How many types of memory cells are there in the immune system?

What percentage of lymphoid tissue in the human body is constituted by Mucosa-Associated Lymphoid Tissue $(MALT)$ (in $\%$)?

Where are $B$-cells and $T$-cells formed? How do they differ from each other?

What is the molecule that stimulates an immune response called?

$A$ newborn infant shows passive acquired immunity against many diseases because:

In higher vertebrates, the immune system can distinguish between self and non-self cells. Due to genetic or other unknown reasons, the immune system sometimes loses this property and starts attacking self-cells, resulting in: