Immunity and cell recognition

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.

• 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.