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====== Immune System ======
===== Terms & Definitions =====
  * **Fever** is a symptom of an anti-infection defense mechanism that appears with body temperature exceeding the normal range due to an increase in the body's temperature. \\ \\
  * **Mechanical barriers** — which include the skin, mucous membranes, and fluids such as tears and urine — physically block pathogens from entering the body. **Chemical barriers** — such as enzymes in sweat, saliva, and semen — kill pathogens on body surfaces. \\ \\
  * **Antibodies** or **immunoglobin** are protective proteins produced by the immune system in response to the presence of a foreign substance, called an antigen. Antibodies recognize and latch onto antigens in order to remove them from the body. \\ \\
  * The **receptors** usually recognize components of microorganisms that are not found on cells of the host, e.g. components of bacterial cell wall, bacterial flagella or viral nucleic acids. \\ \\
  * The **thymus** is a small gland in the lymphatic system that makes and trains special white blood cells called T-cells. The T-cells help the immune system fight disease and infection. The thymus gland produces most of your T-cells before birth. \\ \\
  * **Phagocytosis** is the process by which certain living cells called phagocytes ingest or engulf other cells or particles. It is defensive reaction against infection and invasion of the body by foreign substances (antigens). \\ \\
  * **Inflammation** is a protective response involving immune cells, blood vessels, and molecular mediators. The function of inflammation is to eliminate the initial cause of cell injury, clear out damaged cells and tissues, and initiate tissue repair. \\ \\
  * The **lymphatic system** is a subsystem of the circulatory system in the vertebrate body that consists of a complex network of vessels, tissues, and organs. It helps defend the body against infection by supplying disease-fighting cells called lymphocytes. \\ \\
  * Messenger molecules \\ \\
  * **Agglutination**, is the clumping of red blood cells within the body. It is the body's natural immune response to toxins and pathogens. When red blood cells meet toxins and pathogens within the body, they clump them together. \\ \\
  * Peptides \\ \\
  * **Memory cells** are white blood cells that help the immune system recognize and fight specific pathogens. They are produced after the body encounters a pathogen, and they remain in the immune system to help the body fight the pathogen again. \\ \\
  * **Immunological memory** is the ability of the immune system to respond more quickly and effectively to pathogens it has encountered before. \\ \\
  * **B-plasma cells** are a type of white blood cell that makes infection-fighting proteins called antibodies. \\ \\
  * **Cytotoxic cells** are involved in directly killing intracellular and extracellular pathogens and eliminating mutated and cancerous cells. \\ \\
  * **Helper T cells** are a type of immune cell. When they sense an infection, they activate other immune cells to fight it. They may activate cytotoxic T cells or they may activate B cells, which produce antibodies. \\ \\
  * **Antigen-presenting cells (APCs)** are a large group of various cells that trigger the cellular immune response by processing an antigen and exposing it in a form recognizable by T cells in the process known as antigen presentation. \\ \\
  * Cell clone \\ \\
  * The theory about **clonal selection** states that in a pre-existing group of lymphocytes (both B and T cells), a specific antigen activates only its counter-specific cell, which then induces that particular cell to multiply, producing identical clones for antibody production. This activation occurs in secondary lymphoid organs such as the lymph nodes. \\ \\
  * **Somatic recombination** is a type of gene rearrangement by which cells of the adaptive immune system physically cut out small regions of DNA and then paste the remaining pieces of DNA back together. \\ \\
  * A healthy immune system defends the body against disease and infection. But if the immune system malfunctions, it mistakenly attacks healthy cells, tissues, and organs. Called **autoimmune disease**, these attacks can affect any part of the body, weakening bodily function and even turning life-threatening. \\ \\
  * **Macrophages** are specialised cells involved in the detection, phagocytosis and destruction of bacteria and other harmful organisms. In addition, they can also present antigens to T cells and initiate inflammation by releasing molecules (known as cytokines) that activate other cells. \\ \\
  * **Precipitation** happens when soluble antigens are recognized by antibodies in the immune system. The antibodies bind to these antigens and clump them together into larger, solid complexes that are easier for the body to remove. This clumping is called a precipitate. \\ \\
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| The **innate immune response** is the body’s first, fast, general defense against any invader. It is non-specific, meaning it does not target a specific germ, and it is always ready. | The **specific (adaptive) immune response** is slower at first, but it targets specific invaders. It uses specialized cells (like B and T cells) that remember and attack only the exact germ that caused the infection. |
| In the immune system, "**self**" means the body’s own healthy cells and materials. The immune system knows not to attack these. | "**Non-self**" means anything foreign—like bacteria, viruses, or anything that doesn’t naturally belong in your body. The immune system is trained to recognize and attack non-self things. |
| Found on all body cells (except red blood cells). Shows "self" or infected signals to the immune system. If a cell is infected, **MHC I** shows pieces of the invader to killer T cells (CD8+ T cells), which then destroy the infected cell. | **MHC II** is found only on special immune cells (like macrophages, dendritic cells, and B cells). Shows pieces of non-self (foreign invaders) that they've eaten to helper T cells (CD4+ T cells). This helps activate the immune response. |
| **Humoral response** involves B cells and antibodies. It fights pathogens in body fluids (like blood or lymph). B-cells produce antibodies that stick to invaders (like bacteria and viruses) to block or destroy them. | **Cellular response** involves T cells, especially killer T cells. It fights infected cells or abnormal cells (like virus-infected or cancer cells). T cells directly destroy those infected cells. |
| **Primary response** happens the first time your body encounters a new germ. It is slower and takes a few days to build up antibodies. During this time the body is learning to fight that specific invader. You might feel sick while this is happening. | **Secondary response** happens when your body sees the same germ again. It is faster and stronger because of memory cells (from B and T cells). You might not even feel sick. |
| The **activation phase** is when the immune system detects a threat (like a virus or bacteria). Antigen-presenting cells (like dendritic cells) show the invader to B cells and T cells that get activated and start multiplying. | The **effector phase** is when the activated immune cells go into action. B cells make antibodies and Killer T cells destroy infected cells. The body is now fighting back and removing the invader. |
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activation phase and
effector phase, monoclonal and polyclonal antibodies, antibody and receptor, antibody and
antigen, heavy chain and light chain, active and passive immunization, B- and TLymphocytes