White blood cells The main components of the immune system are lymphocytes, lymphoid tissue, and lymphoid organs, such as the spleen, lymph nodes, and thymus. The immune system protects our body from foreign organisms by fighting infections and conferring immunity to disease. As with other body systems, the immune system must also maintain homeostasis or have some sort of regulation between recognizing material that is “self” (from the body) from material that is “non-self”(foreign). Obviously, the immune system must launch an appropriate immune response against invading antigens. Inappropriate immune responses include hypersensitivities, allergies, and autoimmunity. Diminished immune responses are also detrimental to the body, such as overall immune deficiency and AIDS. The word antigen means anything that can elicit an immune response. It can be a protein or a polysaccharide on the surface of (or released by) a cell bacterium, fungus, virus, etc. An antigen is a substance that is recognized as foreign by the host immune system. In one slide, you see a bacterial cell with several antigens protruding from the cell membrane. Any of these antigens can cause an immune response. Notice that antibody A is only recognizing, and binding to, the antigen depicted as a red triangle. Notice that antibody B recognizes and binds to the antigen depicted as a peach circle. Antibodies are specific; they can only recognize and bind to one particular antigen. The immune system may not necessarily react to, or recognize, the entire polysaccharide or antigenic proteins. Rather, the immune cells may react to only a particular site, perhaps a certain sugar or chain of amino acids on the entire antigenic molecule. These smaller sites on the larger antigenic proteins or polysaccharide are called epitopes. Tolerance If I took your cells and injected them into another person, they would likely a elicit an immune response because your glycoproteins are antigenic it to someone else. However, your own immune system does not react to your antigens. Tolerance to self antigens sometimes malfunctions and becomes the mechanism for autoimmune diseases. We say this tolerance is to self antigens. How does tolerance happen? 1) The immune cells may not express receptors to self antigens 2) T cells do not respond to self antigens a. The thymus kills self reactive lymphocytes b. Other immune cells inactivate lymphocytes that could be reactive to your own cells Formed Elements Formed elements are red blood cells, platelets, and white blood cells. The prefix “leuko” means white and the suffix “cyte” means cell. Therefore, “leukocyte” means white blood cell. White blood cells contain nuclei and organelles, so of all the formed elements, WBCs are the only formed elements that are true cells. Red blood cells lack nuclei and organelles and platelets are only fragments of a larger cell. White blood cells WBCs defend your body against foreign materials, such as bacteria, viruses, or cells from another person. They remove toxins, wastes, and they also remove damaged or dying cells. For example, old red blood cells must be removed from the circulation and macrophages (a type of WBC) help do this. A typical µl of blood contains 6-9000 white blood cells. However, white blood cells circulating in your blood only represent a fraction of the total number present in your body. White blood cells only use the circulatory system to get to their final destination. When needed, they migrate out of the bloodstream and reside in the connective tissues found throughout our body. To remember how many of the white blood cells are neutrophils, basophiles, etc., remember 60-30-6-3-1. Of the white blood cells in the circulation: 60% are neutrophils 30% are lymphocytes (this includes B cells and T cells together) 6% are monocytes 3% are eosinophiles 1% are basophiles If these numbers change, it could suggest pathology. White Blood Cell Count • When a person has a blood test, in addition to RBC information, a “white blood cell count” can be ordered to report the total number of WBCs present. • A slightly more expensive test, but much more informative, is a “white blood cell count with differential”. This test will tell you the total number of WBCs as well as how many of each type of WBC is present. • Usually one orders the cheaper test unless there is a reason to be suspicious of a WBC disorder. Leucopenia means an absolute decrease in white blood cell numbers. The disorder may affect any of the specific types of white blood cells, but most often it affects neutrophils, which, under normal healthy conditions, is the most abundant. A number of conditions may cause leucopenia, including aplastic anemia, treatment with chemotherapeutic drugs, irradiation, and