The body has two types of defences to protect itself from pathogens.
- General and immediate barriers such as skin, to stop entry of pathogens.
- More specific, less rapid and longer-lasting.
Specific defences involve a type of white blood cell; lymphocytes.
- TH cells - Cell-mediated response
- TC cells- Cell- mediated response
- B cells- Humoral response.
How lymphocytes recognise self
- In foetus, lymphocytes are constantly colliding with each other.
- Infection in the foetus is rare- due to placenta.
- Threfore lymphocytes almost exclusively collide with self material.
- Some lymphocytes have receptors that fit those of the body's cells.
- Those lymphocytes either die or are suppressed.
- The lymphocytes that are left must fit non-self material. Therefore only respond to non-self.
- 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 apoptosis (programmed cell death), before they can mature lymphocytes.
- No clones of these anti-self lymphocytes will appear in the blood, leaving only those that might respons to non-self antigens.
- The phagocyte is attracted to the pathogen as it releases chemical products.Therefore the phagocyte then moves towards to the pathogen.
- Phagocytes have serveral receptors on their cell-surface membrane that recognise, and attach to, chemicals on the surface of the pathogen.
- They engluf the pathogen to form a phagosome.
- Lysosomes move towards the phagosome and fuse with it.
- Lysozymes destroy the 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.
B- lymphocytes- They mature in the bone marrow. Involed in humoral immunity- which is immunity involving antibodies that are present in fluids.
T-lymphocytes- They mature in the thymus gland. Involved in Cell-mediated immunity- which is immunity involving body cells.