A little prehistorical perpective might place things in their rightful context, perhaps. Earthly life started out just over 3.5 billion years ago, but (eukaryotic) multicellularity had to wait another 2.5 billion years to come about. Prior to this watershed moment, bellicose or desperate cells could only engage in one-on-one combat with each other, but with multicellarity a host of new possibilities burgeoned. The new, larger organisms could afford to turn individual cells from simple reproduction and survival to more specialised tasks - defensive cells, for instance. Thus multicellularity ushered in the immune system.
Our innate immune system is the direct descendant of such efforts. Fundamentally, it consists of:
- Certain cells: neutrophils, monocytes/macrophages, supported by basophils and eosinophils. Natural killer cells, although a late addition, are also best thought of as belonging to the innate immune system.
- Pattern-recognition molecules
Complement has been dealt with already on this blog, starting here. We'll have to talk more about pattern-recognition molecules later, but these are fundamentally circulating molecules that recognise and bind to certain common pathogen structural motives and thereby hasten the invader's demise. For instance, mannan-binding lectin recognises and binds to a carbohydrate pattern found on many bacteria, viruses, protozoa and fungi, and then activates the complement system.
The innate immune system is capable of reacting within minutes to any invasion by pathogens (or to tissue damage from other causes). However, its response is non-specific; it depends on recognising certain patterns common to groups of organisms, rather than to specific pathogens. For instance, the innate immune system doesn't in principle distinguish between H. influenzae and S. pneumoniae.
In the next post we'll meet the adaptive immune system, and then finally get around to comparing the two systems directly.