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By the end of this section, you will be able to:
  • Describe physical and chemical immune barriers
  • Explain immediate and induced innate immune responses
  • Discuss natural killer cells
  • Describe major histocompatibility class I molecules
  • Summarize how the proteins in a complement system function to destroy extracellular pathogens

The immune system comprises both innate and adaptive immune responses. Innate immunity occurs naturally because of genetic factors or physiology; it is not induced by infection or vaccination but works to reduce the workload for the adaptive immune response. Both the innate and adaptive levels of the immune response involve secreted proteins, receptor-mediated signaling, and intricate cell-to-cell communication. The innate immune system developed early in animal evolution, roughly a billion years ago, as an essential response to infection. Innate immunity has a limited number of specific targets: any pathogenic threat triggers a consistent sequence of events that can identify the type of pathogen and either clear the infection independently or mobilize a highly specialized adaptive immune response. For example, tears and mucus secretions contain microbicidal factors.

Physical and chemical barriers

Before any immune factors are triggered, the skin functions as a continuous, impassable barrier to potentially infectious pathogens. Pathogens are killed or inactivated on the skin by desiccation (drying out) and by the skin’s acidity. In addition, beneficial microorganisms that coexist on the skin compete with invading pathogens, preventing infection. Regions of the body that are not protected by skin (such as the eyes and mucus membranes) have alternative methods of defense, such as tears and mucus secretions that trap and rinse away pathogens, and cilia in the nasal passages and respiratory tract that push the mucus with the pathogens out of the body. Throughout the body are other defenses, such as the low pH of the stomach (which inhibits the growth of pathogens), blood proteins that bind and disrupt bacterial cell membranes, and the process of urination (which flushes pathogens from the urinary tract).

Despite these barriers, pathogens may enter the body through skin abrasions or punctures, or by collecting on mucosal surfaces in large numbers that overcome the mucus or cilia. Some pathogens have evolved specific mechanisms that allow them to overcome physical and chemical barriers. When pathogens do enter the body, the innate immune system responds with inflammation, pathogen engulfment, and secretion of immune factors and proteins.

Pathogen recognition

An infection may be intracellular or extracellular, depending on the pathogen. All viruses infect cells and replicate within those cells (intracellularly), whereas bacteria and other parasites may replicate intracellularly or extracellularly, depending on the species. The innate immune system must respond accordingly: by identifying the extracellular pathogen and/or by identifying host cells that have already been infected. When a pathogen enters the body, cells in the blood and lymph detect the specific pathogen-associated molecular patterns (PAMPs) on the pathogen’s surface. PAMPs are carbohydrate, polypeptide, and nucleic acid “signatures” that are expressed by viruses, bacteria, and parasites but which differ from molecules on host cells. The immune system has specific cells, described in [link] and shown in [link] , with receptors that recognize these PAMPs. A macrophage    is a large phagocytic cell that engulfs foreign particles and pathogens. Macrophages recognize PAMPs via complementary pattern recognition receptors (PRRs) . PRRs are molecules on macrophages and dendritic cells which are in contact with the external environment. A monocyte    is a type of white blood cell that circulates in the blood and lymph and differentiates into macrophages after it moves into infected tissue. Dendritic cells bind molecular signatures of pathogens and promote pathogen engulfment and destruction. Toll-like receptors (TLRs) are a type of PRR that recognizes molecules that are shared by pathogens but distinguishable from host molecules). TLRs are present in invertebrates as well as vertebrates, and appear to be one of the most ancient components of the immune system. TLRs have also been identified in the mammalian nervous system.

Questions & Answers

Is the "growth and maintenance phase" in a cell's life cycle when cell division is about to occur
Somto Reply
what is the common name of Basidiomycetes
Ogechukwu Reply
الاجزاء النباتية لابد من تعقيمها قبل زراعتها في القوارير
yes
tariq
whats this?
tariq
do you speak arabic?!
what are bio elements
Shahzad Reply
which are present In Body And such elements Have Great role in our Body there are 16 bio elements that maintains human Body but on The basis of amount There are 6 bio elements present in Concen. of 99% and More Valuable And Highly Concen. element is Oxygent with 65 %
Haider
how je pollution brought about
Lamina Reply
how je pollution brouhgt about
Lamina
non is pollution brouhgt about
Lamina
describe the anatomy of cell division
Ivanovic Reply
Complex traits such as height result from 
Ruben Reply
what is the difference between chloroplasts and mitochondria
Nkalubo Reply
chloroplast in plants and bacterial cell ; mitochondria in animal cells
aung
Diagram of a living cell
Eliza Reply
what is cell
Sule
A cell is the smallest basic unit of life.
John
what's biology
Ogochukwu Reply
this is da study of living and non-living thing in an eco-system
Nutty
it is the study of living and non living organism in the ecology
Akufia
I agree with you dat biology is d study of living nd nonliving features
Winner
why do plants store carbohydrates in form of starch and not glucose?
Nutty Reply
Describe the structure of starch?
Nutty
wat is diffusion
Winner
water is life!.. Discuss?
Nutty Reply
why do plants store carbohydrates in form if starch not glucose!
Nutty
study of living thing
Dennis Reply
what is beyond a liveing cell
Raymond
what is biology
Gabriel Reply
d study of living nd non living thing
Winner
what is vasectomy
Evelyn Reply
The surgical removal of d spermduct
Eniola

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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