Immunity and infectious disease

• ALSO KNOWN AS: Acquired (adaptive) immunity, cellular immunity, immune response, inflammatory response, innate immunity

Definition

Immunity is the state, quality, or condition of being resistant to pathogens, parasites, and nonliving harmful substances.

Innate and Acquired Immunity

Ubiquitous pathogens are found on surfaces, on food, and in the air. Innate and acquired immunity confer lifelong protective immunity to the body against foreign substances, including harmful toxins, viruses, and bacteria. Three basic components work closely to protect the body: physical barriers such as the various epithelial surfaces, innate immunity, and acquired immune responses. Inherited genes, environment, lifestyle, and acquired characteristics can influence the state of immunity.

Innate, or natural, immunity is the ability inherent from birth to fight infection without adapting to a specific pathogen. Innate immunity is characterized by physical barriers that defend against harmful agents and by more sophisticated defense mechanisms. Sometimes, physical defenses can be triggered through innate immune responses, such as ciliary action or sneezing from histamine production.

Other defenses include bactericidal enzyme action in secreted bodily fluids and more complex complement proteins. The innate immune system is nonspecific, focusing on conserved pathogen-associated molecular patterns so that many organisms are attacked in a similar fashion. Although the quality and efficacy of the initial innate response do not improve after subsequent exposures to the same pathogen, innate immunity includes a number of other defense mechanisms. Epithelial surfaces, including the genitourinary tract, respiratory tract, skin, and gastrointestinal tract, produce antimicrobial peptides such as defensins and cathelicidins that inhibit bacterial and fungal growth. Two nonspecific methods to eliminate microorganisms are phagocytosis and opsonization. In phagocytosis, specialized cells such as neutrophils, monocytes, and macrophages ingest and destroy ingested pathogen particles. In opsonization, phagocytic cells recognize a plasma protein (opsonin) binding to the surface of the pathogen, leading to enhanced phagocytosis.

The hallmark of innate immunity is an inflammatory response (inflammation or edema). Proinflammatory mediators such as cytokines, chemokines, and lipid mediators clear the infection. Inflammation, however, is damaging and painful to tissues, and some chronic diseases possess an inflammatory pathology component. Clinically, drugs can be used to control inflammation. Innate immune response is an early defense mechanism against infection, but it is also essential in boosting subsequent adaptive immune responses.

During adaptive (acquired) immunity, the immune system develops a defense specific to a particular antigen and does so with immunological specificity and long-lasting memory beyond the acute infection. An agent evoking a specific immune response is called an immunogen. Immunogens reacting with antibodies are antigens. Virtually any substance of a certain size, including cell proteins, viral nucleic acids, chemicals, or foreign particles (such as a splinter), can become an antigen. The goal of an acquired immune response is to recognize and destroy substances containing antigens.

To reach this goal, acquired immunity utilizes two sophisticated and flexible mechanisms: cell-mediated immunity and humoral immunity. Cell-mediated response relies on B and T lymphocytes (white blood cells). Following antigen exposure, antigens are taken up and presented to B and T lymphocytes by antigen-presenting cells, such as macrophages from the innate system or by dendritic cells from the acquired system. After recognizing their specific matching antigen, B cells differentiate into plasma cells, which then produce and secrete large amounts of antibodies against the specific antigen.

Likewise, T cells differentiate after antigen recognition into helper T cells (Th) or cytotoxic (killer) T cells (Tc); the T cells release lymphotoxins causing cell lysis. Th’s secrete lymphokines, which further stimulate Tc and B cells to proliferate and divide, attracting neutrophils and improving phagocytes’ ability to engulf and kill pathogens. Although innate immunity is available instantly upon infection, acquired immunity takes approximately seven to ten days to mount an initial response. Parts of the innate system, such as complement or phagocytosis, can also be activated by the acquired system through antibody mediation.

Immunoglobulin (Ig) is another term for antibody; it binds specifically to antigenic determinants or epitopes. Immunoglobulins inactivate antigens by complement fixation, neutralization, agglutination, and precipitation. Immunoglobulins are made of two identical heavy chains and identical light chains. They are classified based on their heavy chain as IgM, IgG, IgA, IgE, and IgD.

Passive and Active Immunity

Antibody-mediated immunity includes passive and active immunity. Exposure to the pathogen/antigen results in active immunity. For example, during vaccination, the antigen is presented (by injection of a weakened, killed, or recombinant pathogenic antigen), resulting in the vaccinated person’s body generating a specific immune response against that antigen.

In passive immunity, “natural” or “artificial,” the body does not manufacture its own antibodies; rather, the body gets antibodies from another person. For example, infants undergo natural passive immunity during the transfer of antibodies through the maternal placenta or milk. These infant antibodies disappear between six and twelve months of age with the replacement of breast milk. Passive immunity is short-lived because these antibodies are degraded in the body over time and because no immunological memory exists to produce more antibodies.

Artificial passive immunity involves the transfusion of antiserum or the injection of antibodies that were produced by another person or animal. Immediate protection against an antigen is achieved through these antibodies, although it is a short-lived immunity. Examples of passive immunization include tetanus antitoxin and purified human gammaglobulin.

Immunity Disorders and Complications

Sometimes, single components of the immune system are inefficient, absent, or excessive. In these cases, the state of protection is not reached adequately. The impaired immune system is considered immunocompromised, and it could leave the host body vulnerable to various opportunistic infections. Acquired immunodeficiency syndrome (AIDS), for example, is a result of the depletion of helper T cells after a viral infection. The failure of host defense mechanisms can lead to conditions such as autoimmune diseases, immunodeficiencies, allergies, delayed hypersensitivity states, and transplant rejections. Immune responses in the absence of infection include allergy or hypersensitivity reactions, autoimmunity, graft rejections, and autoimmune disorders like rheumatoid arthritis, lupus, or type one diabetes. An allergic reaction occurs against innocuous substances. Responses to self-antigens are visible in autoimmune diseases. Immunodeficiencies can be inherited (primary) or acquired (secondary).

Impact

Vaccination is a preventive measure against morbidity and mortality resulting from infectious diseases such as polio, measles, diphtheria, pertussis, rubella, mumps, tetanus, and Haemophilus influenzae type B. It is an artificial method of building immunity by deliberately infecting a person so that the body learns self-protection from a pathogen. Controlling and even eradicating infectious diseases reduces frequent doctor’s visits, hospitalizations, and deaths, leading to improved public health, reduced disease burdens, and reduced healthcare costs. For example, the World Health Organization’s immunization campaign from 1967 to 1977 eradicated smallpox.

Another immunization strategy is herd immunity, in which immunization of a high percentage (a herd) of a population provides protection to unvaccinated persons. This type of community immunity tries to break the chain of infection by having large sections of a population immune. It slows infectious disease transmission and can even stop outbreaks. The decreased prevalence of diseases like polio and measles exemplify herd immunity.

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