29. Summary

Summary
For a summary of the entirety of the immune system I recommend the first chapter in each of the three books on immunity that I mentioned in an earlier post: "Undergraduate Immunology" by C. Erridge, "How the Immune System Works" by L. Sampayrac, and "Cellular and Molecular Immunology" by Abbas et al. Another helpful resource is a website called "Bite Sized Immunology". It has a ton of short articles on most of the topics I've covered in this blog. For those looking to get an overview of the adaptive cellular system, there's a well written summary of the T cell response in an excellent paper that appeared in the journal "Advances in Physiology Education" by Nathan Pennock and others.

Here's my version of the major points covered so far.

• There are two major arms of the immune system: innate and adaptive immunity.
• Innate immunity kicks in quickly in response to microbial invasion. It has taken advantage of evolution to fashion a fixed set of protein devices designed to detect and combat commonly encountered pathogens. Its major cellular components are macrophages, neutrophils, natural killer cells, and mast cells. It also makes use of the complement system. When these players mass at the site of an infection, inflammation results.
• Adaptive immunity is mediated by two types of cells: B and T lymphocytes. Both cell types shuffle a small set of genes to create hundreds of millions of different receptor proteins. Each lymphocyte and its immediate progeny (a clone) carries a set of only one these receptors - the idea being that among this great variety of receptors generated by genetic recombination, one or a few will be capable of binding to virtually any molecule, even novel ones that no organism has ever encountered before.
• The molecules that the adaptive immune system responds to are called antigens.
• B lymphocytes are responsible for humoral immunity. They produce specialized proteins, antibodies, that they secrete into the blood. Antibodies bind to antigens located on the exterior of invading microbes, neutralizing some pathogens, and targeting others for destruction by the innate immune system.
• T lymphocytes are responsible for cellular immunity. They do not secrete antibodies. Instead they make use of millions of different T cell receptors to seek out antigens that are present both inside and outside of cells. They utilize proteins of the major histocompatibility complex that display fragments of intracellular proteins on special antigen presenting cells (mostly dendritic cells) to locate quarry that carry viruses and other internal microbes.
• The innate and adaptive immune systems coordinate their activities. They communicate using cytokines, secreted proteins that direct cells to different pathways and behaviors.
• There are two different populations of T lymphocytes: helper T cells (also called CD4+ cells) and killer T cells (also called cytotoxic T cells or CD8+ cells). Helper T cells use cytokines to tell macrophages to ingest microbes and aid in the B cell response. Killer T's directly cause cells harboring viruses to commit suicide by directly injecting specific proteins into their interiors.
• Both cellular and humoral adaptive immunity display memory. That is, repeated attacks yield an improved response. This is the basis for vaccination.

Of course, you shouldn't get the impression that I've covered all of immunology. In fact, I've left a lot out. In particular, I haven't discussed the  question of how the adaptive immune system doesn't ordinarily react to the substances that naturally occur within one's body. I'll leave that to a later post in conjunction with a discussion of the immune response to cancerous cells.

Now I'm going to switch subjects. It's been a long time since the initial posting, but recall that the ultimate aim of this blog is to understand how the immune system can be engineered to react against cancer cells. Hence the next postings on cancer.