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21.1Immunity and Biophylaxis

21.1.1

General nature of sensation

In many cases, molecular and cellular biological pathways have been elucidated during the process of studying immunological issues. The following are some examples of such findings. With the exception of meiosis during gametogenesis (see Chapter 18), genetic recombination is observed only in the genes of T-cell receptors and immunoglobulins in lymphocytes in the primary lymphoid organs (described below). Circulating lymphocytes are in the G0 phase of the cell cycle but when they encounter antigens, they start to proliferate. Detailed analysis of the process revealed the mitogen-activated protein (MAP) kinase cascade (see Chapter 14). Signal transmission is triggered by the binding of the ligand to the receptor. It is often used for the control of genetic transcription, triggering either gene expression or suppression (see Chapter 15). In some cases, second messengers of signaling cascades can function as direct transcription factors. A particularly relevant example of this is signal transmission following the binding of cytokines (described later) to their cognate receptors. Once foreign microorganisms or allergens are recognized, an immunological response is triggered. The process requires a mechanism for antigen-presenting cells (which internalize, cleave, and present antigens) and lymphocytes (which respond to the antigen) to efficiently encounter each other. To achieve this, numerous chemokines (described later) and their receptors control cell movement and locations. All of these receptors have seven transmembrane domains and are coupled to trimeric G-proteins (see Chapter 14). They trigger depolymerization and polymerization of the cytoskeleton in a specific direction (see Chapter 11) in order to promote the migration of immune cells along a chemokine concentration gradient. Despite many advances in the field, some issues remain unresolved, such as how the mechanisms that regulate cellular life span and cell death function with memory T cells (cells with an exceptionally long life cycle). Elucidation of these issues may lead to molecular cell biology-related discoveries.

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21.1.2

Innate immunity and acquired immunity co-operate in the immune response

Immune phenomena are often described from two aspects: innate immunity and acquired immunity. Innate immunity is present in all multicellular organisms. It senses evolutionarily distant foreign organisms such as microorganisms or altered components of oneself and eliminates them using several concerted mechanisms. In contrast, acquired immunity is seen only in vertebrates. It is designed to discriminate foreign substances (“non-self” substances) based on the images of the “shapes” of all the substances making up its own body (“self”) and triggers an immune response to eliminate the foreign substances that have a different “shape” from any of the “self” substances. In addition, it also has a system to “remember” the shape of the foreign substances encountered, and maintains an immune memory. These “shapes” that are recognized by the immune system are called epitopes; and those molecules that have an epitope and thus evoke immune responses are called antigens. Epitopes are recognized by proteins called T-cell receptors and immunoglobulins, which are both produced by lymphocytes playing a central role in acquired immunity.

In vertebrates, including humans, innate and acquired immunity do not exist as separate response systems but always work as a coordinated system. Generally, the phase before activation of lymphocytes is referred to as the innate immune response, while the second phase is called the acquired immune response. Since organisms (such as insects) that only have innate immunity flourish on earth, it has been inferred that acquired immunity is not essential for survival. However, organisms without acquired immunity generally have rapid reproductive life cycles, thereby ensuring survival by producing large numbers of progeny. In this sense, acquired immunity is more associated with the survival of individual organisms, leading to a stronger focus on individuals rather than the species as a whole.

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21.1.3

Molecular and cellular biological aspects of immunity

In many cases, molecular and cellular biological pathways have been elucidated during the process of studying immunological issues. The following are some examples of such findings. With the exception of meiosis during gametogenesis (see Chapter 18), genetic recombination is observed only in the genes of T-cell receptors and immunoglobulins in lymphocytes in the primary lymphoid organs (described below). Circulating lymphocytes are in the G0 phase of the cell cycle but when they encounter antigens, they start to proliferate. Detailed analysis of the process revealed the mitogen-activated protein (MAP) kinase cascade (see Chapter 14). Signal transmission is triggered by the binding of the ligand to the receptor. It is often used for the control of genetic transcription, triggering either gene expression or suppression (see Chapter 15). In some cases, second messengers of signaling cascades can function as direct transcription factors. A particularly relevant example of this is signal transmission following the binding of cytokines (described later) to their cognate receptors.

Once foreign microorganisms or allergens are recognized, an immunological response is triggered. The process requires a mechanism for antigen-presenting cells (which internalize, cleave, and present antigens) and lymphocytes (which respond to the antigen) to efficiently encounter each other. To achieve this, numerous chemokines (described later) and their receptors control cell movement and locations. All of these receptors have seven transmembrane domains and are coupled to trimeric G-proteins (see Chapter 14). They trigger depolymerization and polymerization of the cytoskeleton in a specific direction (see Chapter 11) in order to promote the migration of immune cells along a chemokine concentration gradient.

Despite many advances in the field, some issues remain unresolved, such as how the mechanisms that regulate cellular life span and cell death function with memory T cells (cells with an exceptionally long life cycle). Elucidation of these issues may lead to molecular cell biology-related discoveries.

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