I attended a primer lecture on tbe immune system. Holy hell.
At least 200 different types of T cells. At least 200 other types of immune cells. Then there are all their interactions, signaling, locating, memory functions. I honestly dont know if one can ever get a handle on the complexity.
It is not complexity, but diversity. The functions those cells perform are mostly understood as are most mechanisms by which they work. There are many variants of a similar function and mechanism.
That said, immunology is still quite young too, but compared to it neuroscience is not even born.
