Question

• 1) How does the microbiome act as a physical and chemical barrier of the innate immune system?
• 2) How do NK cells recognize cells for destruction?
• 4) What is the difference between an antigen and a PAMP?
• 5) What are the similarities and differences between T-cell receptors (TCRs) and antibodies?
• 6) Early research suggests that the COVID-19 virus causes a cytokine storm in more severe cases. In general terms, how is a cytokine storm triggered?

Answer 1

1) The physical and chemical barriers form the first line of defence on invasion by pathogens. A diverse microbial flora is associated with the skin and mucous membranes of every human being. The normal microbial flora provide variety of functions such as digestion of substrates, vitamin production, stimulation of cell maturation, stimulation of the immune system, aid in intestinal transit and colonization resistance. The normal flora prevent colonization by pathogens by competing for attachment sites or for essential nutrients in the oral cavity, the intestine, the skin and the vaginal epithelium. They also antagonize other bacteria by producing variety of substances (bacteriocin, peroxides etc) which inhibit or kill the other bacteria. They also stimulate the production of antibodies and induce antibody-mediated immune response. low levels of these antibodies produced against components of normal flora may cross react with certain related pathogens thus preventing infection or invasion.

2) Natural killer (NK) cells are a type of lymphocyte and a component of innate immune system. When cancer cells or infected cells lose their MHC I molecule, they become vulnerable to N K cell killing. Some infected cells are routinely opsonized with antbodies which are recognized by FcyRIII receptors on N K cells activating them. On activation N K cells release cytotoxic granules containing perforin and granzymes leading to lysis of the target cell.

4) Antigens are any molecules that stimulates immune response. Most antigens are proteins or polysaccharides, though small molecules coupled to carrier proteins (haptens) may also be antigenic. Pathogen-associated molecular patterns (PAMPs) are small molecular sequences found on pathogens that are recognised by Toll-like receptors (TLRs) and other pattern-recognition receptors (PRRs).

5) The T cell receptor (TCR) is a protein complex found on the surface of T cells or T lymphocytes which are responsible for recognizing fragments of antigens as peptides bound to major histocompatibility complex (MHC). An antibody or immunoglobulin is a large Y shaped protein produced mainly by plasma cells in response to pathogen and used by immune system to neutralize them. The difference between TCR and antibody is that antibody recognise antigen via the fragment antigen-binding variable region whereas TCR recognise antigen when presented in bound form with MHC molecules. The similarity between TCR and antibody is that both are highly specific in their action.

6) Cytokine storm is a severe immune reaction in which large number of white blood cells are activated which release too many cytokines into the blood too quickly. Infections caused by viruses, bacteria and other agents trigger the cytokine storm in some people. Cytokines are diverse group of small proteins which are secreted by cells for intracellular signalling and communication. Cytokines perform variety of functions in body such as recruiting immune cells to the site of injury, help in proucing antibody and in pain signaling and also help in produing inflammation. Some cytokines (anti-inflammatory cytokines ) controls the body's inflammatory response. Under normal condition both types of cytokines coordinate the response of immune system to take care of infectious agents suh as viruses and bacteria. When too much inflammatory response is generated, the balance is lost and inflammatory cytokines start stroming out of control without enough feedback from the anti-inflammatory cytokines.