Question

Q1. We are hosts not only to pathogenic microorganisms, but also to a large number of different commensals, for example various gut bacteria. How can the immune system distinguish between commensal and pathogenic microorganisms?

Q2. An experiment studied the role of the complement system for clearance of bacteria from the blood of mice. Two types of mice were used in the study: C1q-knockout mice and wild type mice. The mice were infected with either antibody-coated or un-coated bacteria (in a fully factorial design, i.e, both mouse genotypes were exposed to both types of bacteria), and the clearance rate was measured during two hours after bacterial administration. Explain the protecting mechanisms (not only those involving complement) at work in mice receiving different treatments.

Q3.  A family with two kids, aged 6 and 8, decided it would be nice to have a pet and bought a cat. The kids liked the cat, but after three weeks both kids presented with sneezing and swollen, itchy eyes, suggesting they had developed allergy to the cat. Why did it take several weeks until the symptoms appeared?

Q4. Humans have 6 MHC II loci and 3 MHC I loci. Most individuals are heterozygous at all or at least most of these loci. What is the advantage of having many different MHC alleles? Could there also some drawback of having many MHC alleles?

Q5.

You currently have a mild cold, caused by infection by the coronavirus HCoV-OC43, which is quite closely related to the coronavirus currently causing a pandemic in humans, SARS-CoV-2. How do you think the immune response to HCoV-OC43 might affect your susceptibility to infection by the new corona virus, SARS-CoV-2 (where “susceptibility” means both the probability of contracting an infection with SARS-CoV-2, and the severity of the disease symptoms if you actually get infected)? Discuss which aspects of the response induced by HCoV-OC43 that might

• reduce,
• increase, or
• have no effect

on your susceptibility to SARS-CoV-2.

Q6.  Different strains of mice differ in susceptibility to different kinds of pathogens. For example, C57BL/6 mice are relatively resistant to infection by Plasmodium (an intracellular parasite causing malaria), but relatively susceptible to helminth infection. In contrast, BALBc mice are relatively resistant to helminths but susceptible to Plasmodium. Suggest and describe an immunological mechanism that can explain these differences.

Q7. One important component of the defence against viral infection is cytotoxic T cells, which depends on antigen presentation by MHC I. However, some viruses have evolved strategies to interfere with antigen presentation by down regulating MHC I expression. How can cells infected by such viruses be detected and killed by the immune system?

Answer 1

1- Gastrointestinal tract provide greatest surface of body in contact with external environment. The most pathogens enters the body through mucosal surface of intestine. The intestinal immune system protects the sterile core of organisms present there, against invasion and systemic dissemination of pathogens and also limits the level of penetration of commensal micro organisms.

The intestine is continuously in contact with commensal bacteria of different species. These bacteria confer health benefits to host by participating in dietary digestion, gut immunity devoloping, and prevention of colonisation of pathogens.

For the maintainability of integrity and normal functioning of intestine , a delicate equilibrium must be reached between bacteria flora and intestinal immune system.