IMMUNOLOGY1

Immunology, derived from the Latin word 'immunis' meaning 'exempt,' is the study of an organism's response to antigenic challenges. It focuses on how the body protects itself from foreign macromolecules and invading organisms. Key terms include antigen (substances that trigger antibody production), antigenic challenge (the first encounter with an antigen), monoclonal antibodies (specific antibody preparations), immune system (biological structures protecting against disease), immunity (sufficient biological defenses), and immune response (coordinated reaction to foreign substances).

The immune system is crucial for fighting harmful external elements and internal changes. Its main tasks are to protect against microbes (bacteria, viruses, parasites, fungi), maintain homeostasis by neutralizing harmful substances, and defend against malignant cells like cancer, thus preserving bodily integrity.

The immune system has three layers of defense: organs, cells, and molecules. The first line of defense is the barrier, with the skin being the most important physical barrier, preventing pathogen entry, producing antimicrobial proteins, and containing immune cells. The second line consists of non-specific cells like macrophages, responding to various pathogens. The third line involves specific molecules, primarily antibodies, which respond to particular pathogens. Failure of these defenses leads to disease.

Immune cells perform their functions in specific organs, including tonsils, adenoids, lymph nodes, lymphatic vessels, thymus, appendix, bone marrow, Peyer's patches, and spleen. These organs are strategically located throughout the body to facilitate immune responses, such as filtering pathogens and producing immune cells.

Various cells contribute to the immune system. Neutrophils (60-70% of immune cells) are phagocytes that are first responders to infection. Eosinophils (1-2%) fight multicellular parasites and release toxins. Basophils (0.5-1%) are involved in parasitic infections, blood clotting, and allergic reactions. Mast cells release histamine, aid in wound healing, and defend against parasites. Macrophages engulf and digest pathogens and cancer cells. Monocytes differentiate into macrophages and dendritic cells. Dendritic cells are antigen-presenting cells that trigger adaptive immunity. Natural killer (NK) cells target and kill defective cells, including tumor and virus-infected cells.

Immune system molecules include antibodies (immunoglobulins), complements, cytokines, interleukins, and interferons. Antibodies are Y-shaped proteins that bind specifically to antigens, following a lock-and-key model. There are various types: IgG (main antibody in secondary response, crosses placenta), IgD (receptor on B cells), IgE (mediates hypersensitivity and parasitic infections), IgA (secretory antibody, prevents attachment of pathogens to mucous membranes), and IgM (produced in primary response, does not cross placenta).

Complements enhance the ability of antibodies and phagocytes to clear microbes, promote inflammation, and attack pathogen cell membranes. Cytokines are low-molecular-weight proteins secreted by white blood cells, acting as hormones of the immune system to regulate cellular interactions, anti-inflammation, and hematopoiesis. Interleukins are cytokines secreted by leukocytes that stimulate immune responses, regulate cell growth, differentiation, and motility. Interferons are signaling proteins released by host cells in response to viruses, interfering with viral multiplication and activating immune cells.

Immunity protects organisms from pathogens and toxins, categorized into innate and adaptive immunity. Innate immunity is inborn, present at birth, fast-acting, and non-specific, offering lifelong protection. Adaptive immunity is acquired, specific to particular pathogens, and involves clonal expansion of T and B lymphocytes. Adaptive immunity is further divided into natural and artificial, both having active and passive forms. Natural active immunity occurs through infection, while natural passive immunity involves maternal transfer of antibodies. Artificial active immunity is achieved through immunization, and artificial passive immunity through antibody transfer via injection.