The Immune System – Immunity

The immune system is a structure of bodily safeguards that protect the human body from antigens such as infectious and venomous agents. This system recognizes, processes, and then eliminates antigens. Based on descriptions and memory, the immune system deterrence is categorized into two types: innate and adaptive immunity. The coordination and development of a well-adjusted mechanism to obliterate pathogens are shared by both systems. Specific elements acclimate themselves to each new class of illness encountered and are hence able to assemble pathogen-specific immunity.

In contrast, non-specific components act as extinguishers or chemical mediators of pathogens, regardless of antigenic specificity. Innate immunity, also known as non-specific immunity, is always involved in removing antigens at the first line, without prior exposure to this specific antigen.

This category of the immune system is composed of epithelial barriers, molecules, and cells of innate immunity. Examples include NK cells, neutrophils, basophils, eosinophils, and monocytes-macrophages. In contrast, adaptive immunity, known as specific immunity, requires antigen recognition and a secondary reaction to strengthen. Memory is another crucial element that enhances the reaction in adaptive immunity. Furthermore, there are two major types of protection based on the work area: cellular mediated and humoral mediated adaptive immunity. The cellular mediated adaptive immunity is mostly controlled by the coordination of APCs, adaptive T helper cells, and effector cells. Examples of antigen-presenting cells include dendritic cells, monocytes, macrophages, and resident macrophage lymphocytes.

According to JLM Carrillo (2017), the immune system is composed of a few chains of effector mechanisms that can effectively destroy pathogens such as bacteria, fungi, viruses, and parasites. The innate immune system typically consists of somatic barriers such as mucous membranes and skin, chemical agents, reaction of antimicrobial peptides, reactive oxygen, innate immune cells, and soluble mediators such as the complement structure, innate antibodies, and correlative cytokines. Furthermore, the primary purpose of the innate immune system is to prevent the introduction of pathogens into the body through physical and chemical barriers, inhibit the proliferation of infections through the complement network, eliminate pathogens through the process of phagocytosis, and initiate the adaptive immune system through the process of synthesizing several cytokines and presenting antigens to T cells and B cells. Additionally, the cells of the innate system play various roles in protecting the human body from pathogens. Some cells create physical barriers that delay infections. Still, others have pattern recognition receptors that recognize pathogen-associated molecular patterns and damage-associated molecular patterns, responding by creating inflammatory cytokines to destroy microorganisms or infected cells. These cells include myeloid cells, non-myeloid cells, and some lymphoid cells.

In addition, non-myeloid cells consist of fibroblasts and epithelial cells, which typically form a barrier between the internal and external environments. These cells produce antimicrobial substances that inhibit the entry of pathogens. These antimicrobial substances, known as antimicrobial peptides, are important components of the innate immune response that provide the first line of defense against infection. Myeloid cells include dendritic cells, neutrophils, eosinophils, basophils, mast cells, platelets, and macrophages. Each of these cells has a specific function to protect the human body from invading microorganisms. For instance, when a wound occurs, these cells are stimulated by our innate immune system to protect us from infections entering through the open wound. In addition to these, lymphoid cells consist of natural killer cells, and innate lymphoid cells. Innate lymphoid cells are a new class of hematopoietic cell that play protective roles in the innate immune response to infectious pathogens, aid in lymphoid tissue development, oversee tissue remodeling after damage due to injury or infection, and assist in the homeostasis process of tissue stromal cells.

The detection of pathogens by cells involved in the innate immunity response occurs through nonspecific substances shared by many pathogens known as pathogen-associated molecular patterns. Pathogen-associated molecular patterns are highly conserved and are derived from a variety of microorganisms. These patterns do not exhibit specific form or antigenic variability, and host cells do not share the same molecular structure as pathogens. This leads to memorization by the immune system, allowing it to differentiate between self and non-self. Among the pathogen-associated molecular patterns that identify the pathogens are peptidoglycan, lipopolysaccharide, lipoteichoic acid, unmethylated cytosine phosphor-guanine motifs, double-stranded ribonucleic acid virus, and the cell wall element of yeast. Lipopolysaccharide represents a major component of Gram-negative bacteria, while peptidoglycan is a crucial constituent of Gram-positive bacteria. Recognition of these pathogen-associated molecular patterns is facilitated through pattern recognition receptors, primarily belonging to the family of Toll-like receptors.

