5.41 - Explain how skin and mucous membranes act as barriers against pathogens.
Skin - the outer epithelial layer of skin, the epidermis, provides a tough physical barrier against microbes. The shedding of the epidermal cells help remove microbes on the surface.
Mucous Membranes - the respiratory tract is protected by sticky mucus secreted by glands in the bronchi and bronchioles. Because mucus is slightly viscous, it traps many microbes and other foreign substances. Cilia constantly sweet the mucus upwards to the throat where it is swallowed so that the microbes are later killed by the stomach acid.

5.4.2 Outline how phagocytic leucocytes ingest pathogens in the blood and in body tissues.
Details of the sub-divisions and calssifications of phagocytes are not required.
Infection- when a pathogen successfully invades the body (does not necessarily lead to disease Leucocytes (white blood cells) defend the body agains pathogens after they have entered the body. They are found in blood and body tissues (like lungs).
There are many types of leucocytes - some are phagocytic.
These phagocytize (or eat) any cell which is not recognized as the body's own Phagocytes flow over microbes and surround them to form a phagosome
Phagosome fuses with a lysosome. The lysosome contains lysozymes (to break down the microbial cell wall) and digestive enzymes (to break down organic molecules).
The lysosome kills and digests microbes.
All products of digestion are expelled in exocytosis.

5.43 - State the difference between antigens and antibodies
Antigens are large molecules on the outer surface of a cell that infects the body whereas antibodies are proteins secreted from lymphocytes that destroy pathogen and antigen infections.

5.4.4 Explain antibody production
Many different types of lymphocyte exist. Each type recognizes ones specific antigen and responds by dividing to form a clone. This clone then secretes a specific antibody against the antigen. No other details are required
1. Antigen enters body and they are engulfed by macrophages.
2. The macrophage displays the antigen on its surface and then travels to lymph nodes.
3. There the macrophage will present the antigen to specific helper T cells.
4. Helper T cells can only recognize antigen if the antigen is also presented with Major Histocompatibility Complex (MHC)
5. Helper T cell then grows and divides producing both memory cells and additional active helper T cells.
6. Active helper T cells trigger B cells to become active.
7. Activated B cells divide to form a clone of plasma cells and a clone of memory cells.
8. Plasma cells secrete specific antibodies.
9. Memory cells remain in lymph node.
Summary: Pathogen invades body; B cells bind with antigen presented by Macrophage; B cell becomes activated; Clone of B cells produced; Plasma cells; Antibodies.

5.4.5 Outline the effects of HIV on the immune system
The effects of HIV should be limited to a reduction in the number of active lymphocytes and a loss of the ability to produce antibodies.
1. HIV viral RNA codes for the glycoprotein gp120 which recognizes a protein on helper T-cells named CD4, and physically associates with it
2. CD4 is a specific receptor for HIV
3. As a consequence of interaction with CD4 on helper T cells, the body cannot make antibodies properly, nor can infected cells containing HIV be properly eliminated
4. As such, the virus can
-multiply
-kill helper T cell in which is lives
-infect adjacent helper T cells
-repeat cycle until there is eventually substantial loss of helper T cells
5. In addition, the virus can also infect macrophages
6. As helper T cells and macrophages are at the heart of Cell Mediated Immunity (CMI), HIV/AIDS patients become immune deficient and susceptible to infections