MHC Molecules
• Our immune system has the remarkable ability,
and responsibility, of responding appropriately
to a wide variety of potential pathogens in our
environment.
• The proteins that are used as cell-markers to “flag”
self from non-self are called MHC molecules, and are
coded for by a group of genes called the major
histocompatibility complex (MHC).
• MHC genes are diverse, and vary greatly from individual to
individual.
MHC Molecules
• There are two general classes of MHC molecules,
and at least one or the other, or both, are found on
the surface of all nucleated cells in the body.
• Class I molecules (MHC-I) are built into almost all body
cells and are used to present non-self proteins (from
bacteria or viruses, for example) to cytotoxic T cells.
• Class II molecules (MHC-II) are only found only on APCs.
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Both classes are important for antigen processing and presentation.
MHC Molecules
• When APCs come across foreign antigens, they are broken down
and loaded onto MHC-II molecules of APCs.
• The Class II MHC molecules on the APCs present the fragments to
helper T cells, which stimulate an immune reaction from other
cells.
• Clones of activated T cells (and the antibodies from plasma
cells) are now “competent” to recognize similar antigenic
fragments displayed by infected cells throughout the body
and respond harshly.
MHC Molecules
• Infected body cells present antigens using
MHC-1 molecules
MHC Molecules
• Cytotoxic T cell
destruction of an
infected cell by
release of perforins
that cause cytolysis
• Microbes are
destroyed by
granulysin.
Clonal Selection
• Clonal selection is the process by which a lymphocyte
proliferates and differentiates in response to a specific antigen.
• A clone is a population of identical cells, all recognizing the
same antigen as the original cell.
• Lymphocytes undergo clonal selection to produce:
• Effector cells (the active helper T cells, active cytotoxic T cells,
and plasma cells) that die after the immune response.
• Memory cells that do not participate in the initial immune
response but are able to respond to a subsequent exposure proliferating and differentiating into more effector and
memory cells.
Cytokines
• Cytokines are chemical signals from one cell that
influences another cell.
• They are small protein hormones that control cell
growth and differentiation:
•
•
•
•
Interferon
Interleukins
Erythropoietin
Tumor necrosis factor
Antibodies
• Antibodies (also called immunoglobulins or Igs) are produced by
plasma cells through antibody-mediated immunity.
• Antibodies are composed of 4 peptide chains:
• Two heavy chains and two light chains
• Disulfide bonds link the chains together in a Y-shaped
arrangement.
• The variable region (antigen-binding region) gives an
antibody its specificity.
• The stem is similar for each class of antibody.
Antibodies
• Single-Unit antibody structure
Antibodies
• Some of the ways antibodies are effective
include:
• Neutralizing a bacterial or viral antibody, or a toxin by
covering the binding sites and causing agglutination
and precipitation (making what was soluble,
insoluble)
• Activating the classical
complement pathway
• Enhancing phagocytosis a process called
opsonization
Antibodies
• The complement system is a series of blood
proteins that often work in conjunction with
antibodies – it can be activated by multiple
pathways in a step-wise or cascading fashion. It
encourages vasodilation and inflammation,
antigen opsonization,
and antigen
destruction.
• The main proteins
are C1-C9.