As cited by Janeway Jr., C.A. (2002), the induction of an adaptive immune response begins when a microorganism is consumed by an immature dendritic cell in the infected tissue. These specific phagocytic cells dwell in most tissues and are comparatively long-lived, turning over at a moderate rate. They derive from the same bone marrow progenitor as macrophages, and journey from the bone marrow to their peripheral stations, where their role is to monitor the particular environment for pathogens. Ultimately, all tissue-resident dendritic cells travel via the lymph to the regional lymph nodes where they interact with recirculating naive lymphocytes. If the dendritic cells are unable to be activated, they promote tolerance to the antigens of self that they carry. Additionally, the immature dendritic cell carries receptors on its surface that recognize common features of many pathogens, such as bacterial cell wall proteoglycans. Like macrophages and neutrophils, attachment of a bacterium to these receptors stimulates the dendritic cell to engulf the pathogen and process it intracellularly. Immature dendritic cells are also consistently taking up extracellular material, including potential viral molecules or bacteria, through the receptor-independent mechanism of macropinocytosis.

The primary role of dendritic cells, however, is not to destroy pathogens, but to carry pathogen antigens to peripheral lymphoid organs and then present them to T lymphocytes. In addition to this, a pathogen contained in infected tissue will be intercepted by a dendritic cell to trigger its activation and migration to the adjacent lymph node. Upon activation, the dendritic cell transforms into a highly efficient antigen-presenting cell, undergoing changes that enable it to activate pathogen-specific lymphocytes that it encounters in the lymph node. Moreover, activated dendritic cells produce cytokines that affect both innate and adaptive immune responses, making these cells imperative gatekeepers that determine whether and how the immune system responds to the presence of infectious agents. We will compare the maturation of dendritic cells and their central role in presenting antigens to T lymphocytes.

According to Russell R. (n.d), adaptive immunity, also known as specific immunity, has the ability to identify and protect itself against specific threats and their effects. This aspect of immunity is smart. Its memory allows it to react immediately to a second encounter with a pathogen. Several components form part of specific immunity which are acquired over time. These include antigens which trigger specialized immune responses, a variety of cells, tissues, and organs including B and T lymphocytes with certain receptors, lymphatic circulation system, interactions with the circulatory, nervous, and endocrine systems, as well as interactions with non-specific systems such as antigen presentation. Furthermore, there are five classes of immunoglobulin involved in adaptive immunity: IgA, IgG, and IgM, which are usually produced after a viral infection, IgE, which is involved in the activation of basophils and mast cells against parasites and allergens, while IgD is a type of B-cell receptor. In addition, there are two types of specific immunity acquired during a person's life: natural, which is the immune response that fights against pathogens encountered in everyday life, and artificial, which responds to pathogens in a vaccine, also known as active or passive immunity. An active response stimulates our own immune system, such as through vaccination, to create our own immunity, while a passive response is already present in our body and is inherited from the mother via the placenta or colostrum.

When evaluating a patient's immune response to infection episodes, it's observed that the immune system has evolved to handle infectious antigens. There are several lines of host defense. Mechanical barriers act as a crucial first defense, followed by the disruption of drainage functions as found in tears, saliva, bile, pancreatic fluid, mucus and even sebaceous secretions, which protect the surfaces they cover. The obstruction is often linked to infections. Moreover, secretions containing a variety of enzymes and factors can slow down bacterial growth and the normal microbial flora helps to prevent the overgrowth of pathogenic bacteria.

According to Fink L. S. and Cookson T.B. (2005), in most cases, cell death is classified as either apoptosis or necrosis. Apoptosis is an active, programmed activity of self-destruction within a cell to prevent inflammation. It describes a significant form of cell death seen when cells are lost during embryonic development, the transformation of normal cells in healthy adult tissue, or atrophy as a result of hormone release. The process of apoptosis is characterized by the condensation of the nucleus and cytoplasm, and cellular fragmentation into membrane-bound fragments, which are then removed by the process of phagocytosis. Conversely, necrosis is passive, where cell death occurs unexpectedly due to environmental changes, resulting in the uncontrolled release of inflammatory cellular materials. Necrosis is also used to describe dead tissues and the changes that occur in body cells after their death. In the absence of phagocytosis, apoptotic bodies can lose their integrity and become necrotic. Necrotic cell death often occurs in response to injuries such as trauma and infarction, which are significant results of a pathological process.

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