Phygocytosis may be defined as — ‘the engulfing of microorganisms or other cells and foreign
particles by phagocytes’.
Alternatively, phagocytosis (from the Greek words for eat and cell) referts to — ‘the phenomenon
of ingestion of a microorganism or any particulate matter by a cell’.
Interestingly, the human cells which critically carry out this ardent function are collectively
known as phagocytes, such as : all types of WBCs, and derivatives of WBCs.
Actions of Phagocytic Cells : In this event of a contracted infection, both monocytes* and
granulocytes** usually get migrated to the infected area. Interestingly, during this process of migration,
the monocytes do get enlarged to such a dimension and size that they finally develop into the actively
phagocytic macrophages.
Types of Macrophages : There are, in fact, two major categories of the macrophages, such as :
(a) Wandering Macrophages : Based on the glaring fact that these cells (monocytes) do have
a tendency to leave the blood and subsequently migrate via the tissue cells to the desired
infected areas, they are commonly known as wandering macrophages.
(b) Fixed Macrophages (or Histocytes) : A monocyte that has eventually become a resident in
tissue. Fixed macrophages or histocytes are invariably located in certain specific tissues
and organs of the body. In fact, they are found abundantly in various parts of a human body,
for instance :
• Bronchial tubes ; • Lungs (alveolar macrophages) ;
• Bone marrow ; • Nervous system (microglial cells ) ;
• Lymph nodes ; • Peritoneal cavity (surrounding abdominal organs) ;
• Liver (Kupffer’s cells ) ; • Spleen ;

Importantly, the macrophage variants critically present in the body strategically constitute the
mononuclear phagocytic (reticuloendothelial) system.
 Functions of Phagocytes (or Phagocytic Cells) : It has been duly observed that when
an infection gets contracted one may apparently observe a distinct shift taking place predominantly in
the particular types of WBC which runs across the blood stream. Thus, the following cardinal points
may be noted, carefully :
􀁑 Granulocytes – particularly the ‘neutrophils’ occur overwhelmingly in the initial phase of
infection, at this point in time they are found to be extremely phagocytic in nature.
􀁑 Distinct aforesaid dominance is evidently shown by the presence of their actual number in a
differential WBC count.
􀁑 With the progress of contracted infection, the macrophages also predominate – scavenge –
phagocytize remaining live/dead/dying microorganisms.
􀁑 Enhanced number of monocytes, that eventually develop into the corresponding macrophages,
is adequately reflected in the WBC-differential count explicitely.
􀁑 Blood and lymph containing bacteria when made to pass via various organs in the body
having fixed macrophages, cells of the mononuclear phagocytic system ultimately get rid
of the bacteria by phagocytosis.
􀁑 Mononuclear phagocytic system also helps in the critical disposal of the worn-out blood

Mechanism of Phagocytosis : In order to understand the exact and precise mechanism
of phagocytosis, we may have to divide the phenomenon of phagocytosis, as illustrated in Fig. 9.6,
into four cardinal phases, such as : chemotaxis, adherence, ingestion, and digestion. These four distinct
phases shall now be treated briefly in the sections that follows from [A] through [D] :
[A] Chemotaxis [Syn : Chemotropism] :
Chemotaxis may be defined as — ‘the movement of additional white blood cells to an area of
inflammation in response to the release of chemical mediators by neutrophils, monocytes, and
injured tissue’.
In other words, chemotaxis refers to the chemical attraction of the phagocytes to microbes.
Importantly, the various ‘chemotactic chemical susbtances’ which specifically attract the
phagocytes happen to be such microbial products as components of :

• white blood cells (WBCs),
• damaged tissue cells, and
• peptides derived from complement.
[B] Adherence :
Adherence refers to the act or condition of sticking to something. In fact, it represents the ensuing
adherence of antigen-antibody complexes or cells coated with antibody or complement to cells
bearing complement receptors or Fe receptors. It is indeed a sensitive detector of complement-fixing
Because, adherence is intimately related to phagocytosis, it represents the attachment of the
later’s plasma membrane onto the critical surface of the bacterium or such other foreign material.
Nevertheless, adherence may be hampered by the specific presence of relatively larger capsules or
M protein*. Besides, in certain instances adherence takes place quite easily and conveniently, and the
microbe gets phagocytized rapidly.

[C] Ingestion :
In usual practice adherence is followed by ingestion. One may vividly notice that during the
phenomenon of ingestion, the plasma membrane belonging to the phagocyte gets extended in the form
of distinct projections usually termed as pseudopods which eventually engulf the bacterium. Thus,

once the bacterium gets duly surrounded, the pseudopods meet and fuse ultimately, thereby surrounding
the bacterium with a particular ‘Sac’ known as phagocytic vesicle or phagosome.
[D] Digestion :
Digestion refers to the particular phase of phagocytosis, wherein the respective phagosome*
gets detached from the plasma membrane and duly enters the cytoplasm. Later on, within the cytoplasm
the phagosome meticulously gets in touch with the lysosomes** which essentially comprise of two
important components, namely :
• digestive enzymes, and
• bactericidal substances.
Modus Operandi [or Mode of Action] : The various steps involved are as given below :
(1) Both phagosome and lysosome membranes upon contacting each other invariably gets fused
to result into the formation of a ‘single larger structure’ termed as ‘phagolysosome’.
(2) Interestingly, the integral contents of the phagolysosome usually ‘kills’ most types of
microorganisms within a span of 10–30 minutes. The most plausible and possible reason for
such a marked and pronounced bactericidal effect is perhaps due to the specific contents of
the lysosomes.
(3) Residual body : After completion of the process of digestion the actual contents of the
phagolysosome are duly brought into the cell by ‘ingestion’ ; and, therefore the
phagolysosome essentially and exclusively comprises of the indigestible material, which is
usually known as the ‘residual body’.
(4) Residual body subsequently takes a step forward toward the cell boundary and critically
discharges its ‘waste products’ very much outside the cell.
A Few Exceptions : These exceptions are as stated below :
(a) Toxins of certain microorganisms viz., toxin-producing Staphylococci plus the bacterium
Actinobacillus (present in dental plaque, may actually exert a cidal effect upon the phagocytes.
(b) Some other microbes, for instance : Chlamydia, Leishmania, Mycobacterium, and Shigella
together with the ‘malarial parasites’ may possibly dodge and evade the various components
of the immune system by gaining an access into the phagocytes.
(c) Besides, the said microorganisms may virtually block the ultimate fusion between phagosome
and lysosome, as well as the adequate process of acidification (with HCl) of the digestive