Understanding the Immune System: Types and Functions
Types of Immunity
– First: External barriers that prevent microorganisms from entering the body.
– Second: Unspecified internal defenses fighting the invaders.
– Third: The system directs immune attack, an immune response against microbes.
Innate Immunity:
Mechanisms that fight the microbe before infection occurs. Major components of innate immunity include physical barriers, chemical barriers, phagocytic cells, and plasma proteins.
A) Physical Barriers or Mechanical:
The skin prevents or hinders the entry of pathogens. It is highly unlikely that bacteria enter through intact skin, but when there is a cut or burn, it becomes a risk. Membranes lining the body cavities that communicate with the outside (digestive tract and airways) also serve as barriers. Mucosal cells produce mucus that lubricates and traps germs.
B) Chemical Barriers:
Secretions that affect the development or death of microbes occur (tears, saliva, mucous secretions of the digestive and circulatory systems). Tears and saliva contain an enzyme called lysozyme that kills bacteria. Sweat provides lysozyme and helps drag pathogens outside the body.
C) Phagocytic Cells:
These include macrophages, neutrophils, and NK (natural killer) cells. Macrophages are leukocytes (white blood cells) derived from monocytes. When monocytes leave the blood and become macrophages, they originate in tissues and take shape, being present in the central nervous tissue, alveolar epithelium, intestines, liver, and bones. Neutrophils are white blood cells that phagocytose dead cells and contain antibacterial substances. NK cells are a variant of lymphocytes that phagocytose virus-infected cells and tumors.
D) Plasma Proteins:
Specific proteins called cytokines, interleukins, or lymphokines are involved in the induction of the inflammatory response and regulation of white blood cell production. Interferons are proteins released by cells affected by viruses. They are taken up by specific cells with receptors for them and respond by secreting peptides that inhibit viral replication, also stimulating NK cells. The complement system consists of two dozen plasma proteins that enhance certain immune, allergic, and inflammatory responses, contributing to the body’s defense.
A) Humoral Adaptive Immunity:
The production of proteins that are very specific against an antigen, called antibodies. These are immunoglobulins that circulate through the blood and body fluids in search of antigens; they can neutralize the ability of infecting pathogens and mark them for easy removal.
B) Cell-Mediated Adaptive Immunity:
Intracellular pathogens can survive and proliferate within phagocytes and other cells, where antibodies fail. This immunity is characterized by the presence of T lymphocytes. The adaptive cellular immune response occurs in three phases:
A) Recognition of the Antigen:
Selected preferentially through clonal theory, a wide variety of cells are generated randomly, and some are selected for the immune response. Each cell will react with an antigen determined by proteins on the cell membrane surface of lymphocytes, which act as receptors.
B) Activation of Lymphocytes:
This requires two different signals: 1) The presence of the antigen ensures that the immune response is specific. 2) The presence of some manifestation of the innate immune response.
C) Effector Phase:
Lymphocytes that have been activated specifically by the antigen carry out the removal of the latter. Antibodies eliminate extracellular microbes while T cells target intracellular ones. As a result of these three phases, the antigen is removed, and a record is made by cells for future infections.
Leukocytes: White blood cells.
Immunity: The set of mechanisms for defense against external agents.
Edward Jenner: Improved the technique of variolation by designing an experimental procedure in which a healthy child was inoculated with a fluid sample obtained from a pustule of bovine chickenpox. When the child was infected with human varicella, he did not develop the disease.
Louis Pasteur: Isolated and cultured in the laboratory the bacterium that causes cholera (inoculated chickens). He concluded that aging the bacteria decreases its virulence and that these variants may be administered attenuated to protect the body. This variety is called the vaccine.
Types of Immunity: The body has three lines of defense against microbial attacks:
– First: External barriers that prevent microorganisms from entering the body.
– Second: Nonspecific internal defenses fighting the invaders.
– Third: The immune system directs its attack, an immune response against specific microbes.
Innate Immunity: All mechanisms that exist before infection, corresponding to the first line of defense. Main components include physical barriers, chemical barriers, phagocytic cells, and plasma proteins.
– Physical Barriers: The epithelia that form the skin and mucosal tissues (digestive tract and airways). Mucosal cells produce mucus, which lubricates and traps germs. The hair cells (lining of the nose and trachea) sweep germs trapped in mucus to the pharynx, where they move to the stomach to be destroyed by hydrochloric acid.
– Chemical Barriers: Tears, saliva, mucous secretions, secretions from sebaceous glands, and sweat. The enzyme lysozyme (found in tears and saliva) destroys the cell walls of bacteria, leading to their death.
– Phagocytic Cells: Specific responses are represented by cells with phagocytic capacity, including macrophages, neutrophils, and natural killer cells (NK).
– Plasma Proteins: The last barrier is formed by specific proteins called cytokines, interleukins, or lymphokines. All participate in the induction of the inflammatory response and in regulating the production of white blood cells. Other blood proteins, such as interferons (released by infected cells and macrophages), also play a role.
2) Acquired (Specific): Composed of lymphocytes and their products, characterized by specificity and the ability to remember and respond to repeated exposures to the same microbe.
– B Lymphocytes: Antibodies are proteins that recognize, inactivate, and facilitate the destruction of foreign agents (extracellular microbes).
– Helper T Lymphocytes: Secrete cytokines that stimulate the proliferation and specialization of other lymphocytes, in addition to the inflammatory response, eliminating intracellular microbes.
– Natural Killer Cells: Destroy cells infected by intracellular pathogens such as viruses.
A) Adaptive Humoral Immunity: Involves very specific proteins called antibodies, which correspond to immunoglobulins.
B) Adaptive Cellular Immunity: Characterized by the participation of T cells, which help destroy intracellular pathogens (viruses and bacteria that grow within cells).
Adaptive Immune Response: This process occurs in three stages: recognition of antigen (clonal selection theory), activation of lymphocytes (hypothesis of the two signals), and effector phase (elimination of the antigen). As a result, the antigen is destroyed, and the cells are stored for future destruction.
Immunity and Vaccines: Used to produce samples of whole viruses or bacteria and dead or attenuated variants. Once inside the body, they produce immunity (adaptive or acquired) because they generate a specific immune response. The aim is to produce antibodies and cytotoxic cells capable of destroying cells infected by the virus.
Lymphoid Organs: These are part of the immune system and are responsible for the formation of lymphocytes. They include blood vessels, lymphatics, glands, adenoids, lymph nodes, thymus, spleen, Peyer’s patches, and bone marrow.
Allergies: Can be considered a wildly exaggerated immune response against seemingly innocuous substances. Characterized by early encounters with the allergen, the body does not show any response after a second exposure, leading to a rapid inflammatory reaction. As a result of this reaction, an increase in mucus secretion, dermatitis (itching), eczema (skin irritation), and even diarrhea can occur. Treatment often involves the use of antihistamines, which are sometimes combined with decongestants.