What immune defenses does the fetus have?

The fetus's first line of defense is a primitive form of immunity called toll-like receptors (TLRs). They work a bit like an alarm system; they recognize cells as evil or friendly and then send alerts to other immune cells (namely T and B cells) to do the dirty work.

Immune cells called B-lymphocytes appear in the fetal liver, and T-lymphocytes develop in the thymus (a gland in the chest, which is large in babies, but small in adults) by the end of the first trimester. By this time, the fetus is much better able to withstand sophisticated invaders and potential threats than it was when just the TLRs were in place. Along with the timing of the fetus's major organ development, the fact that the fetal immune system isn't fully up and running until the second trimester is why you have to be especially careful about exposure to toxins and infectious agents during your first trimester.