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By the end of this section, you will be able to:
  • Describe hypersensitivity
  • Define autoimmunity

A functioning immune system is essential for survival, but even the sophisticated cellular and molecular defenses of the mammalian immune response can be defeated by pathogens at virtually every step. In the competition between immune protection and pathogen evasion, pathogens have the advantage of more rapid evolution because of their shorter generation time, large population sizes and often higher mutation rates. Thus pathogens have evolved a diverse array of immune escape mechanisms. For instance, Streptococcus pneumoniae (the bacterium that causes pneumonia and meningitis) surrounds itself with a capsule that inhibits phagocytes from engulfing it and displaying antigens to the adaptive immune system. Staphylococcus aureus (the bacterium that can cause skin infections, abscesses, and meningitis) synthesizes a toxin called leukocidin that kills phagocytes after they engulf the bacterium. Other pathogens can also hinder the adaptive immune system. HIV infects T H cells using their CD4 surface molecules, gradually depleting the number of T H cells in the body ( [link] ); this inhibits the adaptive immune system’s capacity to generate sufficient responses to infection or tumors. As a result, HIV-infected individuals often suffer from infections that would not cause illness in people with healthy immune systems but which can cause devastating illness to immune-compromised individuals.

A colored scanning electron micrograph of a lymphocyte with cytoplasmic extensions, and many small spheres coming out of the lymphocyte and scattered around it.
HIV (green) is shown budding from a lymphocyte cell (red) in culture. (credit: modification of work by C. Goldsmith, CDC; scale-bar data from Matt Russell)

Inappropriate responses of immune cells and molecules themselves can also disrupt the proper functioning of the entire system, leading to host-cell damage that can become fatal.

Immunodeficiency

Immunodeficiency is a failure, insufficiency, or delay in the response of the immune system, which may be acquired or inherited. Immunodeficiency can allow pathogens or tumor cells to gain a foothold and replicate or proliferate to high enough levels so that the immune system becomes overwhelmed. Immunodeficiency can be acquired as a result of infection with certain pathogens that attack the cells of the immune system itself (such as HIV), chemical exposure (including certain medical treatments such as chemotherapy), malnutrition, or extreme stress. For instance, radiation exposure can destroy populations of lymphocytes and elevate an individual’s susceptibility to infections and cancer. Rarely, primary immunodeficiencies that are present from birth may also occur. For example, severe combined immunodeficiency disease (SCID) is a condition in which children are born without functioning B or T cells.

Hypersensitivities

A maladaptive immune response toward harmless foreign substances or self-antigens that occur after tissue sensitization is termed a hypersensitivity    . Types of hypersensitivities include immediate, delayed, and autoimmune. A large proportion of the human population is affected by one or more types of hypersensitivity.

Questions & Answers

What are eukaryotic cells?
Nwosueke Reply
where does the cell get energy for active transport processes?
A'Kaysion Reply
what is synapsis
Adepoju Reply
how many turns are required to make a molecule of sucrose in Calvin cycle
Amina Reply
why Calvin cycle occurs in stroma
Amina
why do humans enhale oxygen and exhale carbondioxide?
Maryam Reply
why do humans enhale oxygen and exhale carbondioxide? For the purpose of breaking down the food
dil
what is allele
uzoka Reply
process of protein synthesis
SANTOSH Reply
what is cell
Zulf Reply
what is cytoplasm
uzoka Reply
cytoplasm is fluid of cell.
Deepak
how many major types of Cloning
Saeed Reply
two
amir
two
Zulf
comparative anatomy of gymnosperms?
Meenakshi Reply
anatomy of gymnosperms
Meenakshi
how genes are regulated
Ainjue Reply
what is storage of glycogen
Student Reply
glycogen is a protein content
Najeem
how many times breathing a day normally does a person have
Vernalyn Reply
100
Aadil
on average 18000 times a day when resting.
gagan
the -10 and -35 regions of prokaryotic promoters are called consensus sequences because
Michelle Reply

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Source:  OpenStax, Concepts of biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11487/1.9
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