Primary and Secondary Immune Defences
Primary Defence - Skin
- Primary defences are the initial barriers that prevent us from being harmed by pathogenic organisms entering our body.
- There are 2 main types of primary defences and these are the skin and mucous membranes.
- Our bodies are covered by the skin and this is the first primary defence that we have.
- Skin cells called keratinocytes are produced in the base of the skin's epidermis and then mitose out to the surface of the skin.
- Keratinisation is a process that takes about 30 days and is when the cytoplasm of the keratinocyte cells is replaced by keratin as the cells move to the surface.
- When the cells have reached the surface of the skin they are no longer alive and they fall off as dead skin cells.
- This is very useful in the protection against pathogens as it acts as a barrier to prevent them from entering our body.
Primary Defence - Mucous Membranes
- Mucous membranes are another type of primary defence.
- In places like the digestive system and the lungs, the barrier between our blood and the environment is reduced and this leaves us more vulnerable to infection. Pathogens can just as easily enter our blood in the same way that, for example, oxygen can.
- Because of this, things like our lungs are protected by mucous membranes.
- Goblet cells are cells that secrete mucus and are found in places like our trachea.
- The mucus that the goblet cells secrete traps any pathogens that might be in the air and then cells called ciliated epithelial cells 'wave' the mucus to the back of our throat so that we can swallow it.
- When we swallow the pathogens the acidity of our stomach acid denatures the enzymes within the pathogen and renders it harmless.
Secondary Defence - Phagocytes
- If pathogens make it past the primary defences, in order for them to be killed the non-specific phagocytes, that are manufactured in our bone marrow, must destroy them!
- The two different types of phagocyte are neutrophils and macrophages.
- Macrophages are relatively large cells and travel in the blood as monocytes. They are then transported in the blood to various organs but the majority accumulate in the lymph nodes and there in the lymph nodes, they mature into macrophages.
- When cells are infected they release chemicals, for example histamine, and this attracts surrounding neutrophils to the area.
- Histamine will also cause capillaries to become more leaky and this is why infected areas can be red and swollen.
- Another result of the leaky capillaries is that there is more tissue fluid going through the lymphatic system and the pathogens are transported to the lymph nodes where the macrophages can kill them.
- Neutrophils are the most common phagocyte, they travel around in the blood but because they are relatively small they are able to fit through capillary pores and squeeze out into the tissue fluid.
- They don't live for very long but when an infection occurs they will be made in very large numbers to try to combat it.
How do they work?
1 - When a pathogen invades the body they are detected as foreign by the antigens on their cell surface membrane.
2- Complementary antibodies attach to the antigens.
3 - The receptors (membrane -bound proteins) on phagocytes bind to the antibodies.
4 - The phagocyte then engulfs the pathogen by folding it's membrane inwards and creating a phagosome vacuole.
5- Lysosomes that are inside the phagocyte then fuse with the phagosome and release enzymes into it.
6 - These enzymes digest the bacteria into products that are harmless nutrients that can then be absorbed by the cytoplasm of the cell.