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. 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.