from same document - first paragraph
. Introduction
The immune system is a defense system that protects organisms from
invading pathogens, such as viruses or bacteria. It comprises a
heterogeneous group of cells, i.e., immune cells, as well as
cell-independent mechanisms. Immune cells can be broadly divided into
two main categories according to their properties and defense
mechanisms: cells of the innate and cells of the adaptive immune
system. Cells from the innate immune system, namely macrophages,
neutrophils, eosinophils, basophils, mast cells, natural killer cells,
and dendritic cells, are the first cellular line of defense. Their
mode of action is generally fast but with limited specificity. Cells
from the adaptive immune system, namely B cells and T cells, have a
higher level of specificity, but their activation is delayed. However,
cells from the adaptive immune system develop memory against pathogens
after a first confrontation, and their speed and efficiency against a
previously faced pathogen is greatly enhanced during a second
encounter.
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