Immunology 3: The Immune Response to Infection Flashcards Preview

• Pathogen-Associated Molecular Patterns (PAMPs)

• Innate defences include Natural Killer (NK) cells ( large, granular lymphocyte)
  • NK cells lyse virally infected cells
  • Recognizable histologically by abundant granules and cytoplasm (ready to react immediately)
  • Cytokines (messenger molecule-INF-A and INF-B) are produced by virally infected cells leading to the recruitment of NK cells and later, T cells if necessary

• Specific antibodies can “neutralize” viruses when they are outside the cell preventing their attachment (steric interference) and colonisation of cells. They can also interfere with viral process inside the cell
• Cytotoxic T cells kill virally infected cells- Resulting in cell death

• Are a subset of all T cells
• May lead to tissue injury
• Express CD8 on their surface
• Other T cells (HELPER T cells) express CD4 on their surface
• Express T cell receptors (TCR) on their surface
• T-cells (both CD8 positive and CD4 positive) have incredible diversity (and specificity) like B cells.
• Better killers than NK cells
• Develop memory just like B cells – subsequent exposure leads to a faster, stronger response

Cytotoxic T cells “see” antigen in association with MHC (major histocompatibility complex). Antibodies (like those on the surface of B cells) recognize epitopes on the surface of antigens. In contrast, the epitopes recognized by TCRs are often buried inside the protein structure. Hence, the antigen must be broken down into peptide fragments before binding to a MHC molecule of the self. The T cell receptor then binds to the complex of MHC molecule and epitope peptide.

Cytotoxic T cells (CD8 positive) “see” antigen in association with MHC class  Helper T cells (CD4 positive) “see” antigen in association with MHC class 2.

All cells express MHC class 1, allowing them to “call for help when virally infected.

Lymphocytes (including T and B cells) generally get activated in the lymph nodes. For this to happen, both antigen and lymphocytes must read the lymph node.
 

Lymphatic vessels:

• Drain fluid from tissues into circulation
• Transport macromolecules (including antigen) from tissue to lymph nodes
• Provide pathway for the migration of cells

Lymph nodes are found at regular intervals along the lymphatic vessels. They act as “sampling stations” (organized tissues that facilitate cells of the immune system and antigen).

• Lymphocytes leave the blood and enter lymph nodes via the High Endothelial Vessels (specialized blood vessels within the lymph node).
•  Lymphocytes and lymph return to blood via the thoracic duct

If immune cells encounter antigen in the lymph nodes, an immune response develops in the node.

Dendritic cells are “antigen presenting cells”. They allow the T cells to “see” the antigen in association with them and hence mount a response.

Innate defences are often triggered by Pathogen Associated Molecular Patterns (PAMPs). PAMPs are recognized by Pattern Recognition Receptors. Different PAMPs trigger different kinds of responses.

Bacterial cell wall constituents:

• Peptidoglycan
• Lipopolysaccharide (LPS) – found in gram negative bacteria
• Mannan (Carbohydrate found in the wall and capusle of bacteria)

Formylmethionine – the first amino acid added during prokaryotic protein synthesis. Not seen in eukaryotes.

Over twenty proteins, some of which are labelled C1-9, that work together to defend us from bacterial invasion. Cascade of interactions between the different proteins, three triggers: lectin pathway, alternate pathway, classical pathway.

3 outcomes:

            - Recruitment of inflammatory cells (concentration gradient)

            - Opsonization of pathogens- facilitate attachment

            - Perforation of pathogen cell membranes by the MAC

Both B cells and T cells are involved in the adaptive defence against bacteria. T cells ‘help’ B cells.

• Steric hindrance – antibody binds to an antigen and hinders it’s molecular function
• Neutralization of toxins and bacterial virulence factors (such as enzymes that act as ‘spreading factors’)
  • Toxins may cause disease and also major virulence factors
• Opsonisation – allows phagocytic cells to ‘see’ antigens 
• Activation of the complement cascade
  • MAC – lyses bacteria
  • Opsonisation
  • Recruits leukocytes