Cell recognition and the immune system
Defence mechanisms
ù Non-specific
® Physical barriers e.g. skin
® Phagocytosis
ù Specific
® Cell-mediated response (T cells)
® Humoral response (B cells)
Phagocytosis
• Phagocyte is attracted to the pathogen by chemoattractants, and moves towards it
along a concentration gradient.
• Phagocyte binds to the pathogen.
• Phagocyte engulfs the pathogen by endocytosis, forming a phagosome.
• Lysosomes within the phagocyte migrate towards the phagosome and fuse with it.
• The lysosomes release their lysozymes into the phagosome, where they hydrolyse
the bacterium.
• The hydrolysis products of the bacterium are absorbed by the phagocyte and
displayed on its surface or released by exocytosis.
Cell mediated immunity
• Pathogen’s antigens are presented on the surface of infected cells.
• T cells are activated when their receptors bind with the presented antigens.
• Activated T cells divide rapidly (mitosis) to form a clone of identical T cells.
• T cells differentiate into:
Cytotoxic T cells
- Destroy pathogens and infected cells by enzyme action
- Secrete chemicals which attract and stimulate phagocytes
Helper T cells
- Stimulate the activity of the cytotoxic T cells and B cells
Suppressor T cells
- Switch off the T and B cell responses when infection clears
Memory T cells
- Some activated T cells remain in circulation and can respond quickly if the
same pathogen is recognised
• T cells kill infected cells by producing a protein that makes holes in the cell surface
membrane.
• Cells become freely permeable to everything and die.
Defence mechanisms
ù Non-specific
® Physical barriers e.g. skin
® Phagocytosis
ù Specific
® Cell-mediated response (T cells)
® Humoral response (B cells)
Phagocytosis
• Phagocyte is attracted to the pathogen by chemoattractants, and moves towards it
along a concentration gradient.
• Phagocyte binds to the pathogen.
• Phagocyte engulfs the pathogen by endocytosis, forming a phagosome.
• Lysosomes within the phagocyte migrate towards the phagosome and fuse with it.
• The lysosomes release their lysozymes into the phagosome, where they hydrolyse
the bacterium.
• The hydrolysis products of the bacterium are absorbed by the phagocyte and
displayed on its surface or released by exocytosis.
Cell mediated immunity
• Pathogen’s antigens are presented on the surface of infected cells.
• T cells are activated when their receptors bind with the presented antigens.
• Activated T cells divide rapidly (mitosis) to form a clone of identical T cells.
• T cells differentiate into:
Cytotoxic T cells
- Destroy pathogens and infected cells by enzyme action
- Secrete chemicals which attract and stimulate phagocytes
Helper T cells
- Stimulate the activity of the cytotoxic T cells and B cells
Suppressor T cells
- Switch off the T and B cell responses when infection clears
Memory T cells
- Some activated T cells remain in circulation and can respond quickly if the
same pathogen is recognised
• T cells kill infected cells by producing a protein that makes holes in the cell surface
membrane.
• Cells become freely permeable to everything and die.
