0909 - Immune Response to encapsulated Organisms Flashcards
Explain how encapsulated microorganisms evade phagocytosis
Prevent encounters with phagocytes
Avoid recognition and attachment
Survive within phagocyte
(each dealt with on separate card)
How do encapsulated microorganisms prevent encounters with phagocytes?
C5a Peptidase. Destroys C5a, which attracts phagocytes.
Membrane-damaging toxins - destroy phagocytes and other cells.
How do encapsulated microorganisms avoid recognition and attachment by phagocytes?
Capsules can bind proteins to mimic a host cell, inactivate C3b and prevent the complement cascade or opsonisation. M protein can do the same thing for pyogenes.
Fc receptors bind to the Fc (tail) region of antibodies, reversing their orientation.
How do encapsulated microorganisms survive within phagocytes?
Escape from the phagosome before it fuses with the lysosome.
Prevent phagosome-lysosome fusion
Survive within phagolysosome
Describe host immune responses to encapsulated microorganisms
As TCRs recognise peptides, not polysaccharides, body needs a T-independent response. T-independent antigens can activate B-cells without needing T-cells. 2 types
TI-1 Antigens - lipopolysaccharide (Gram negative) and bacterial DNA. These antigens directly induce B-cell division with, but without affinity maturation or memory B-cells being produced. The concentration of TI-1 antigen is inversely proportional to the specificity of the antibody response.
TI-2 Antigens - highly repetitive structures (LPS, viral envelopes) can induce an immune response in particular types of B-cells. These are spleen B-1 cells and Marginal Zone B cells, non-circulating B-cells in the spleen, rare at birth but increasing with age. When IgG and IgM from MZ B-cells bind to the bacterium, they can induce antibody-enhanced phagocytosis. Still no memory or somatic hypermutation.
Discuss how vaccines targeted against encapsulated bacteria work
You use a conjugate vaccine, which links the peptide with the polysaccharide. The peptide allows you to have the benefit of a T-cell response, which means you get memory B-cells and high-affinity antibody. However, the vaccine is only effective against one capsule - a different capsule type will not get the benefits.
