the bane, part two

i know i promised to write something more metaphor-friendly this time around, but i lied. immunology continues to run circles around me, and i need to fight back. what i will do, though, is (try to) place this otherwise intrinsically meaningless discussion of lymphocytes in the context of HIV/AIDS. even if the letters and numbers are still greek to me, at least they'll seem at least collectively more significant in the setting of a truly frightening disease. on that cheery note, here's my basic understanding of lymphocytes, aka T cells and B cells:

in high school health class, you probably learned something about how HIV and AIDS work. the virus kills "helper" T cells, which are part of the body's natural defense system, and the debilitated victim eventually dies from an infection. along with B cells (bone marrow-derived), T cells (thymus-derived) are lymphocytes, specialized cells that are responsible for adaptive immunity--the body's specific response to (presumably) foreign invaders. although functionally distinct, B cells and T cells communicate extensively, signalling one another with dozens of molecules in countless scenarios.

"helper" T cells are called helpers because they don't do any killing--they just signal. appropriately, cytotoxic "killer" T cells do a fair share of the killing. in other words, helper T cells are pussies, albeit clever, essential ones. moreover, helper T cells are further classified into two groups: Th1 and Th2. respectively, two signaling molecules induce the development of a baby (naive) helper T cell into these two distinct types: interleukin-12 (IL-12) and IL-4. the best way for me to remember that IL-12 induces Th1 development is that both IL-12 and Th1 have the number "1" in their names. incidentally, this is one of the prime examples of why immunology makes me want to light myself on fire.

but i digress. Th1 is a key player in cell-mediated immunity, the principle mechanism through which the body defends against viruses and certain bacteria. IL-12 essentially gives helper T cells a heads up that a virus or bacterium is wreaking havoc somewhere, and in turn the activated Th1 cell releases two signaling molecules--IFN-gamma and IL-2--to activate and direct killer cells. IFN-gamma activates macrophages, and IL-2 activates killer T cells. as far as i can tell, the "cell" in "cell-mediated" refers to these killer cells. maybe i'm wrong. boo freaking hoo if i am.

on the other hand, Th2 cells, via IL-4 activation, anchor the antibody-mediated immune process. you can't spell antibody without the letter "b," and so it follows that B cells produce antibodies and are the targets of Th2 signals. again, this is why i hate immunology. most of my classmates, if they see "antibody," they automatically think "B cell." for me, when i see "antibody," i think, "shouldn't it be written 'anti-body'? and why didn't they call it "contra-body" instead? that way, a molecule called ABAB would carry some intrinsic value." it's all i can do not to write "anti-body" in my notes. but again i digress. Th2 cells produce and send out IL-4 and IL-5 molecules to help B cells make antibodies, which then control particular extracellular pathogens.

this brings us back to HIV. HIV does in fact destroy helper T cells, but which ones: Th1 or Th2? i had to google this question for an answer, and even then i couldn't find anything definitive. it appears HIV preferentially invades Th1 cells, but Th1 cells are better at controlling the internalized virus, whereas Th2 cells are less often infected but are weaker upon infection. a third possibility is that HIV infects naive T cells (Th0), but i recently read an abstract arguing against this idea. still, between the two of them, Th1 and Th2 cover the body's chief defense mechanisms against infection, which is why HIV's destruction of both helper T cell-types is so devastating.