1. Immune System Basics
Immunity: The capacity to resist infectious pathogens.
Pathogens: Disease-causing organisms
Self vs. Non-self recognition
Major Histocompatibility Complex (MHC 1)
Antigen- a particle or piece of pathogen an immune system recognizes as foreign.
MHC 1
Antigens

3. 1st Defense: Non-specific Immune System
Reacts immediately after infection- does not need to ID pathogen.
Barrier Defenses: Skin and Mucous membranes
Inflammatory Defenses:
Histamine is released at the sign of damage
Blood vessels leak fluid and WBC’s
Cellular and Molecular Defenses:
Macrophages: Use pocket transport (phagocytosis) to destroy foreign particles.
Natural Killer Cells (NK): Release hydrolytic enzymes onto target cells to rupture/destroy them.
Interferon
Complement

4. Final Defense: Specific Immune System
Recognizes pathogens and develops a sustained immune response.
Comprised of two parts:
Cell- Mediated Response
Humoral Response
White blood cells characters (lymphocytes):
Helper T cells (Th)
Killer T cells (Tc)
B cells
Macrophage

5. Specific Immunity- The Battle Begins!
Macrophages search body tissues for pathogens.
Consume pathogens with phagocytosis, kill it with lysosomes, and save the antigens.
Antigens placed into MHC 2 receptors and displayed on macrophage’s membrane.
The macrophage is now considered an antigen-presenting cell (APC).

7. Specific Immunity Cont.
Macrophage chemically signals Helper T to attach to it.
Helper T attaches to the MHC 2 receptor (with foreign antigen stuck in it) with its CD4 receptor.
Helper T cells have incredible variety of receptors that act like a “lock and key” in regards to the displayed antigen.
If the Helper T’s “key” fits the displayed antigen’s “lock”, the Helper T is activated.
Activation results in Helper T releasing cytokines (ex. Interleukin)- chemicals that cause lymphocytes to start mitosis.

8. Antigen- presenting cell Peptide antigen Bacterium
Fig
Antigen-
presenting
cell
Peptide antigen
Bacterium
Class II MHC molecule
CD4
TCR (T cell receptor)
Helper T cell
Cytokines +
Humoral
immunity
(secretion of
antibodies by
plasma cells) +
Cell-mediated
immunity
(attack on
infected cells)
Figure The central role of helper T cells in humoral and cell-mediated immune responses + +
B cell
Cytotoxic T cell

9. Cell-Mediated Response Seek and Destroy
Body cells can be infected by viruses that will hide inside the cell.
As the virus reproduces inside cells, pieces of it fall off and are put into new MHC 1 receptors that the cell puts on its own membrane.
Killer T cells can bind to an infected cell’s MHC 1 receptors with their CD8 receptors.
If Killer T binds to MHC 1 receptors with antigen attached, it releases a chemical called perforin.
Perforin ruptures the infected cells membrane and exposes the virus to other immune cells.

11. Top view: binding surface exposed to antigen receptors
Fig
Top view: binding surface
exposed to antigen receptors
Antigen
Class I MHC
molecule
Antigen
Figure Antigen presentation by an MHC molecule
Plasma
membrane of
infected cell

12. Released cytotoxic T cell
Fig
Released cytotoxic T cell
Cytotoxic T cell
Perforin
Granzymes
CD8
TCR
Dying target cell
Class I MHC
molecule
Pore
Figure The killing action of cytotoxic T cells
For the Discovery Video Fighting Cancer, go to Animation and Video Files.
Target
cell
Peptide
antigen

13. Infected cell Microbe Antigen- presenting cell 1 Antigen associates
Fig
Infected cell
Microbe
Antigen-
presenting
cell 1
Antigen
associates
with MHC
molecule
Antigen
fragment
Antigen
fragment 1 1
Class I MHC
molecule
Class II MHC
molecule 2 2
T cell
receptor
T cell
receptor 2
T cell
recognizes
combination
Figure The interaction of T cells with antigen-presenting cells
(a)
Cytotoxic T cell
(b)
Helper T cell

14. Humoral System Bring in the artillery!
B cells have receptors called antibodies (100,000/cell).
Different B cells have uniquely shaped antibodies that match specific antigens.
If a B cell’s antibody is able to bind with a specific antigen (lock and key effect), the B cell receives a message from Helper T’s to become activated.
Activated B cells divide into Plasma B and Memory B cells.

15. Humoral System Cont.
Plasma B cells produce and secrete 10,000 “keyed” antibodies per hour.
Due to their shape, each can bind to several antigens at once.
Antigen/Antibody binding has three effects.
Neutralization
Macrophage signaling
Complement pore formation

16. Figure 43.21 Antibody-mediated mechanisms of antigen disposal
Viral neutralization
Opsonization
Activation of complement system and pore formation
Bacterium
Complement proteins
Virus
Formation of
membrane
attack complex
Flow of water
and ions
Macrophage
Pore
Figure Antibody-mediated mechanisms of antigen disposal
Foreign
cell

17. Memory B cells These cells do not actively produce antibodies
Instead, they remain in the bloodstream and maintain their cell life cycle independently from Th commands.
If the same pathogen/antigen complex presents itself in the future, these cells are already activated and ready to produce antibodies.
There are also Memory versions of Th and Tc cells that serve a similar function.