Introduction to Immunology: Infection, Immunity, and Hypersensitivity
a) Define the following terms: 1) Infection. 2) Virulence. 3) Toxin.
b) Name three examples of pathogens, indicating in each case: 1) the type of organism, 2) the disease it causes, 3) the route of infection.
c) Explain the significance of the allergic reaction and autoimmunity. Cite any situation in which these processes occur.
a)
- The term infection involves the competitive action between two beings, a host and a parasite. If the parasite is successful, it causes disease. Infection is a type of parasitism.
- The degree of pathogenicity of a microorganism is known as virulence and is measurable. Pathogenicity refers to the ability of microorganisms to enter the host and cause anatomical and physiological changes that result in disease.
- A toxin is a proteinaceous substance that determines the functional specificity of some bacteria. Toxins are characterized by their ability to cause damage to the host and are the primary causes of bacterial diseases.
b)
c) Hypersensitivity is an inappropriate or exaggerated response of the immune system. Individuals who experience hypersensitivity are said to be hypersensitive. Importantly, hypersensitivity reactions are expressions of immune responses, both from pathogens and harmless substances, that occur inappropriately and cause inflammatory changes and tissue damage. There are two types of hypersensitivity: immediate and delayed.
The allergic reaction is now considered synonymous with immediate hypersensitivity. It is characterized by an exaggerated immune response against environmental antigens that, for most individuals, are safe (such as pollen, dust mites, and animal dander). The allergy-causing substances are called allergens and may be natural or synthetic products that cause tissue damage by triggering the inflammatory response.
Autoimmunity is a type of acquired immunity against the body’s own tissues. Autoimmunity occurs as a consequence of the failure of tolerance that lymphocytes and antibodies have towards own tissues. The diversity of the immune system is extraordinary, and since all the specificity expressed by T and B cells are generated at random, including many that are specific to the body’s own components. Mechanisms to distinguish between self and non-self determinants may fail, leading to phenomena of self-recognition. Autoimmunity is the source of many known autoimmune diseases, which generate abundant autoreactive antibodies and cells.
There are several autoimmune diseases with diverse clinical features, which are classified into two main groups:
- Organ-specific diseases: The immune response is directed against antigens located in specific organs, such as the thyroid, adrenals, stomach, and pancreas.
- Non-organ-Specific diseases: Autoantibodies and injuries are not specific to an antigen located in a single organ but affect certain organs more intensely.
a) Explain what the antigen-antibody reaction is.
b) What is the major histocompatibility complex?
c) Explain the role it plays in the immune response of T lymphocytes and B lymphocytes.
a) The humoral immune response involves antibody production in response to antigens that enter the bloodstream. The presence of antigens in the body triggers the production and release of antibodies by lymphocytes into the blood and tissue fluids. Antibodies are specific because they are designed to bind with their corresponding antigens through a process called the antigen-antibody reaction. During this process, the antigens are destroyed or rendered useless. Antibodies can combine with other chemicals, collectively referred to as the complement complex, which are characterized by their inactive enzymatic precursors that become active upon the antibody binding with the antigen. These activated enzymes then attack the antigens.
There are different types of antigen-antibody reactions:
- Precipitation reaction: Occurs when the antigen molecule is soluble in plasma, and the formed antigen-antibody complex is insoluble, leading to precipitation.
- Agglutination reaction: When the antigens are on cells or molecules, there is an aggregation of cells (agglutination) with the antibody molecules acting as links between them.
- Neutralization reaction: Occurs mainly with viruses and involves a reduction in the infectivity of the virus when antibodies bind to the antigenic determinants of the viral envelope.
- Opsonization reaction: Antibodies called opsonins bind to infected cells, making them more easily recognized and engulfed by phagocytic cells.
b) There is a set of personal and individual proteins in the plasma membrane of cells that serve to identify an individual, similar to a fingerprint. These proteins are specific to each person. This set of proteins is encoded by the major histocompatibility complex (MHC), which is polymorphic and polygenic: each gene has many different allelic forms among individuals of a species. This complex comprises at least 20 different genes and, given their genetic information, are used, among other things, to clarify family relationships.
c) The two main types of blood cells that recognize antigens are B lymphocytes and T lymphocytes.
The cellular immune response is mediated by T lymphocytes, which specifically bind to antigens. These white blood cells have receptor molecules on their membranes that allow them to recognize antigens. T cells activated by this recognition divide and secrete lymphokines, which are chemicals that activate other immune system components. B lymphocytes respond to these signals. In the humoral immune response, B cells that recognize the antigen are activated. Once activated, B cells divide and differentiate into plasma cells that secrete antibodies. By binding to the antigens they encounter, antibodies can neutralize them or trigger their destruction either by complement enzymes or by natural killer cells.
Some T and B cells become memory cells that persist in the circulation and can quickly stimulate the immune system to eliminate the same antigen if it is encountered again in the future.