2.2.2 Health and Disease Define the terms immune response, antigen and antibody; Describe the primary defences against pathogens and parasites (including skin and mucus membranes) and outline their importance. (No details of skin structure are required);

2.2.2 Health and Disease Immune response is any defense mechanism of the body that is initiated against a perceived foreign object. Some primary defences against invasion are: Skin – water proof and impervious to most types of bacterial attack. Oil on surface of the skin is toxic to bacteria. Mucous membranes: Trap dirt hair and bacterial particles. This is then ejected from the body helped by cilia which waft to aid movement. Tears, saliva and urine: Contain chemicals toxic to bacteria.

2.2.2 Health and Disease Antigen: Large, complex molecule such as a protein, long sugar molecule and glycoprotein which allows immune system to recognise pathogens. Antibody: Protein molecule secreted by B lymphocytes. Made of 4 polypeptides joined together.

2.2.2 Health and Disease Describe, with the aid of diagrams and photographs, the structure and mode of action of phagocytes; Describe the structure and mode of action of T lymphocytes and B lymphocytes including the significance of cell signalling and the role of memory cells

The immune response If our body’s barriers to infection are bypassed our immune system will step in and try to fight off the pathogen. There are 2 main types of cells in our immune system: Phagocytes and lymphocytes

Phagocytes There are two types of phagocytic white blood cells: Neutrophils and macrophages which engulf and digest foreign particles of almost any size or type

Macrophages Also leave the blood When IN the blood called “monocytes” When in liver (which they often are in large quantities) called “Kupffer cells” Long lived, surviving after taking in foreign particles Break foreign particles up into their component molecules then place some of these in their plasma membrane. Cells that behave in this way are called “antigen-presenting cells” (APCs) This helps other cells identify invaders

Neutrophils 60% of white blood cells Short lived Often destroyed after killing bacteria Constantly being remade in the bone marrow Often leave the blood to actively patrol parts of the body with invaders

Phagocytosis Pathogens are taken in by phagocytosis and fused with lysosomes that break the bacteria up and then removed from the cell.

Lymphocytes There are 2 types of lymphocytes: B Lymphocytes and T Lymphocytes Which look identical but differ in their functions, both are stimulated by ANTIGENS and produce ANTIBODIES (this action is SPECIFIC)

B Lymphocytes Develop in bone marrow Meets specific antigen in blood or on an APC Responds by dividing repeatedly to form a large number of clones (1) Some will differentiate into plasma cells which rapidly produce more antibody which is released by exocytosis Antibodies spread around the body and bind with bacterial antigen, resulting in their destruction (2) Other cloned cells don’t secrete antibodies but instead remain as MEMORY CELLS which persist and can respond quickly if the same pathogen invades again

T Lymphocytes Spend time developing in thymus gland during childhood (this gland disappears by teenage years) Only respond to their specific antigen if presented in the plasma membrane of another cell, then replicate to make clones There are different types of T lymphocytes; Memory cells T helper cells divide to replicate themselves then secretes CYTOKINES, chemicals that stimulate other cells to fight the invaders. Helper cells also stimulate killer cells T Killer cells actually destroy the cell they’re bound to by secreting chemicals such as hydrogen peroxide. These are our main defence.