The primary defences are those that attempt to prevent pathogens from entering the body.
The main primary defence is the skin. The outer layer of the skin is called the epidermis. At the base of the epidermis, cells called keratinocytes are produced by mitosis. As they migrate outwards, their cytoplasm is replaced by the protein keratin so they are dead by they reach the surface of the skin. They act as an effective barrier to pathogens.
Mucous membranes are also a primary defence. Goblet cells secrete mucus which traps dirt and particles which may be pathogens. The cilia then move in a co-ordinated fashion to waft the mucus along to the top of the trachea, where it can enter the oesophagus and enter the stomach where their enzymes are denatured.
Other primary defences are antibodies in the tear ducts to protect the eyes, wax in ear canals and acidic conditions in the vagina.
Secondary defence - phagocytes
Non-specific phagocytes have the job to kill pathogens before they reproduce. There are two types of phagoyctes:
- Neutrophils - These are the most common phagocytes. They are produced in the bone marrow and have a short life span. They are released in large numbers in response to an infection. They travel in the blood and can be found on epithelial surfaces.
- Macrophage - They travel in blood in the form of monocytes and settle in body organs, such as the lymph node. They have a role in the specific response to pathogens.
The action of phagocytes
When pathogens enter the body, their antigens are recognised as foreign. Antibodies attach to these antigens. Phagocytes have receptors which bind to the antibody attached to the pathogen. This can be assisted by proteins called opsonins.
Once the antibody has been attached to the phagocyte surface receptors, the phagocytic membrane folds inwards - engulfing the pathogen. The pathogens are then trapped inside a vacoule called a phagosome. Lysosomes inside the phagocyte fuse with the phagosome and release lysins which are enzymes to digest the pathogen. The end products are harmless nutrients.
As neutrophils are short lived, they die after digesting a few pathogens.