Immunology is a biomedical science focused on studying the immune responses of organisms to foreign substances and the associated mechanisms. The immune response encompasses not only the reaction to antigenic stimulation but also the biological processes involved in recognizing and eliminating antigens. Many elements of the immune system are cellular in nature, not restricted to specific organs but rather distributed and circulating throughout various tissues in the body. Immunology has extensive applications across multiple medical disciplines, including oncology, organ transplantation, rheumatology, bacteriology, virology, dermatology, and parasitology.
Given the current capability to employ high-throughput and high-bandwidth methods for the unbiased exploration of diverse immune responses, researchers have identified key participants involved in health and disease and generated new hypotheses and mechanistic studies. The immune system can be investigated using various techniques applied to peripheral blood samples from healthy donors, as well as samples taken at specific time points during infection, disease, or post-vaccination. Characterizing innate cell subsets can be achieved through techniques such as flow cytometry, RNA-seq, phospho-flow cytometry, and cytometry by time-of-flight (CyTOF), which together reveal mediators of early inflammation and their associated signaling pathways.
Modern immunology has evolved into an independent discipline with its own theoretical framework and has specialized research methodologies, offering new tools for biological research. In the early 20th century, immunology was employed to differentiate human blood groups. Immunological methods have also been utilized in the study of animal and plant toxins. For instance, these methods were initially used to investigate diphtheria and tetanus toxins and subsequently adapted to study plant toxins like ricin and Croton toxin. Early 21st-century advancements, such as enzyme immunoassay, radioimmunoassay, and immunofluorescence, have furnished biology with practical research tools.
The development of monoclonal antibody technology represents a significant breakthrough in immunology. Monoclonal antibodies play a crucial role in identifying surface molecules on different immune cells and other tissues, enabling the characterization, isolation, and categorization of immune cells, and advancing the exploration of the structure and function of various membrane surface molecules.
The adaptive immune system, also referred to as the acquired or specific immune system, is a subsystem of the immune system consisting of specialized cells and processes designed to eliminate pathogens or inhibit their growth. This system generates immunological memory following an initial response to a specific pathogen, resulting in a heightened response upon subsequent encounters with the same pathogen. The acquired immune system represents one of the two primary immunity strategies present in vertebrates.
Fig.1 Adaptive immune response to SARS-CoV-2 antigen exposure.1, 3
Cluster of differentiation (CD) markers are surface proteins that identify specific cell differentiation lineages, recognized by monoclonal antibodies. Originally defined on the surfaces of leukocytes and recognized by specific antibodies, CD antigens now include some intracellular molecules and proteins on non-leukocyte cells. CD numbers are assigned to these molecules following consensus at international workshops.
Immune disorders can be categorized based on the affected components of the immune system, the level of immune activity (overactive or underactive), and whether the condition is congenital or acquired. These disorders primarily fall into three main categories: autoimmune diseases, immunodeficiencies, and allergies.
In immunology, an Fc receptor is a protein present on the surface of various cells–including eosinophils, follicular macrophages, dendritic cells, and human platelets–that play a role in immune protection. The receptor's name derives from its binding affinity for the Fc (fragment crystallizable) region of antibodies.
Fig.2 Schematic diagram of Fc receptor.2
Cytokines comprise a heterogeneous array of small proteins critical for cellular signaling. Due to their size, they cannot cross the lipid bilayer of cells to enter the cytoplasm, and thus, they exert their functions by binding to specific cytokine receptors on the surface of target cells. Cytokines participate in paracrine, endocrine, and autocrine signaling, functioning as regulators of the immune system.
The innate immune system, referred to as the non-adaptive immune system, constitutes one of the fundamental defense mechanisms in vertebrates. This innate defense mechanism is predominant in fungi, invertebrates, plants, and prokaryotes alike.
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