5.4

//**A diagram of the skin is not required**// The skin and mucous membranes form a barrier that prevents most pathogens from entering the body. The outer layers of the skin are tough and form a physical barrier. Sebaceous glands in the skin secrete lactic acid and fatty acids, which make the surface of the skin acidic. This prevents the growth of most pathogenic bacteria. Mucous membranes are soft areas of skin that are kept moist with mucus. Mucous membranes are found in the nose, trachea, vagina and urethra. Although they do not form a strong physical barrier, many bacteria are killed by lysozyme, an enzyme in the mucus. In the trachea pathogens tend to get caught in the sticky mucus and cilia then push the mucus and bacteria up and out of the trachea.
 * 5.4.1 Explain how skin and mucous membranes act as barriers agains pathogens.**

//Details of the sub-divisions and classifications of phagocytes are not required.//** Some of the leucocytes in blood are phagocytes. 1. These cells identify pathogens. 2. Then ingest them by [|Endocytosis] 3. The pathogens are then killed and digested inside the cell by enzymes from lysosomes. Phagocytes can ingest pathogens in the blood. They can also squeeze out through the walls of blood capillaries and move through tissues to sited of infection. They then ingest the pathogens causing the infection. Large numbers of phagocytes at a site of infection form pus.
 * 5.4.2.Outline how phagocytic leucocytes ingest pathogens in the blood and in body tissues


 * 5.4.3 State the difference between antigens and antibodies.**

- A large molelcule (protein, glycoprotein, lipoprotein or polysaccharide) on the outer surface of a cell - All living cells have these antigens as part of their cell membrane or cell wall - Their purpose is for cell communication and cells from different individuals have different antigens, while all the cells of the same individual have the same antigens
 * Antigen:** Usually defined as a molecule recognised as foreign by the immune system that has infected the body, and antigen is actually:

- B-cells make them - Each antibody binds to a specific antigen - Comprised of a constant region and variable regions (where the antigens bind)
 * Antibody:** (also known as immunoglobulin) A globular protein secreted from lymphocytes that destroy pathodgen and antigen infections




 * 5.4.4 Explain antibody production.**

Antibodies are produced by B-lymphocytes (B-cells), which are one kind of leucocyte (white blood cell). All blood cells are produced in the bone marrow. B-cells also differentiate in the bone marrow (B for bone marrow) before moving to to the lymph nodes.

Different Types of lymphocytes exist and each type recognizes one specific antigen. After recognition of the specific antigens, the lymphocytes begin to divide and form clones, which then secrete specific antibodies.

Steps for Antibody production:

1) Antigen enters the body and is engulfed by macrophages 2) The macrophage displays the antigen on its surface and then travels to the lymph nodes 3) Here the macrophage will present the antigen to a specific helper T-cell 4) Helper T-cells can only recognise the antigen if it is also presented with MHC (major histocompatibility complex - distinguishes self from non-self cells) 5) Helper T-cell is activated - it grows and divides producing both memory cells and additional active helper T-cells 6) Active helper T-cells trigger B-cells to become active - see point on clonal selection 7) Activated B-cells divide to form a clone of plasma cells and a clone of memory cells 8) Plasma cells secrete specific antibodies while memory cells remain in the lymph nodes for defense against future infection.

Clonal Selection: A B-cell is selected by binding to its specific antigen and this then results in it forming a clone. Each B-cell only has one specific type of antigenic receptor on its surface whose shape is identical to the antibodies that the cell can make. The receptor will react only to a single antigen. The term clonal selection is used because the antigen selects the B-cells that will proliferate.


 * 5.4.5 Outline the effects of HIV on the immune system**

When the HIV virus infects a person, the virus will specifically infect and destroy helper T cells, also known as CD4+ T cells. The HIV virus also infects other cells that express CD4+ like macrophages and dendritic cells, but not to the same extent as T helper cells.

T helper cells activate cytotoxic T cells and other helper T cells. They are also necessary for B cell activation. Therefore the destruction of helper T cells impairs the immune system as the number of active lymphocytes has been reduced and the person’s ability to produce antibodies is impaired.

The infected person is then much more susceptible to most diseases.This often leads to a number of ‘opportunistic diseases’ like rare forms of pneumonia and skin cancer.