Immunity to Microorganisms

Question 1

Take home points for immunity to microorganisms

Answer 1

• only those organisms that can successfully evade the immune response are successful pathogens
• the primary adaptive immune response to bacteria is an antibody response. The primary local immune response is an IgA response. The serum antibody response is primarily IgG
• in most cases the microorganisms is actually destroyed in a phagocytic cell. Only infected host cells are killed by cytotoxic T cells
• specific immunity acts to enhance the uptake of microorganisms by phagocytic cells or to enhance the activity of phagocytic cells

Question 2

Innate immunity

Answer 2

-numerous non-specific barriers to infection exist that limit the entry of, or aid in the rapid clearance of microorganisms

• skin: barrier to infection and produces fatty acids toxic to some bacteria
• pH barriers: many organs (stomach, vagina) are acidic leading to the destruction of bacteria
• flushing: many epithelial surfaces are cleaned by ciliary action or flushing such as the urinary tract
• lysozyme: present in tears , nasal secretions and saliva, it destroys cell walls
• phagocytes: when bacteria penetrate the skin or mucus membranes macrophages and neutrophils may phagocytize and kill the microorganisms. They have receptors for many bacterial products that allow them to respond to these bacterial without specific immune recognition
• complement- can be fixed on the surface of many bacteria and yeast due to alternative pathway activation. This may lead to opsonization of direct lysis

Question 3

Rules of microorgansims

Answer 3

1) Most bacteria die inside a phagocyte
2) The primary adaptive immune response to bacteria is antibody
3) If the macrophage does not kill a bacteria at first, make it better

Question 4

Virulence Factors

Answer 4

• ability to survive the acid environment of the stomach as demonstrated by Salmonella typhose
• production of spreading factors
• production of toxins that inhibit or kill immune cells
• the presence of an antiphagocytic capsule such as S. pneumonia
• proteins like Staph aureus protein A which binds to and blocks the opsonizing action of IgG
• other antiphagocytic factors such as M protein
• some bacteria are not flushed from the respiratory tract or the digestive tract due to their ability to attach to the endothelium
• ability to survive inside the phagosome

Question 5

Antibody overview

Answer 5

• prevent attachment to epithelium. This tends to be mediated by secreted IgA
• trigger complement leading to increased opsonization or lysis
• bind to antiphagocytic M proteins or capsules, preventing the antiphagocytic activity and acting as an opsonin
• opsonized bacteria are not only taken up better but are also killed faster than non-opsonized bacteria
• neutralized toxins
• neutralize spreading factors such as tissue damaging enzymes

Question 6

Facultative intracellular parasites

Answer 6

• ability to survive within the phagosome. This may be accomplished by surviving the digestive enzymes of the phagolysosome, or by preventing the fusion of the lysosomes with the phagosomes
• this is important for Listeria monocytogenes, mycobacterium tuberculosis and leprae

Question 7

Immunity to Listeria

Answer 7

• developed a strain of Listeria that will kill mice when a high dose of bacteria is injected
• low inoculation will induce short term growth but will induce immunity
• these mice will be immune to the high dose of Listeria
• phagocytosed by macrophages but not killed
• antibody increases the rate of phagocytosis but not the kiling
• macrophages from the site of infection are able to kill Listeria: ACTIVATED MACROPHAGES
• long term immunity is specific
• immunity is cell mediated
• TH1 response

Question 8

Cell-Mediated Immunity

Answer 8

• specific immunity is a function of T cells
• non- specific killing is a function of activated macrophages
• it was noted that only live listeria induced CMI
• both live or dead bacteria produced antibody

Question 9

Two types of helper T cells

Answer 9

• helper cells are CD4+
• some stimulate Cell Mediated Immunity (TH1): IL2, IFN-gamma, macrophage activation, B-cell activation and production of opsonizing antibodies such as IgG1
• some stimulate Antibody production (TH2): IL-4, IL-5, general activation of B cells to make antibodies

Question 10

Immunity to Parasites

Answer 10

• The T cell response is the most important in the clearance (or control) of parasites
• cytotoxic T cells are only involved in a limited manner- may kill the cells infected with parasites
• T cells activate macrophages that kill many different parasites
• T cell dependent granulomata formation
• T cells produce factors that cause the infiltration and perhaps the increased production of eosinophils
• T cells and B cells are both involved in the production of specific antibodies to parasite antigens
• IgG and IgM blood borne parasites (Plasmodium)
• IgE- immune response to helminths)

Question 11

Mechanisms by which parasites escape the immune response

Answer 11

• inaccessibility (hide in host cells)
• avoidance of recognition- vary cell surface expression, pick up and display host antigens
• immunosupression
• survive inside of macrophages

Question 12

Immunity to viruses

Answer 12

• T cell mediated immunity important
• antibody prevents infection
• CTLs kill infected targets
• NK cells stimulated by T cells
• activated macrophages can kill infected cells