July 31, 2012

Q&A: Concerning Physiological Defence Mechanisms

Humans live in an environment teeming with micro-organisms and could not exist as a species without highly effective mechanisms of host defense. The innate immune system constitutes the first-line barrier, the rapid-response mechanism, to prevent microbial invasion. Its components are inherited from parent to child and directed against molecules expressed only by micro-organisms. These host defense components are evolutionarily ancient, found in all multicellular organisms, and expressed in humans as conserved elements (homologs) shared with other vertebrates and, in some form, with insects and plants.

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This is a Question & Answer revision article designed for medical students and professionals preparing for the PLAB, MRCP or USMLE examinations. They are based on actual questions from these examinations. You may find the Allergies, Blood & Immune System articles more useful, or one of our many articles on Diseases & Conditions, Medical Syndromes, Health & Wellness or Home Remedies.
In this article:
Innate Vs adaptive immunity
MCQ: clinical scenario
MCQ: answer
MCQ: explanation

Innate Vs adaptive immunity

"Innate" immunity refers to immune responses that are present from birth and not learned, adapted, or permanently heightened as a result of exposure to micro-organisms, in contrast to the responses of T and B lymphocytes in the adaptive immune system [6]. The importance of innate immunity can be appreciated by considering that the generation time of most bacteria is 20 to 30 minutes, whereas the development of a specific adaptive immune response with antibody and T cells takes days to weeks. The innate immune system protects the host during the time between microbe exposure and initial adaptive responses.

MCQ: clinical scenario

An enzyme found in tears and saliva is known to damage bacterial cell walls. It does this by:

a) Dissolving the cytoplasmic membrane
b) Breaking the covalent links between teichoic acid and muramic acid
c) Hydrolyzing the link between N-acetyl glucosamine and N-acetyl muramic acid
d) Inhibiting cell wall protein synthesis
e) Converting D-alanine to L-alanine

MCQ questions & answers on medicalnotes.info

MCQ: answer

The correct answer is C.
It does this by hydrolyzing the link between N-acetyl glucosamine and N-acetyl muramic acid.

MCQ: explanation

Lysozyme is a small 14 kDa protein found in tears. It contains 4 disulfide bonds and its secondary structure is both helical and sheet. It hydrolyzes the bond between NAG subunits in bacterial cell walls and therefore serves as a defence mechanism.

1). UpToDate: An overview of the innate immune system. Available online: https://www.uptodate.com/contents/an-overview-of-the-innate-immune-system

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