11. T Cells & Cell-Mediated Immunity
• T Cells must be activated by exposure to
antigen
• T Cells recognize antigens when they are
bound to membrane receptors of other cells
• Antigen membrane receptors are called
– MAJOR HISTOCOMPATIBILITY COMPLEX
PROTEINS
• Class I MHC
• Class II MHC
12. T Cells & Cell-Mediated Immunity
• MHC Proteins
• Class I
– Found on the surface of all nucleated cells
– Peptides produced inside the cell are displayed on
the surface by Class I MHC proteins.
• Class II
– Found on the surface of lymphocytes & phagocytes
• Called ANTIGEN-PRESENTING CELLS (APCs)
13. Types of T Cells
1. Cytotoxic T Cells
2. Helper T Cells
3. Memory T Cells
4. Suppressor T Cells
16. Helper T Cells
• Activated by exposure to antigens presented by
Class II MHC proteins
• Activated helper T cells perform two functions:
1. Coordinate specific & nonspecific defenses
2. Stimulate both cell-mediated & antibody-mediated
immunity
• Activated helper T cells divide to produce:
– More activated helper T cells
– Memory T cells
• helper T cells clip
17. Memory T Cells
• Produced upon initial exposure to antigen, but
do not respond to the antigen at that time.
• Provide a no-delay response to any FUTURE
exposure to the same antigen
– Immediate differentiation into cytotoxic T cells
and helper T cells
18. Suppressor T Cells
• Activated suppressor T cells dampen the
responses of other T cells and B cells
– Secrete suppression factors
• Act after the initial immune response
28. Antibody (IgG) Function
• Neutralization
• Agglutination and precipitation
• Activation of complement
• Attraction of phagocytes
• Enhancement of phagocytes
• Stimulation of inflammation
Innate Immunity
Genetically determined
Present at birth
Independent of previous exposure to antigens
Acquired Immunity
Not present at birth
Appears after exposure to an antigen
Can be passive or actived
T Cell activation does not usually result from direct lymphocyte-antigen interaction.
T Cells are seldom activated by the mere presence of antigens alone.**
**T Cells recognize antigens when those antigens are bound to membrane receptors of other cells.
MHC Proteins
Class I
Class II
Cytotoxic T Cells = Killer T Cells
Among the many T cells produced upon initial exposure to an antigen.
Do not respond to the antigen during initial exposure.
Remain on reserve until subsequent exposure.
Immediately differentiate into killer T cells upon subsequent exposure.
This rapid response generally destroys invading pathogens before they can cause the body to produce signs and symptoms of disease.
Suppression does not occur immediately bc suppressor T cells are activated more slowly than other types of T cells.
A B Cell is sensitized by exposure to antigens (step 1). Once antigens are bound to antibodies in the B cell membrane, the B cell displays those antigens in its cell membrane. Activated helper T cells encountering the antigens on the sensitized B cell membrane then release cytokines that trigger the activation of the B cell (step 2). The activated B cell then divides, producing memory B cells and plasma cells that secrete antibodies (step 3).
Antibody aka IMMUNOGLOBIN (IgG).
antigen-antibody complex
Antibodies do not bind to the entire antigen as a whole.
They bind to certain portions of its exposed surface, regions called ANTIGENIC DETERMINANT SITES.
The specificity of that binding depends on the 3D “fit” between variable segments of the antibody molecule and the corresponding sites of the antigen.
A complete antigen has at least two antigenic determinant sites, one for each arm of the antibody molecule.
Exposure to a complete antigen can lead to B cell sensitization and an immune response.
Most environmental antigens have multiple antigenic determinant sites; entire microorganisms may have thousands.
