"What's an 'Immune System?'"

While my eyes were glazing over in front of the television yesterday, a grating child's voice stated "I think salmonella is a kind of dinosaur!" Then another one said "I don't know how not to get sick!" and another asked "What's an immune system?" It turns out it was a commercial for Purell or some other we're-all-scared-of-dying gel. This particular company used ignorant, snuffling, touching, sneezing children to spread its message of fear-- but I found myself not worried about encountering germs, but worried about how much I know about what happens when my body encounters them.

I suppose that last child's question, combined with the fact that I'm currently stumbling unassisted through a series of failing immunopreciptation experiments here at the CDC, made me wish I'd had an immunology course at Skidmore. What IS an immune system?!

Yikes!

My biggest question is: what are the genetics involved in specific antibody production? I have a sketchy idea about innate vs. adaptive immunity, and the hideously complicated cascade of events that is kicked off when a pathogen is encountered and its antigens are presented to this cell that turns into that cell and yadda yadda and eventually antibodies are secreted. But are there genes just sitting in these immune cells that are waiting to be transcribed when a certain signal is given? And then you get a perfectly specific antibody? Just like that? The array of antigens that pathogens possess is so vast and diverse, how are our immune cells "told" what to make? I looked it up. All day. And I think I've found an answer in "Immunobiology" which is a text book available online through PubMed.

"The startling feature that emerged from the biochemical studies was that an antibody molecule is composed of two distinct regions. One is a constant region that can take one of only four or five biochemically distinguishable forms; the other is a variable region that can take an apparently infinite variety of subtly different forms that allow it to bind specifically to an equally vast variety of different antigens."

Okay, but how do you end up with one of the infinite options when you only have so many genes?

"This question was answered in 1976, when Susumu Tonegawa discovered that the genes for immunoglobulin variable regions are inherited as sets of gene segments, each encoding a part of the variable region. During B-cell development in the bone marrow, these gene segments are irreversibly joined by DNA recombination to form a stretch of DNA encoding a complete variable region. Because there are many different gene segments in each set, and different gene segments are joined together in different cells, each cell generates unique genes for the variable regions of the heavy and light chains of the immunoglobulin molecule. Once these recombination events have succeeded in producing a functional receptor, further rearrangement is prohibited. Thus each lymphocyte expresses only one receptor specificity."

Oh! So it's like fast-paced "evolution" with many options being produced, and only one is successful and gets the job done.

I still don't really understand how that type of "natural selection" works so quickly and keeps us from dying so well, and I don't understand the pressures that enhance the production of the successful antibody, but I do understand a LITTLE bit more about the immune system. And now I'm going home to watch more shitty commercials.

So put that in your pipe and smoke it, Purell.