M

macrophage A MONOCYTEthat leaves the BLOOD
circulation and takes up residence in the tissues.
Once there, the cell undergoes several changes:

• It greatly enlarges.


• It develops pseudopods (footlike projections)
  that permit it to move through tissue.


• It increases the amount of lysozyme its gran-
  ules contain, increasing its ability to consume
  cellular debris.

Macrophages, the IMMUNE SYSTEM’s tissue-based
scavengers, are part of the MONONUCLEAR PHAGOCYTE
SYSTEM. They engulf, dismantle, and consume the
carcasses of cells that other immune cells destroy
and of cells that die naturally (APOPTOSIS). They
also absorb and breakdown the particulate debris
from toxins, BACTERIA, viruses, and other sub-
stances. Macrophages also respond to the inflam-
matory process, contributing to INFLAMMATIONand
the formation of granulomas. As well,
macrophages are key to ANTIGENpresentation and
processing, the mechanism by which T-cell lym-
phocytes and B-cell lymphocytes recognize non-
self antigens. As the macrophage dismantles a
substance, it displays the substance’s antigens,
along with the relevant MAJOR HISTOCOMPATIBILITY
COMPLEX(MHC), on the surface of its cell membrane
for lymphocytes to detect. Lymphocytes ignore
debris that belongs to the body. Debris that is for-
eign activates an IMMUNE RESPONSE.
For further discussion of macrophages within
the context of the structures and functions of the
immune system, please see the overview section
“The Immune System and Allergies.”
See also B-CELL LYMPHOCYTE; GRANULOMA; LYMPHO-
CYTE; NATURAL KILLER(NK) CELL; T-CELL LYMPHOCYTE;
VIRUS.

major histocompatability complex (MHC) The

group of genes, located on CHROMOSOME6, that

determine the HUMAN LEUKOCYTE ANTIGENS(HLAS)

the body’s cells carry on their cell membranes.

HLAs are unique proteins that cell membranes

display to identify themselves to the IMMUNE SYS-

TEM. There are three types of MHC:

• Class I MHC encodes the HLAs that all nucle-
  ated cells and platelets in the body carry to
  identify them as self cells (the body’s own
  cells).


• Class II MHC encodes the HLAs that lympho-
  cytes, macrophages, dendritic cells, and other
  substances involved in ANTIGENprocessing carry.
  These HLAs are fundamental to ANTIBODY-MEDI-
  ATED IMMUNITYand are also responsible for GRAFT
  VS. HOST DISEASEand organ rejection in people
  who undergo ORGAN TRANSPLANTATION.


• Class III MHC encodes the immunoglobulins
  from which the immune system forms antibod-
  ies.

MHC is central to antigen processing. When a

MACROPHAGE or dendritic cell (phagocytes, also

called scavenger cells, in the MONONUCLEAR PHAGO-

CYTE SYSTEM) consumes cellular debris, it displays

the antigens of the debris alongside its own HLA.

This comparison display allows T-cell lymphocytes

to recognize the cellular debris as self or nonself

and respond accordingly. Self antigen evokes no

reaction; nonself antigen mobilizes the IMMUNE

RESPONSE.

See also ANTIBODY; CELL-MEDIATED IMMUNITY;COM-

PLEMENT CASCADE; GENE; IMMUNOGLOBULIN; LYMPHO-

CYTE; MACROPHAGE; PHAGOCYTE; PHAGOCYTOSIS;

PLATELET; T-CELL LYMPHOCYTE.

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