Lymphatics and Immunity 2: Innate Immunity

What is innate or non-specific immunity?

Innate immunity got its name because of the fact that it always works in the same way and is against any type of invading agent. It is a non-specific defence. There are seven major categories of non-specific defence:

  1.   Physical Barriers

These include skin, hair, epithelial layers of internal passageways, secretions to flush away materials and secretions to kill or inhibit pathogens.

  2.   Phagocytes

There are two types of phagocytes; microphages, which are small and leave the bloodstream to fight infection, e.g. neutrophils, and macrophages, which are large cells distributed through the body. Macrophages are produced from monocytes and once activated, respond to pathogens in several ways:

  a.   Engulf pathogens and destroy using lysosomal enzymes

  b.   Bind to pathogens so other cells can destroy it

  c.   Destroy pathogens by releasing toxic chemicals into the interstitial space

There are two types of macrophages; fixed macrophages stay in specific tissues or organs and free macrophages travel throughout the body.

  3.   Immunological Surveillance

This is carried out by natural killer cells, which recognise and destroy abnormal cells. Once they identify the abnormal cell, they attach to it and form perforin vessels from their own golgi apparatus. They release the perforin by exocytosis which lyses the abnormal plasma membrane and attacks the cancerous cells.

  4.   Interferons

Interferons are proteins (cytokines) released by activated lymphocytes and macrophages. Cytokines are chemical messengers which coordinate local activities and can act as hormones to expand their site of effect. Interferons are small proteins which can diffuse into a cell and bind to receptors within the cell. Once inside the cell, more interferon proteins are transcribed which stimulates the production of antiviral proteins. These prevent the replication and integration of virus into our DNA. There are three classes of interferons:

  a.   Alpha interferons are produced with cells already affected by a virus. They attack viruses and stimulate natural killer cells.

  b.   Beta interferons are secreted by fibroblasts and slow down inflammation.

  c.   Gamma interferons are secreted by T and natural killer cells to stimulate macrophage activity.

  5.   Complement

Complement proteins (11 in the plasma) induce the lysis of foreign cells, which works in both non-specific and specific defence mechanisms. They complement antibody action. There are two pathways which activate the complement system:

In the classical pathway, C1 attaches the bacterial cell wall antibodies, catalysing a series of reactions involving other complement proteins.

In the alternative pathway, there are no antibodies and the classical pathway is activated by in a much slower, less effective manner involving properdin, factor B and factor D.

However, what is common to both pathways, is that an inactive C3 protein becomes activated into C3b, which attaches to the cell wall. 5 compliment proteins then join to form the membrane attack complex with a hole in the centre. This causes the destruction of plasma membranes, increasing phagocytic activity and making the cell easier to engulf through opsonisation (opsonisation is the destruction of plasma membranes and phagocytosis of infected cells through the use of complement proteins in non-specific immunity). Mast cells and basophils release histamine and inflammation and blood flow to the area is increased.

  6.   Inflammation

Inflammation is only a localised response to an injury. It is triggered by any stimulus that kills or injures a tissue. Due to increased tissue fluid we get swelling and pain. Due to increased blood flow, we get redness and heat.

  7.   Fever

Fever is any body temperature which is maintained at a lever about 37 degrees. It is caused by the release of pyrogens from the hypothalamus.