10 | Chapter 1 The immune system is a collection of molecules, cells, and tissues, which protect an individual from infectious microbes and eliminate foreign substances, and consists of two main branches: the innate immune system and the adaptive immune system. The innate immune system provides immediate, nonspecific defense mechanisms, including physical barriers like the skin, as well as immune cells that engulf and destroy pathogens. The innate immune system responds always in the same way to repeated infections and it is not able to escalate the responses with each successive exposure to a particular microbe or antigen. The innate immune system is phylogenetically the oldest system of host defense. The adaptive immune system, on the other hand, develops a more specific and targeted response over time, that can recognize and “remember” specific pathogens, providing a more tailored and long-lasting defense. The most characteristic feature of adaptive immunity is its unique specificity for distinct molecules. Lymphocytes and antigen-presenting cells (APC) are the main components of adaptive immunity. There are distinct subpopulations of lymphocytes that differ in their function and how they recognize antigens. B lymphocytes (B cells) recognize extracellular antigens, and are endowed with the unique property to produce antibodies. Antibodies are effectors of humoral immunity; they are serum proteins that initiate processes leading to the neutralization of antigens. T lymphocytes (T cells), on the other hand, mediate cellular immunity. T cells are able to directly kill the infected cells and they also direct immune responses by helping B cells to eliminate the pathogens. The communication between immune cells is crucial for an effective immune response, and various signaling molecules, such as cytokines, help coordinate these interactions. The immune system’s ability to distinguish self from non-self prevents it from attacking the body’s own cells, a condition known as autoimmunity. B cells are specialized white blood cells that are responsible for producing antibodies, which are proteins that recognize and neutralize specific pathogens like bacteria and viruses. The total of antibodies with different specificities is called the B-cell repertoire, a vast and unique collection of distinct receptor molecules on the surface of B cells. The B-cell repertoire is generated through a process known as V(D)J recombination, occurring in the bone marrow during the early stages of B- and T-cell development. This genetic rearrangement process creates an enormous variety of potential antibody specificities, allowing the B and T cells to recognize a wide range of antigens. Upon maturation in the bone marrow, B cells migrate to the bloodstream and lymphoid tissues, where they can be activated, e.g. in response to foreign invaders. The diversity and adaptability of the B-cell repertoire contribute significantly to the immune system’s ability to defend the body against a myriad of pathogens. This intricate system ensures that the immune response is tailored to the specific challenges posed by different infectious agents, providing a key defense mechanism for maintaining overall health and well-being.
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