Numerous cells are able to ingest foreign materials, but the ability to increase this activity in response to opsonization by antibody and/or complement, so as to acquire antigen specificity, is restricted to cells of the myeloid series, principally polymorphs, monocytes and macrophages; these are sometimes termed ‘professional’ phagocytes.

1. Phagocytosis
Numerous cells are able to ingest foreign materials, but the ability to increase this
activity in response to opsonization by antibody and/or complement, so as to acquire
antigen specificity, is restricted to cells of the myeloid series, principally polymorphs,
monocytes and macrophages; these are sometimes termed professional phagocytes.
Apart from some variations in their content of lysosomal enzymes, all these cells use
essentially similar mechanisms to phagocytose foreign objects, consisting of a
sequence of attachment, endocytosis or ingestion and digestion. In the figure this
process is shown for a typical bacterium. In general, bacteria with capsules are not
phagocytosed unless opsonized, whereas many non-capsulated ones do not require
this. There are certain differences between phagocytic cells; e.g. polymorphs are very
short-lived (hours or days) and often die in the process of phagocytosis, while
macrophages, which lack some of the more destructive enzymes, usually survive to
phagocytose again. Also, macrophages can actively secrete some of their enzymes,
e.g. lysozyme. There are surprisingly large species differences in the proportions of
the various lysosomal enzymes.