Chapter 38 The Human Defence System

1. Chapter 38 The Human Defence System

2. A pathogen is an organism that causes disease. • The general defence system: • non specific • acts against all pathogens • is not specific to any one pathogen • consists of skin, mucous membranes & their secretions • also consists of white blood cells that destroys pathogens that enter the body. • The general defence system acts as a barrier to all pathogens attempting to gain entry to the human body

3. First line of general defence • Skin – provides a structural barrier to infection • Clotting – blood clotting prevents entry of further pathogens if skin broken • Lysozyme – enzyme found in sweat, tears which dissolves bacterial cell walls. • Sebaceous glands – in skin produce chemicals that kill bacteria • Mucous – sticky substance in body systems to trap pathogens

4. 6. Cilia – tiny hairs that line respiratory system. They beat and move mucous. 7. Acid – HCL in the stomach kills bacteria 8. Beneficial bacteria – in the vagina produce acid to prevent growth of bacteria.

5. Second line in general defence system: Phagocytic white blood cells – When cells are damaged by invading white blood cells they release chemicals which attract white blood cells (phagocytes) which engulf (surround) and destroy the pathogen.

6. Some macrophages move around the body in body fluids and act as scavengers Other macrophages remain fixed in places such as the spleen, lymph nodes, tonsils etc. These macrophages filter out and destroy pathogens in lymph.

7. Defence Proteins Complement is a set of about 20 proteins found in blood plasma. These proteins are activated by infection and destroy viruses and other pathogens. Interferons are anoth set of defence proteins They prevent viral multiplication and help limit spread of virus infections e.g. colds Inflammation When cells are infected they release chemicals that result in blood capillaries opening (dilating). This causes swelling, redness, heat etc and causes more white blood cells to come to the site of infection. Sometimes inflammation can occur around the whole body and cause a temperature rise i.e. fever

8. The specific defence system: • acts against only one particular type of pathogen • is called the immune system • acts by forming antibodies and by killing infected cells The specific defence system attacks particular pathogens

9. Lymphocytes and monocytes: • are white blood cells • are found in lymphatic tissue • both react to antigens • Monocytes (which develop into macrophages) destroy pathogens and display antigens on their membranes • (some phagocytes are very large and are called macrophages) • Lymphocytes: • attack body cells displaying antigens • produce antibodies

10. An antigen is a foreign molecule that stimulates the production of antibodies. An antibody is a protein produced by lymphocytes in response to an antigen. Each type of antibody is highly specific to a single antigen.

11. Antibodies inactive antigens and allow them to be destroyed

12. Some lymphocytes survive for many years as memory cells, allowing long-term immunity. • Induced immunity is the ability to resist disease (by producing antibodies) caused by specific antigens.

13. Active immunity means that a person makes his/her own antibodies. This provides long-term immunity in two ways: • when pathogens naturally enter the body • when antigens are artificially placed in the body due to vaccination e.g. MMR vaccine • Passive immunity occurs when foreign antibodies are introduced into the body. This provides short-term immunity in two ways: • a child getting antibodies in a natural manner from the placenta or mother’s milk through breast feeding • getting an injection of foreign antibodies

14. B-cells (or B-lymphocytes) mature in the bone marrow. • T-cells (or T-lymphocytes) mature in the thymus gland. • B-cells: • recognise an antigen • produce antibodies • control mostly bacterial infections (and some virus infections)

15. The four types of T-cells all recognise specific antigens: • helper T-cells stimulate B-cells (to produce antibodies) • killer T-cells (cytotoxic cells) produce perforin, which causes abnormal body cells to burst • suppressor T-cells turn off immune responses • memory T-cells survive a long time to trigger immunity to the same antigen in later years