Cell Recognition And The Immune System

HOW LYMPHOCYTES RECOGNISE CELLS BELONGING TO THE BODY:

• There are probably around 10 million different lymphocytes present at any time, each capable of recognising a different chemical shape.
• In the fetus, these lymphocytes are constantly colliding with other cells.
• Infection in the fetus is rare because it is protected from the outside world by the mother and, in particular, the placenta.
• Lymphocytes will therefore collide almost exclusively with the body's own material (self).
• Some of the lymphocytes will have receptors that exactly fit those of the body's own cells.
• These lymphocytes either die or are suppressed.
• The only remaining lymphocytes are those that might fit foreign material (non-self), and therefore only respond to foreign material.
• In adults, lymphocytes produced in the bone marrow initially only encounter self-antigens.
• Any lymphocytes that show an immune response to these self-antigens undergo programmed cell death (apoptosis) before they can differentiate into mature lymphocytes.
• No clones of these anti-self lymphocytes will appear in the blood, leaving only those that might respond to non-self antigens.

PHAGOCYTOSIS:

• Chemical products of pathogens or dead, damaged and abnormal cells act as attractants, causing phagocytes to move towards the pathogen (e.g. a bacterium).
• Phagocytes have several receptors on their cell-surface membrane that recognise, and attach to, chemicals on the surface of the pathogen.
• They engulf the pathogen to form a vesicle, known as a phagosome.
• Lysosomes move towards the vesicle and fuse with it.
• Enzymes called lysozymes are present within the lysosome. These lysozymes destroy ingested bacteria by hydrolysis of their cell walls.
• The soluble products from the breakdown of the pathogen are absorbed into the cytoplasm of the phagocyte.

CELL-MEDIATED…