymphocytes are a type of white blood cell that serves as a key mediator of immune function. By creating substances that trigger the activation of almost all other immune system cells, they play a crucial part in regular immunological responses. They are involved in almost all adaptive immune responses, as they secrete cytokines for the proliferation and stimulation of other immune cells. Helper T lymphocytes have a receptor on their surface called CD4, which binds to class II major histocompatibility complex (MHC) molecules on antigen-presenting cells. Antigen-presenting cells are cells that ingest and process foreign substances and display fragments of them on their surface.
Helper T lymphocytes recognize these fragments and become activated. They then differentiate into two main subtypes: TH1 and TH2 cells, which have different functions and secrete different cytokines.
TH1 cells primarily stimulate cell-mediated responses, such as those involving cytotoxic T cells and macrophages, which kill infected cells or pathogens.
TH2 cells primarily stimulate humoral responses, such as those involving B cells, which produce antibodies that neutralize pathogens or mark them for destruction.
2.2.3. Cytotoxic T lymphocytes
Cytotoxic lymphocytes are a type of white blood cell that can kill target cells that are infected, damaged, or cancerous. They include cytotoxic T cells (also known as CD8+ T cells or CTLs) and natural killer cells (also known as NK cells). They are part of the adaptive immune system and respond to antigens presented on MHC class I molecules. They use two main mechanisms to kill target cells: releasing cytotoxic granules or activating the CD95 pathway.
2.2.4. Regulatory T lymphocytes
Regulatory T lymphocytes (Tregs) are a subpopulation of T cells that modulate the immune system, maintain tolerance to self-antigens, and prevent autoimmune disease. Additionally, they play a role in putting an end to immune responses once the invaders have been successfully neutralized. They express the biomarkers CD4, CD25, and FOXP3 and can be produced by the thymus (natural Tregs) or by differentiation of naive T cells outside the thymus (adaptive Tregs). They can suppress the activation, proliferation, and cytokine production of other T cells, B cells, and dendritic cells. They can also produce soluble messengers that have a suppressive function, such as TGF-beta, IL-10, and adenosine.
2.2.5. Memory T lymphocytes
A particular type of T lymphocytes called memory T cells may perform some of the same tasks as memory B cells. Their ancestry is unknown. Memory T lymphocytes can remember a specific antigen and quickly respond to it when they encounter it again. They are important for long-term immunity against infections and diseases. They can persist after an illness has been treated, mounting a quicker and more effective defense when exposed to antigens again.
There are different subsets of memory T cells, such as Central memory T cells (T-CM), effector memory T cells (T-EM), tissue-resident memory T cells (T-RM), stem cell memory T cells (T-SCM), and virtual memory T cells (T-VM), each with different functions and locations in the body.
... and that's the Armed Forces in our Body.