idiosyncratic drug reactions. Idiosyncratic is a term used to describe drug reactions that are different from the effects obtained in most persons and that cannot be explained in terms of allergies. In other words, a patient reacts very badly to a particular drug for no known reason. They probably have a different gene expression than other people. A number of drugs can cause idiosyncratic reactions, such as chloramphenicol, which is used as an antibiotic. Obviously, if a person has a reduced white blood cell count, they would be more prone to infection. Leukocytosis is a term to describe excessive numbers of white blood cells. Leukocytosis is normal when you are fighting an infection. Leukocytosis also occurs with leukemia, except that with leukemia, the excessive white blood cells are not normal, or functional. Leukemia is a word to describe uncontrolled production of white blood cells of a myelogenous or lymphogenous cell lines. What does that mean? Remember that the hemocytoblast is a multipotent stem cell that gives birth to any type of blood cell. If it differentiates into a myelogenous cell line, it will further differentiate into neutrophils, eosinophiles, basophiles, macrophages, and monocytes. If it differentiates into a lymphogenous cell line, it will further differentiate into either a B cell or a T cell. A cancer-causing mutation in the myelogenous or lymphogenous cell lines causes leukemia which is characterized by greatly increased numbers of abnormal white blood cells. The leukemic cells from the bone marrow may reproduce so greatly that they invade the bone marrow where they are being made, and overwhelm normal white blood cells that are trying to develop. Although there are greater numbers of white blood cells in circulation during leukemia, these cells are not functioning properly. Additionally, the cancerous cells within the bone marrow consume too many of the nutrients, impeding the development of normal blood cells (including platelets and red blood cells). Almost all types of cancer cause both anorexia ( a reduction in food intake caused primarily by diminished appetite) and cachexia ( a metabolic disorder of increased energy expenditure leading to weight loss greater than that caused by decreased food intake alone). In other words, the cancer cells are too metabolically active and consume too many nutrients. TYPES OF WHITE BLOOD CELLS 1. Basophiles (mast cells) 2. Eosinophils 3. Neutrophils 4. Monocytes (macrophages) 5. Lymphocytes (T cells and B cells; discussed later in this lecture) BASOPHILES Basophiles migrate to injury sites and release the contents of their granules. The granules contain histamine and heparin among other components. Histamine is a vasodilator and increases capillary permeability. Collectively this means the capillary will have more blood flow through it and the capillary itself will be more leaky, to allow more white blood cells to reach the tissue infection. Heparin is an anticoagulant, so the blood will be less likely to clot, allowing better movement of the white blood cells to the infection site. When a basophile leaves the circulation and enters the tissues, it is called a mast cell. The function of the basophiles augments the inflammation response initiated by the mast cells. Remember “B for basophiles and B for begin”, as in beginning the inflammatory response. Basophils and mast cells bind to an IgE antibody/antigen complex to begin the anti-inflammatory response known as an allergic reaction. EOSINOPHILES Eosinophils attach themselves to parasites, particularly parasitic flukes invading the GI tract. They release hydrolytic substances and digest the parasite from the outside. During a parasitic infection, there is an increase in eosinophil numbers. Eosinophiles also seem to play a role in preventing the excessive spreading of local inflammatory factors (cytokines) from mast and basophiles. Clearly, if the mast cells and basophiles were left unchecked, then widespread histamine and heparin release would cause excessive vasodilatation. This is what happens during anaphylactic shock. Cytokines • Soluble molecules released mainly by leukocytes to communicate and coordinate / direct the immune response • Include: – Interleukins (IL-1, IL-2, etc.) – Chemokines: signals for leukocyte extravasation and migration within tissues (chemotaxis) – Interferons – Tumor necrosis factors NEUTROPHILS (a type of phagocyte) Neutrophils are the most abundant of the circulating white blood cells. This white blood cell is extremely mobile and is the first WBC to leave the circulation to assist the macrophages in their fight of an infection in the tissues. They are excellent phagocytes; they ingest large particles or even entire cells (such as bacteria). MONOCYTES AND MACROPHAGES (types of phagocytes) A monocyte that leaves the circulation and enters the tissues is called a macrophage. A macrophage can ingest many more foreign particles than a neutrophil and is the first line of defense against invading microbes, followed by neutrophils. The macrophages and neutrophils are nonspecific immune cells since they do not discriminate between the types of material they ingest. They can engulf any type of bacteria, other microbes, or even dead or dying body cells. They both use opsonization to help engulf foreign material. Opsonization means that when antibodies (released by a mature B lymphocyte) bind to a foreign particle (antigen), they bind the antigen at the stem (Fc receptor) of the antibody. The macrophage or neutrophil can grab onto this FC receptor and more easily wrap itself around the bacterium to engulf it. This is especially important when the bacterium has a protective capsule around it that prevents the phagocyte from getting a grip on it. Opsonization with an antibody helps it to grab on to the slippery bacterium. The difference between macrophages and neutrophils Macrophages and neutrophils both use hydrolytic enzymes to break apart the foreign particles that they ingest. However, only the macrophage then takes some of the particles of the antigen and embeds them in its cell membrane, then goes to a lymphocyte to present the antigen pieces to it. The lymphocyte then “feels” the shapes of the antigen pieces, and launches further defensive measures. Because of this, macrophages are considered antigen presenting cells (APC), which activate T cells and B cells into action. The immune system is divided into two categories, innate and adaptive immunity: INNATE IMMUNITY (Nonspecific immune defenses) 1. Simple barriers 2. Inflammation 3. Fever 4. Antimicrobial substances (complement proteins and interferons) 5. Generic reactions to antigens and foreign microbes i. Phagocytes (macrophages and monocytes) ii. Basophils iii. Eosinophils ADAPTIVE (ACQUIRED) IMMUNITY (specific) 1. Cell mediated (T cells) 2. Humoral mediated (B cells) INNATE IMMUNITY (Nonspecific immune defenses) Simple barriers include cutaneous and mucous membranes. The secretions from these membranes can prevent the spreading of invading microbes. Mucus can trap microbes, or serosal secretions may contain lysozymes that can destroy the microbes. Inflammation is also a nonspecific host defense. The characteristics of inflammation include dolor (pain), calor (heat) rubor (redness), and tumor (swelling). Fever is another nonspecific host defense, and this elevates metabolic rates for all cells of the body, including the activity of white blood cells. Anti-microbial substances are also nonspecific. These include complement proteins and interferons. Another non-specific defense is the phagocytic white blood cells (neutrophils, tissue macrophages, eosinophiles, and basophiles). These are generic in their responses to invading microbes. Inflammation involves three main stages: 1) Vasodilatation and increased permeability of blood vessels 2) Phagocyte migration and phagocytosis 3) Tissue repair THE INFLAMMATORY REACTION On one slide, see a picture of a knife protruding through the integument system down to the dermis. The mast cells and basophiles release copious amounts of histamine and heparin in response to the injury. These chemicals result in dilation of the local blood vessels, leading to increased blood flow and increased blood vessel permeability. As a result, the area will become red, swollen, warm, and painful to the touch. As a result of increased blood flow, there will be more white blood cells, specifically monocytes and neutrophils, attracted to the injury site. The macrophages in the dermis release cytokines. A cytokine is a chemical which recruits more white blood cells to the injury site. The increased blood flow will also bring with it more clotting factors. A clot will form in the tissue that stops the bleeding into the wound, but also walls off the wound to surrounding tissue so any microbe introduced into the warm should be localized and prevent it from spreading. When more white blood cells arrive at the injury site, T cells and antibodies also arrive, providing specific defenses against any microbe that entered the wound. The macrophages and neutrophils will phagocytize any debris and also aid in the repair of the tissue. ADAPTIVE (ACQUIRED) IMMUNITY (non-specific) Adaptive immunity is from lymphocytes (B cells and T cells). These cells attack in different ways. T cell lymphocytes (cell-mediated immunity) B cell lymphocytes (humoral immunity) Both of these types of cells recognize antigens, but they can only recognize whatever single antigen or epitope they were exposed