Understanding Infection, Immunity, and Hypersensitivity: A Comprehensive Overview
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)
- Infection involves the competitive interaction between two organisms: a host and a parasite. If the parasite succeeds in establishing itself, it leads to disease. Infection is a form of parasitism.
- Virulence refers to the degree of pathogenicity of a microorganism. It is a measurable characteristic. Pathogenicity describes the ability of a microorganism to enter a host and cause anatomical and physiological changes that result in disease.
- A toxin is a proteinaceous substance that contributes to the functional specificity of some bacteria. Toxins are characterized by their ability to cause damage to the host and are the primary cause of bacterial diseases.
b)
c) Hypersensitivity is an inappropriate or exaggerated response of the immune system. Individuals who exhibit such responses are said to be hypersensitive. Importantly, hypersensitivity reactions are simply expressions of immune responses, directed against both pathogens and harmless substances. These responses are inappropriate in their intensity and can lead to inflammatory changes and tissue damage. There are two main 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 are typically harmless to most individuals (e.g., pollen, dust mites, animal dander). The substances that trigger allergies are called allergens and can be either natural or synthetic products. They cause tissue damage by initiating an inflammatory response.
Autoimmunity is a type of acquired immunity directed against the body’s own tissues. It arises from a failure of the tolerance that lymphocytes and antibodies normally have towards self-tissues. The immune system is incredibly diverse, and the specificity of T and B cells is generated randomly. This includes cells that are specific to the body’s own components. Mechanisms that distinguish between self and non-self determinants can fail, leading to self-recognition. Autoimmunity is the underlying cause of many autoimmune diseases, which are characterized by the production of abundant autoreactive antibodies and cells.
There are numerous autoimmune diseases with diverse clinical features. They can be broadly 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 tissue damage are not limited to a single organ but may affect certain organs more severely.
a) Explain the antigen-antibody reaction.
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 the production of antibodies in response to antigens that enter the bloodstream. The presence of antigens triggers the production and release of antibodies by lymphocytes into the blood and tissue fluids. Antibodies are specific, meaning they are designed to bind to their corresponding antigens through a process called the antigen-antibody reaction. This process either destroys or neutralizes the antigens. During this process, antibodies can combine with other chemicals, collectively known as the complement system. The complement system consists of inactive enzyme precursors that become activated when an antibody binds to an 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. The resulting antigen-antibody complex is insoluble and tends to precipitate.
- Agglutination reaction: When antigens are present on cells or molecules, the antibody molecules act as links, causing the cells to clump together (agglutinate).
- Neutralization reaction: Primarily occurs with viruses. It involves a reduction in the infectivity of the virus when antibodies bind to antigenic determinants on the viral envelope.
- Opsonization reaction: Antibodies called opsonins bind to infected cells, making them more susceptible to phagocytosis by phagocytic cells.
b) The plasma membrane of cells contains a set of unique proteins that serve as an individual’s identifier, similar to a fingerprint. These proteins are encoded by the major histocompatibility complex (MHC), which is both polymorphic and polygenic. Polymorphic means that each gene has many different allelic forms among individuals of a species, and polygenic means that the complex is composed of at least 20 different genes. Due to their genetic information, MHC proteins are used, among other things, to determine 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 enable them to recognize antigens. When activated by antigen recognition, T cells divide and secrete lymphokines, which are chemicals that activate other components of the immune system. B lymphocytes are among the cells that respond to these signals.
The humoral immune response involves B cells that recognize and are activated by antigens. Once activated, B cells divide and differentiate into plasma cells, which secrete antibodies. By binding to antigens, antibodies can neutralize them or trigger their destruction by either complement enzymes or natural killer cells.
Some T and B cells become memory cells that persist in the circulation. These memory cells can rapidly stimulate the immune system to eliminate the same antigen if it is encountered again in the future.