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Overview of Innate and Adaptive Body Defenses

Pearson
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Because there are such a wide variety of pathogens, we need a corresponding breadth and depth of defense. Like a medieval castle, the immune system has three main lines of defense against the invading hordes that besiege it. Let’s examine each one in the order a pathogen would encounter them. The first line of defense consists of the surface barriers to entry, also called innate external defenses. Like the walls of the castle, the skin and mucous membranes form this barrier. Like the moat surrounding the walls, many of the body’s barriers are coated in secretions such as mucus and tears. The second line of defense in a castle consists of the guards who check everyone they encounter to determine if they are friend or foe. The body has similar defenses, cells and chemicals in body fluids, that are always on the ready to attack and destroy anything they identify as foe. These defenses are called the innate internal defenses. The guards can also call on the third line of defense, the army. The body’s armies are called the adaptive defenses. The adaptive defenses consist of two kinds of lymphocytes: B cells and T cells. It takes time to mobilize them and train them to fight an identified enemy. When surface barriers and secretions are penetrated by an enemy, the innate internal defense mechanisms, acting as guards, step into action. The innate defenses identify enemies by recognizing a limited number of markers unique to pathogens. When they recognize enemies, they attack immediately and often manage to eliminate the threat. When the innate defenses are overwhelmed, they secrete chemical messengers to mobilize the armies of adaptive defenses. Unlike the medieval soldiers, B cells and T cells don’t have eyes. Instead, they play a game of blind-man’s bluff, touching everything they encounter, and searching for a special shape that they recognize and to which they can bind. The special shape that a particular lymphocyte recognizes is called an antigenic determinant and is formed by the three-dimensional structure of a large molecule called an antigen. Antigens are usually proteins, but can also be large carbohydrates or nucleic acids. The surface of any given pathogen is studded with many different antigens, each usually having many different antigenic determinants. In our soldier analogy, the enemy is the pathogen, the antigen is the enemy's face, and the antigenic determinant is just part of his face, in this case, the nose. Let’s look at one antigen’s antigenic determinants. For simplicity, most people refer to antigenic determinants simply as antigens. We will do the same. Both B and T cells bind antigens. Protein molecules called antibodies also bind antigens. These antibodies, which play an important role in destroying pathogens, are secreted by the clonal descendants of B cells, called plasma cells. Antibodies float freely in extracellular fluids such as the blood. These fluids, once called humors (like the vitreous and aqueous humors of the eye) can be transferred from one person to another, carrying the antibodies with them and thereby transferring immunity from one person to another. The branch of adaptive immunity that can be transferred via body fluids is called humoral immunity and involves B cells and the antibodies they ultimately produce. Humoral immunity is directed against extracellular pathogens. In contrast, the other branch of adaptive immunity cannot by transferred via transferring body fluids. It is called cellular immunity because it involves cells, the T cells, which directly attack other cells. The cells attacked by these T cells are cells of our own body. Think about which of these circumstances cause your T cells to attack cells of your own body? Let’s see if you’re right. While you might not have thought of this at first, cells from another individual are like the cells of one’s own body but have differences in their surface proteins because of individual variation. Let’s summarize what you have learned. The type of cells involved in humoral immunity are B cells. T cells are involved with cellular immunity. Humoral immunity involves antibodies, while cellular immunity does not. And finally. Humoral immunity deals with extracellular pathogens; while the pathogens targeted by cellular immunity are intracellular. While it is important for you to keep these key points in mind, note that there are many interactions between cellular and humoral immunity. In fact, humoral immunity cannot function without cellular immunity in the form of helper T cells.
Because there are such a wide variety of pathogens, we need a corresponding breadth and depth of defense. Like a medieval castle, the immune system has three main lines of defense against the invading hordes that besiege it. Let’s examine each one in the order a pathogen would encounter them. The first line of defense consists of the surface barriers to entry, also called innate external defenses. Like the walls of the castle, the skin and mucous membranes form this barrier. Like the moat surrounding the walls, many of the body’s barriers are coated in secretions such as mucus and tears. The second line of defense in a castle consists of the guards who check everyone they encounter to determine if they are friend or foe. The body has similar defenses, cells and chemicals in body fluids, that are always on the ready to attack and destroy anything they identify as foe. These defenses are called the innate internal defenses. The guards can also call on the third line of defense, the army. The body’s armies are called the adaptive defenses. The adaptive defenses consist of two kinds of lymphocytes: B cells and T cells. It takes time to mobilize them and train them to fight an identified enemy. When surface barriers and secretions are penetrated by an enemy, the innate internal defense mechanisms, acting as guards, step into action. The innate defenses identify enemies by recognizing a limited number of markers unique to pathogens. When they recognize enemies, they attack immediately and often manage to eliminate the threat. When the innate defenses are overwhelmed, they secrete chemical messengers to mobilize the armies of adaptive defenses. Unlike the medieval soldiers, B cells and T cells don’t have eyes. Instead, they play a game of blind-man’s bluff, touching everything they encounter, and searching for a special shape that they recognize and to which they can bind. The special shape that a particular lymphocyte recognizes is called an antigenic determinant and is formed by the three-dimensional structure of a large molecule called an antigen. Antigens are usually proteins, but can also be large carbohydrates or nucleic acids. The surface of any given pathogen is studded with many different antigens, each usually having many different antigenic determinants. In our soldier analogy, the enemy is the pathogen, the antigen is the enemy's face, and the antigenic determinant is just part of his face, in this case, the nose. Let’s look at one antigen’s antigenic determinants. For simplicity, most people refer to antigenic determinants simply as antigens. We will do the same. Both B and T cells bind antigens. Protein molecules called antibodies also bind antigens. These antibodies, which play an important role in destroying pathogens, are secreted by the clonal descendants of B cells, called plasma cells. Antibodies float freely in extracellular fluids such as the blood. These fluids, once called humors (like the vitreous and aqueous humors of the eye) can be transferred from one person to another, carrying the antibodies with them and thereby transferring immunity from one person to another. The branch of adaptive immunity that can be transferred via body fluids is called humoral immunity and involves B cells and the antibodies they ultimately produce. Humoral immunity is directed against extracellular pathogens. In contrast, the other branch of adaptive immunity cannot by transferred via transferring body fluids. It is called cellular immunity because it involves cells, the T cells, which directly attack other cells. The cells attacked by these T cells are cells of our own body. Think about which of these circumstances cause your T cells to attack cells of your own body? Let’s see if you’re right. While you might not have thought of this at first, cells from another individual are like the cells of one’s own body but have differences in their surface proteins because of individual variation. Let’s summarize what you have learned. The type of cells involved in humoral immunity are B cells. T cells are involved with cellular immunity. Humoral immunity involves antibodies, while cellular immunity does not. And finally. Humoral immunity deals with extracellular pathogens; while the pathogens targeted by cellular immunity are intracellular. While it is important for you to keep these key points in mind, note that there are many interactions between cellular and humoral immunity. In fact, humoral immunity cannot function without cellular immunity in the form of helper T cells.