to. This is like a specific key (T cell or B cell) will unlock only a specific lock (antigen) in your house. Each flu virus, bacterial infection, or fungal infection you have ever had in your lifetime is like a different lock in your body. This means we need millions of different types of B and T cells in our body. When one specific B or T cell encounters its antigen that it recognizes, it is activated and quickly creates copies of itself, like duplicating keys. During the infection battle, many of the duplicated B or T cells die, but a few continue to live on as memory cells. These memory cells will allow you to mount a quicker response and more aggressive response should you ever become infected with the same antigen in the future. MEMORY CELLS In the slide, the cell at the top has a brown nucleus, and represents a virgin cell. A virgin B or T cell is one that has never been exposed to an antigen. When the virgin lymphocyte encounters an antigen, it starts to make clones of itself. Some of the clones will be active cells (those shown with the red nucleus) and fight the infection. The other clones (yellow nucleus) will remain dormant and serve as memory cells. When the infection has been conquered, the red nucleated activated cells will die. But the yellow nucleated memory cells will continue to live in the lymph tissues. If the memory cells encounter the same antigen again, they can quickly generate another army of activated cells. Thus, the second response to the antigen will be much faster and more aggressive. Why? At the first exposure, a single virgin cell had to replicate to create clones of itself. With the second exposure, multiple memory cells were available for cloning. Sheer numbers dictate that the formation of activated cells will occur more rapidly and more abundantly. Using a graph to describe a quicker response to a second exposure to the same antigen, you’ll notice on the x-axis, 7 days elapse after an antigen has been introduced into the body. On the y-axis you’ll notice the immune response. It is a gauge of intensity, and you’ll notice there are no units associated with this axis. An antigen (A) is introduced into the body on day five, and it takes 7-10 days before your immune response reaches its peak. If, at a later date, antigen A is reintroduced into your body, because you caught the same cold again from your child or spouse, you’ll notice the immune response takes a much shorter time to respond to that antigen. Additionally, it responds with more vigor. In other words, the immune response occurs faster and with more intensity. If however, you were exposed to a different antigen, say antigen B, you’ll notice that your immune response will again take 7-10 days upon first exposure to this antigen to mount an impressive response worthy to contain and eradicate antigen B. ADAPTIVE (ACQUIRED) IMMUNITY (specific) 1. Cell mediated (T cells) 2. Humoral mediated (B cells) 1. CELL MEDIATED IMMUNITY: T CELLS There are three main T cells: A. Helper T cells B. Cytotoxic T cells (Killer cells) C. Suppressor T cells. HELPER T CELLS The helper T cell category is by far the most numerous. As their name implies, they help the immune system direct its response. Helper T cells are activated when their receptors bind to an antigen on the surface of an antigen presenting cell (macrophage). Helper T cells unite cell mediated and humoral responses. When the helper T cell is activated, it will replicate to create many clones of itself, it recruits more white blood cells into action, can stimulate cytotoxic T cells, and can stimulate B cells to mature into plasma cells that can then secrete antibodies. CYTOTOXIC T CELLS (KILLER T CELLS) The cytotoxic T cell must have direct contact with its target cell (a microorganism or a cell infected with a virus). For this reason, cytotoxic T cells are also called killer T cells. When a cytotoxic T cell binds to an antigen on a virally infected cell, the cytotoxic T cell releases hole-forming proteins on the surface of the infected cell. These proteins are called perforins and they literally create holes in the infected cell’s membrane. This causes the infected cell to begin a process called apoptosis. Apoptosis is cellular suicide (cell mediated cell death). The infected cell starts to degrade its nuclear DNA and cellular proteins. When the process is over, the cell is dead and the virus cannot exist without it. NOTE: without your helper T cells, your B cells and cytotoxic T cells would be largely unresponsive to the infection in your body. If you could pretend to be HIV (the virus that causes AIDS), which of the T cells would you infect first and wipe out? If you guessed helper T cells, if you would be correct. In fact, HIV infects helper T cells and depletes their population. All the while, the cytotoxic T cells are unresponsive to the infection going on. HIV is truly an insidious virus. SUPPRESSOR T CELLS Suppressor T-cell function is not well understood. But we do know they are capable of suppressing the functions of cytotoxic and helper T cells. They prevent the cytotoxic T cells from generating excessive immune reactions that could destroy healthy surrounding tissue. 