The nucleus is the most obvious organelle when looking at a cell. In a macrophage, the nucleus is an odd shape, almost looking like a horseshoe or a jelly bean. The surface of the nucleus is called the nuclear envelope, which consists of 2 membranes, and is covered in tiny structures called nuclear pores, which let molecules in and out of the nucleus. Within the nucleus is a smaller structure, called the nucleolus, which is made up of RNA and produces ribosomes, which produce proteins that the cell needs.
The nucleus also contains DNA, which contains all of the information needed to give each cell it's unique characteristics. Although the DNA found in every cell of an organism is very similar, some unnecessary parts may be inactive, which is why macrophages are different from muscle cells, and so on. This is very important when considering cell specialization; DNA gives cells the specific properties they need to do their job.