The immune system is ubiquitous throughout the body. It is composed of soluble molecules, spread throughout the body fluids (blood, lymph, extracellular fluid, etc.):
- complement proteins
- the antibodies
- antimicrobial peptides
- cytokines
- amines such as histamine
- etc
Another of its components are immune cells, such as:
- neutrophils
- B and T lymphocytes
- eosinophils and basophils
- mast cells
- monocytes and macrophages
- dendritic cells
- natural killer cells
- microglial cells
These cells, in addition to being found in some lymphoid tissues and organs such as the spleen, thymus, lymph nodes, bone marrow and mucosa-associated lymphoid tissue (MALT), are also found throughout our fluids, tissues and organs, where they perform all the functions mentioned above.
I will not go into much detail about how the immune system works in the face of infection, but we can say that the immune response to micro-organism aggression is divided into an innate immune response and an adaptive immune response. The former is a response mediated by most immune cells, with the exception of lymphocytes, which destroy invading micro-organisms by different mechanisms, although in their wake they leave a significant inflammatory reaction that causes "collateral damage" to tissues. The second is a much more specific immune response that has memory. If our body has already been attacked by a micro-organism, the lymphocytes will have "taken note" of which germ it was and will have produced specific antibodies. It will take some time the first time, but on re-exposure, these antibodies will immediately recognise the offending agent and trigger a much more specific and effective response on entry.
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