2. HUMORAL IMMUNITY Humoral immunity is conferred by the B cells in your body. It is called that because the B cells generate antibodies that stay in the blood ( blood is a humor, or fluid) and it is the antibodies that will latch onto a specific antigen. B cells do not engage in cell to cell combat like T cells. B cells provide protection against foreign antigens that are in your circulation or outside of the cell. These antigens may arise from bacteria, viruses, or parasites, just to name a few. When the B cell becomes activated by an antigen presenting cell (a macrophage) or activated by a helper T cell, the B cell will mature into a plasma cell. The plasma cell can make antibodies. Each antibody made will only be able to recognize one specific antigen. That means that you would need many different types of B cells, each capable of making only one type of antibody, to defend against the endless number of foreign antigens you may encounter. When you come into contact with an antigen for a second or third time, the memory cells will allow you to respond much more quickly and with greater quantity, including speeding up the production of antibodies. Memory cells are what make a vaccine work. A vaccine is made from a dead or weakened microbe, or just portions of them (their epitopes). When introduced into the body, these antigens stimulate the immune system. Vaccines cause you to create memory cells. If you should happen to come into contact with the antigen later in life, your memory cells are already in place, and can rapidly respond with great quantities of antibodies, limiting how severe your illness will be. Remember, B cells, T cells, memory cells, and antibodies can only recognize one particular antigen. In one slide, you can see B cells with integral proteins of various shapes. Notice that one B cells is square shaped and can recognize only a square antigen. Once it binds its antigen, the B cell becomes activated, and begins to replicate clones of itself. Additionally, that B cells that was stimulated will turn into a plasma cell. It is the plasma cell that can now generate antibodies that will specifically blind to that square shaped antigen. This is similar to the T cells, which also only respond to one particular antigen. You would need many different B cells and T cells in order to fight al the invading microbes and other antigens and that you might encounter throughout your life span. ANTIBODIES (immunoglobins) An antibody is a protein that is secreted into the plasma by a plasma cell (mature B cell). They are Y- shaped proteins with two long peptide chains (dark brown in the picture) and two short chains (blue). The stem region determines the classification type of the antibody. There are five main classes (Ig stands for immunoglobulin): IgG, IgA, IgM, IgE, and IgD. Remember this as GAMED. The arm regions that project outward from the stem of the antibody consist of amino acids that allow the antibody to bind to one particular antigen. There are usually only to variable regions, one at the end of each arm, and it is here that the antibody gets its specificity in binding to one particular epitope on an antigen. The variable region is the part of the antibody that attaches to the antigen. Although most antibodies only have two variable regions, larger antibodies, such as IgM, have multiple binding sites for a particular antigen. Levels of antibodies can be measured in the serum. In fact, we can take a patient’s blood sample, loaded into a gel, and separate the antibodies according to their size. IgG IgG is the most abundant of all antibodies and their functions include: opsonization, neutralizing toxins and attacking viruses. They can cross the placenta and cause destruction of fetal red blood cells. This is called hemolytic disease of the newborn. IgG is the second antibody made during an infection (IgM is the first. IgA These antibodies are found on the surface of your body. They are found on mucosal surfaces, sweat, tears, bronchiole secretions, and even in your saliva. IgM These antibodies are extremely large and can have 10 binding sites. In other words, 10 of the variable regions of each IgM antibody are capable of binding the same antigen. They are the first antibody that is produced, and they can bind to antigens, covering up their toxin-producing