The immune system of our body consists of two parts, the innate immunity and the adaptive immunity. 

Innate Immunity - the first line of defense.

Innate immunity, also called natural immunity, is a basal immune system that automatically recognizes and destroys threats. It has a swift response time.
The Innate immune system consists of leukocytes, a type of white blood cell. Natural Killer cells, Mast cells, Phagocytes, Macrophages, Neutrophils and Dendritic cells are all innate leukocytes and function within the immune system by identifying and eliminating pathogens that might cause infection.
Natural Killer cells – primarily attack antigens such as infected cells, cancer cells, bacteria and viruses but can also influence other parts of the immune system of the adaptive immunity system, for example T-cells.

Adaptive Immunity - the second line of defense.

The adaptive immunity consists of T-Cells and B-Cells- the leukocytes or the white blood cells.
This part of the immune system adapts and learns from previous infections and contains heavy lifters such as T-Cells that are very good at destroying threats but also cause a lot of collateral damage sometimes.
It takes longer to activate the adaptive immunity, and it is only called upon if there is an ongoing infection that the first line of defense cannot handle. The adaptive immunity has a memory and adapts to get better every time it encounters a new threat, virus or bacteria or our own infected cell.
However, the adaptive system sometimes malfunctions and wrongly recognizes our own body as a threat; this can lead to an autoimmune disease.