Innate immunity

As mentioned above, the innate immune system responds in a generic fashion to the presence of pathogens. The simplest and most basic form of the innate immune system is the physical barriers, such as the skin, which I also mentioned above. As we saw there, many of these anatomical barriers are augmented by additional mechanisms, such as the antibacterials present in saliva. It is also noteworthy that many barriers have the ability to expel pathogens by flushing them away, as with tears and sweat. If these barriers are overcome, one of the first responses is inflammation. The other main component of innate immunity is the complement system.

In this section, I will treat both in some detail, but to begin at the beginning, I should like to discuss the cells of the innate immune system. Once we understand what they are and what they do, we can start to think about the processes - like inflammation - in which they participate.

The cells which participate in the innate immune system are types of white blood cell (also referred to as leukocytes). Not all leukocytes participate in the innate immune system, but the ones that do are mast cells, phagocytes, basophils, eosinophils and natural killer cells. (Sometimes γδ T cells are also included, but we don't need to consider them here.) Below, we look at each of these in turn. At this stage in our discussion, I will need to be cursory, but we will be able to look at them in more detail later on, once we have the knowledge to do so.

• Mast cells. Mast cells are located in certain tissues, predominantly connective tissue and mucous membranes. If a mast cell is damaged, or if it detects a pathogen, it releases granules containing histamine and heparin, as well as other molecules involved in regulating and mediating the immune response. Cells which contain granules in their cytoplasm are called granulocytes.
  
  So, to put it very briefly, the mast cell is very much an initiator of the response. Recall, as this is very important, that the innate immune response is non-specific. Should a mast cell be damaged at all, or should it recognise a pathogen, it will always respond in the same way. It will release a number of molecules which effect responses in other cells.


• Phagocytes. A phagocyte is a cell which destroys pathogens (and, also, dead cells) by literally engulfing and digesting them. That is, they "eat" pathogens - hence the name (which is, ultimately, from the Greek phageîn, meaning 'to eat'). This process is referred to as phagocytosis.


• Macrophages. (Abbreviation: MΦ) The macrophage is a type of phagocyte - specifically it is a large phagocyte. Quite simple, really. The role of the macrophage is to phagocytose pathogens and to release a number of enzymes and complement proteins, which mediate the immune response.


• Neutrophils. The neutrophil is also a phagocyte and it also phagocytoses. As well as this, the neutrophil is a granulocyte, releasing - when stimulated to do so - granules of substances (specifically antimicrobials) which serve to destroy some pathogens.


• Dendritic cells. (Abbreviation: DC) The dendritic cells are located in areas such as the skin, which are in frequent contact with the world around us and, therefore, are vulnerable. They also phagocytose pathogens, but they play an important role, too, in the adaptive immune system. We will come to this in due course but, for now, suffice it to say that, having engulfed and digested the pathogen, the dendritic cell then contributes to the activation of cells of the adaptive immune system.


• Basophils. Basophils are another type of granulocyte which release histamine and heparin when they encounter a pathogen.


• Eosinophils. Eosinophils are very similar to basophils. When they encounter a pathogen, they release highly toxic substances which do a good job of dispatching pathogens. Unfortunately, they do quite a lot of damage to the body they're defending, as well. Which is not ideal.


• Natural killer cells. (Abbreviation: NK cells) Natural killer cells are fascinating cells which do not actually attack pathogens themselves, as such. Instead, natural killer cells serve to destroy cells of the body which are damaged in some way. Thus, they can destroy tumours, as well as any cells which are infected by viruses.

These, then, are our cells which are involved in the innate immune response. At the end of this section, I will return to them and give a much more detailed description of these cells and a fuller explanation of how they work.