sites, or just weigh the entire antigen down so that it precipitates out of solution (plasma) so the entire agglutinated compex can be removed from the circulation. IgE These are the antibodies responsible for causing your allergic reactions. If you have severe allergies, IgE’s are not your friend. They stimulate histamine release from basophiles and mast cells. They are also important in the lysis of parasitic worms. IgD These antibodies are believed to help activate B cells in some unknown fashion. HOW ANTIBODIES ATTACK Because of the bivalent nature of the antibodies (a stem and two arms, each attack in different ways), they can inactivate the invading agent in one of several ways: 1. Agglutination, in which multiple large particles with antigens on their surfaces, such as bacteria or red blood cells, are bound together into a clump held together by the antibodies that are attached to the surface antigens. This large clump can then precipitate. In other words, the antibody and antigen complex becomes so large that it is rendered in soluble and precipitates out of solution. 2. Neutralization, in which the antibodies don’t cover the entire cell; they just cover the toxic sites of the antigen agent. Neutralization is also a factor in hemolytic disease of the newborn, during which a mother is Rh negative and pregnant with an Rh positive fetus. 3. Lysis, in which some antibodies are capable of directly attacking membranes of cellular microbes, and therefore cause them to rupture (lyse). 4. Opsonization, in which the antibody binds to an antigen that the phagocytes are having difficulty engulfing, such as a bacterium with a capsule (like TB). 5. Complement Cascade, in which the antibody pops the cell membrane of an invading organism. PATHOLOGICAL STATES OF THE IMMUNE SYSTEM Remember, the immune system must also have homeostasis. Too much or too little reactivity to antigens is not a healthy balance. HYPERSENSITIVITY AND ALLERGIES A child ingests a peanut for the first time, and is not aware that he is allergic to the proteins on them. 1. A macrophage ingests the peanut antigens, and break them down. 2. Some of the peanut antigens are embedded into the cell membrane of the macrophage. 3. The antigenic peptide from the peanut is presented to a T helper cell and activates it. 4. The activated T helper cell now releases many cytokines. 5. The cytokines activate cytotoxic T cells. 6. The T helper cell activates B cells. 7. The B cells replicate clones of themselves a. Some of these clones become memory cells b. Some of these clones mature into plasma cells i. Plasma cells secrete IgE antibodies ii. IgE antibodies bind to mast cells and basophiles iii. The mast cells and basophils release histamine granules iv. Severe vasodilatation occurs v. Anaphylactic shock: blood pressure drops AUTOIMMUNITY Autoimmunity describes the loss of self tolerance by the lymphocytes. The T cells and B cells now attack the body’s own tissues. For some reason, women are more afflicted with autoimmune disorders than men. Examples of autoimmune diseases 1. Graves’ disease 2. Myasthenia Gravis 3. Systemic Lupus Erythematosus (SLE, or “Lupus”) 4. Rheumatoid arthritis 5. Multiple sclerosis 6. Hashimoto’s thyroiditis 7. Insulin-dependent diabetes mellitus. GRAVES’ DISEASE This is an autoimmune disorder in which antibodies called TSI ( thyroid stimulating immunoglobulin) stimulate the thyroid inappropriately and this leads to hyperthyroidism. HASHIMOTO’S THYROIDITIS Another autoimmune disorder of the thyroid, except the antibodies destroy the thyroid gland and this leads to hypothyroidism. MYASTHENIA GRAVIS Antibodies destroy the nicotinic-acetylcholine receptor on the surface of skeletal muscle. As a consequence, the patient has progressive muscle weakness due to the loss of nervous stimulation of the muscle cells. INSULIN-DEPENDENT DIABETES MELLITUS Antibodies destroy the beta cells of the islets of Langerhans in the pancreas. This leads to the inability for insulin-dependent cells of the body to import glucose from the plasma. MULTIPLE SCLEROSIS Antibodies attack the myelin sheath around neurons, leading to a decline in the speed of the action potential. SYSTEMIC LUPUS ERYTHEMATOSUS (SLE) This is a chronic inflammatory disease that affects many systems of the body. The initial manifestation is a red rash around the nose and mouth and almost looks like a butterfly mask on the patient’s face. RHEUMATOID ARTHRITIS This is a collagen disease and often ends up deforming the patient’s joints. This is one the antibodies chronically attack and destroy the collagen within the joints of the patient. The patient ends up with hands and feet severely flexed and curled. Extension of the joints is extremely difficult, if not impossible.