This episode of Forgotten Physiology accepts the challenge of explaining cell mediated immunity in 5 minutes without the aid of caffeine! Grab some popcorn and don’t miss the action.
The body is a lot like an exclusive club in downtown D.C. No one gets in without an invitation (MHC receptor…every self respecting cell has one) and I do mean NOBODY. There are bouncers always watching the entrances and the exits (Dendritic cells, Macrophages) for shady characters looking to start trouble (viruses, bacteria, fungi, parasites). They are also constantly carding at the door for minors (immature red and white cells) or anyone who has an expired membership card (malformed cells, tumor cells, or any infected cells that make the wrong receptor).
Now these bouncers are pretty fierce. Good guys, but you don’t want to ever cross them because they will go Green Hulk on you expanding to sometimes 3 times their normal size and will literally (no really literally) eat you alive. They also have one hell of a temper and not only will they eat you but they’ll steal your wallet pass your picture to their friends so that anyone who even looks like you gets the gangland treatment. (sampling antigenic determinants from the bacterial cell for display to helper Tcells)
A Day in the Life of the Macrophage (cell-mediated immunity)
Let’s ride along with the macrophage. This guy never stops working he pounds the pavement patrolling our peripheral blood circulation for strangers and shady characters (pathogens, toxins, foreign substances) that don’t belong and when it finds them it swallows them whole and digests them with a kind of industrial strength cellular stomach called a lysozyme, but it doesn’t stop there. Our immune system hasn’t learned about the infection yet it hasn’t hit the news. The macrophage must now present that antigen (foreign substance which elicits an immune response) to another kind of specialized cell the T-cell. That’s what a macrophage does it acts as an antigen presenting cell (APC to his friends). The way it does this is by binding some of the peptides of that antigen it swallowed with its own proteins (MHCII class) displays them on the surface of its membrane as receptors. Our macrophage now takes a little field trip to a nearby lymph node (a hot spot in town where all the young lymphocytes hang out) but he’s not just looking for any ole T-cell. Macrophage can only present antigen to a cell with matching receptors for it. He’s looking for “Misses Right.” Warning this next part is a little graphic. If and when our hero finds a T-cell they dock receptors (MHC II complexed with antigen T-cell’s CD4 receptor w/MHCII binding site) Macrophage passes along some IL-1 that stimulates that cell to switch on divide into daughter cells release its own IL-2 that stimulates those cells to divide. So the macrophage has now informed your T-cells, your effector cells about the infection and your T-cells produce an army of messenger clones all hard wired to manage the same infection.
Now the infection has reached the front page news. Meanwhile B-cell with the right receptors has encountered the same antigen that everybody is talking about. He has already processed the antigen bound it with his own MHC II protein. Now he’s watching for T-cell to switch him on, give him the software he needs (IL-2 helper cytokines) so that he can upgrade – switch from an IgM to a IgG antibody producing cell and divide into an army of plasma cell clones. So at this point you can imagine your lymph nodes are becoming a very crowded place and they are. Those lymph nodes begin to swell as thousands of activated lymph cells fight the active infection. These cells are short lived though many of your plasma cells spit out antibody and then die shortly after. Some of those cells live on inactive in your lymphatic system as memory cells. If that antigen returns they’ll switch back on and start spitting out high specificity IgG. This specialized antibody is many times more efficient at binding than the store brand IgM. Phew….now I need a nap. Until next time my friends. Stay classy and never stop learning.