IMMUNE SYSTEM AND IMMUNOSUPPRESSANTS.
The immune system is a defense system of the body comprising many
biological structures and function within an organism that protects against disease.
To function normally, an immune system must detect a wide
variety of organisms, known as pathogens, from viruses to
parasitic worms, and distinguish it from the healthy
tissues of an organism.
In many species, there are two major systems
of the immune system: the innate immune system
and the adaptive immune system.
Both subsystems use humoral immunity and cell-mediated immunity
to perform their functions. In humans, the blood–brain barrier(BBB),
blood–cerebro-spinal fluid barrier (BCSFB), and similar
fluid–brain barriers(FBB), separate the peripheral immune system
from the neuroimmune system, which protects the brain.
Pathogens can rapidly evolve and adapt, and thereby avoid detection
and neutralization by the immune system; however, multiple defense
mechanisms have also evolved to recognize and neutralize pathogens.
Even simple unicellular organisms such as bacteria possess a
rudimentary immune system in the form of enzymes that protect against
bacteriophage infections.
Other basic immune mechanisms evolved in ancient eukaryotes and
remain in their modern descendants, such as plants and invertebrates
These mechanisms include phagocytosis, antimicrobial peptides
called defensins, and the complement system. Jawed vertebrates,
including humans, have even more sophisticated defense mechanisms,
including the ability to adapt over time to recognize specific
pathogens more efficiently. Adaptive (or acquired) immunity
creates immunological memory after an initial response to a
specific pathogen, leading to an enhanced response to subsequent
encounters with that same pathogen. This process of acquired
immunity is the basis of vaccination.
Disorders of the immune system can result in autoimmune diseases,
inflammatory diseases and cancer. Immunodeficiency occurs
when the immune system is less active than normal, resulting
in recurring and life-threatening infections. In humans,
immunodeficiency can either be the result of a genetic
disease such as severe combined immunodeficiency,
acquired conditions such as HIV/AIDS, or the use of
immunosuppressive medication. In contrast, autoimmunity
results from a hyperactive immune system attacking normal
tissues as if they were foreign organisms. Common
autoimmune diseases include Hashimoto's thyroiditis,
rheumatoid arthritis, diabetes mellitus type 1, and
systemic lupus erythematosus. Immunology covers the
study of all aspects of the immune system.
Immunosuppressant drugs are used to treat autoimmune
diseases. With an autoimmune disease, the immune system
attacks the body's own tissue. Because immunosuppressant
drugs weaken the immune system, they suppress this reaction.
This helps reduce the impact of the autoimmune disease on the body.
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