Immunology Basics - Immunological Computation: Theory and Applications - page 7

polypeptides from inside the cells they are a part of, and display them on the cell's
exterior surface for recognition by T cells.
MHC proteins can be classifi ed into three classes: MHC class I molecules are
found on almost every nucleated cell of the body. Class I molecules present “endog-
enous” antigens to cytotoxic T cells. Endogenous antigens may be fragments of
viral proteins or tumor proteins. Presentation of such antigens would indicate inter-
nal cellular alterations, which if not contained could spread throughout the body.
MHC class II molecules are found only on a few specialized cell types, includ-
ing macrophages, dendritic cells, activated T and B cells. Moreover, class II mol-
ecules present “exogenous” antigens to helper T cells. Exogenous antigens might
be fragments of bacterial cells or viruses that are engulfed and processed by, for
example, a macrophage and then presented to helper T cells.
MHC class III region encodes for other immune components such as comple-
ment components (e.g., C2, C4, or factor B) and some that encode cytokines (e.g.,
TNF- α ).

1.1.2.5 Macrophages and Dendritic Cells

Macrophages are specialized cells, which engulf large particles such as bacteria,
yeast, and dying cells by a process called phagocytosis. When a macrophage ingests
a pathogen, the pathogen becomes trapped in a food vacuole, which then fuses with
a lysosome. Enzymes and toxic oxygen compounds digest the invader within the
lysosome.
Dendritic cells are immune cells that form part of the mammal immune sys-
tem. h ese cells are present in small amounts in those tissues that are in contact
with the external environment such as the skin (where they are often called Lang-
erhans cells) and the inner covering of nose, lungs, stomach, and intestines. In their
immature state, they can also be found in blood. Once activated, they migrate to
the lymphoid tissues, where they interact with T- and B cells to initiate and drive
an immune response.

1.1.3 The Complement System

h e complement system is a part of humoral immunity; when an infection occurs,
this system complements the antibacterial activity of antibody. It consists of a set
of plasma proteins; if one gets activated, it triggers a sequence of reactions on a
pathogen's surface that helps to destroy the pathogen and eliminate the infection.
h e three main consequences of the complement system are the recruitment of
infl ammatory immune cells (phagocytes), opsonization of pathogens by antibod-
ies, and killing of pathogens by creating pores in the bacterial membrane leading
to their death. So the complement activation helps to amplify the eff ects of the
classical pathway.