Antibacterial responses

Question 1

What are the features of a bacterial infection

Answer 1

• In general, bacterial pathogens live and replicate in extracellular spaces with exceptions
• Several of the most acute and dangerous bacterial
diseases are caused not by the bacteria themselves but
by the toxins they produce
• Infection is an interaction between the pathogen and the host
• Key steps in infection: entry, invasion and colonisation of host tissue, evasion of immunity, tissue damage

Question 2

Do all bacteria cause of disease?

Answer 2

• Bacteria do not always cause disease
• The intestine in a healthy adult contains about 1014
essential bacteria
• With about another 1012 on the skin
• Microbiota is a mechanism of protection to infection
both ecological and immunological

Question 3

General features of immunity to bacteria

Answer 3

• Defense mechanisms against microbes comprise both the innate and the adaptive immune system
• The immune system responds in specialised and different ways to different types of bacteria
• The pathogenicity and survival of the bacteria is critically influenced by the ability to evade the effector mechanism of immunity
• Some bacteria establish latent or persistent infection and the immune system does not clear the microbe
• In many cases tissue damage is associated to immunity not to infection
• Defects in the immune system associate to succeptibility.

Question 4

First line of defence (innate)

Answer 4

mechanical, chemical and microbiological - in the skin, GI tract, Respiratory tract, Urogenital tract, Eyes

diagrams

Question 5

Anti-bacterial peptides: defensins

Answer 5

• Anti-microbial peptides capable of killing by penetrating microbial
membranes thus disrupting their integrity.

• They are active against bacteria, fungi and many enveloped and non-enveloped viruses.

• There are two types: ᵬ -defensins and β-defensins.
• ᵬ -defensins are secreted mainly by neutrophils and by Paneth cells

• β-defensins are secreted by a broad range of epithelial cells, in particular, those in the respiratory tract, the skin and the urogenital tract.

Question 6

If these initial barriers are crossed what happens?

Answer 6

Complement activation by microbial cell wall components
Complement being a PRR
= alternative pathway of complement

Mannose on the surface of bacteria
= Lectin pathway of complement

Question 7

The complement system

Answer 7

Key effector function of the humoral response.

Serum and cell surface proteins that interact with one another to generate products that eliminate extracellular bacteria

diagram

Question 8

The complement functions

Answer 8

diagrams

Question 9

The role of complement receptors in phagocytosis

Answer 9

diagrams

Question 10

Before and after complement mediated

lysis

Answer 10

before: alive
after: killed

Question 11

Pathogen Recognition Receptors

Answer 11

diagrams

Question 12

Toll-like receptors TLRs

Answer 12

diagrams and tables

Question 13

PRR and PAMPs interaction leads to

Answer 13

PHAGOCYTOSIS and CYTOKINEs

= inflammation (diagram)

Question 14

Neutrophils and bacteria infection

Answer 14

• Phagocytosis and 
degranulation of granules- 
intracellular killing of bacteria
• Phagocytosis and oxidative 
burst kills the bacteria
• Neutrophils can also kill 
bacteria phagocytosing 
bacteria - Neutrophils 
Extracellular Traps (NETS)

Question 15

Humoral response: the function of the

antibodies

Answer 15

• Neutralise bacterial toxins
• Trigger classical complement pathway by
binding of IgM to the bacterial cell surface
• Opsonisation; coating of bacteria with antibody thereby aiding phagocytosis

Question 16

Toxin neutralisation

Answer 16

• Many bacteria secrete protein toxins that cause disease by disrupting the normal function of host cells
• In order to do this a bacterial toxin must bind to a receptor
• Thus, antibodies that bind to that receptor block the function of the toxin

Question 17

Opsonisation

Answer 17

diagrams

Question 18

Triggering of the complement cascade

Answer 18

• IgM bound to the surface 
of a bacterium binds to a 
complement component 
that initiates the classical 
pathway of complement 
activation.
• As a consequence of this 
the bacterial cell surface 
is coated in C3b facilitating its phagocytosis.

Question 19

Anti-bacterial qualities of

immunoglobulins

Answer 19

Different classes of immunogliobulins have different anti-bacterial qualities - diagrams

Question 20

The adaptive immune response leads to

Answer 20

inflammation and bacteria eradication
-Antigen uptake and presentation by dendritic cells
-Dendritic cells present antigen to T cells in the
secondary lymphoid organs

Question 21

Some pathogens adapt to survive inside macrophages

Answer 21

Micobacterium tuberculosis
inside a macrophage

A time course of innate and adaptive immunity to intracellular bacteria

The MHC class II pathway presents bacterial antigens
derived from extracellular infections to helper CD4 T cells - Helper T cells enhance phagocytosis of extracellular pathogens by
activating macrophages

Cooperation of CD4+ and CD8+ T cells during intracellular infections

diagrams