1. Humoral and Cell Mediated Immunity
Chapter 35
Humoral and Cell Mediated Immunity

2. You Must Know The differences in humoral and cell-mediated immunity.
Why helper T cells are central to immune response.

3. Pathogens (such as bacteria, fungi, and viruses)
INNATE IMMUNITY
(all animals)
Barrier defenses:
Internal defenses:
B and T lymphocytes produce a humoral immune response and a cell-mediated immune response. In the humoral immune response, antibodies help neutralize or eliminate toxins and pathogens in the blood and lymph In the cell-mediated immune response specialized T cells destroy infected host cells.
ADAPTIVE IMMUNITY
Humoral response:
(vertebrates only)
Antibodies defend against infection in body fluids.
Cell-mediated response:
Cytotoxic cells defend against infection in body cells.
Crash Course Video 3 3

4. Helper T Cells: A Response to Nearly All Antigens
A type of T cell called a helper T cell triggers both the humoral and cell-mediated immune responses.
Helper T cell
Antigen receptor

5. Cell-mediated immunity
The central role of helper T cells in humoral and cell-mediated immune responses
Antigen- presenting cell
Antigen fragment
Pathogen
Class II MHC molecule 1
Antigen receptor
Helper T cell
Antigen receptor
Helper T cell
Cytokines
B cell
Cytokines 2
Cytokines
Cytotoxic T cell
An antigen must be displayed on the surface of an antigen-presenting cell and bind specifically to the antigen receptor of a T cell. Antigen-presenting cells (e.g. macrophages) have class I and class II MHC molecules on their surfaces. Antigen-presenting cells are recognized based on their class II MHC molecules. Antigen receptors on the surface of helper T cells bind to the antigen and the class II MHC molecule. Signals are then exchanged between the two cells. The helper T cell is activated, proliferates, and forms a clone of helper T cells, which then activate the appropriate B cells and cytotoxic T cells.
Humoral immunity
Cell-mediated immunity
video 5

6. Cytotoxic T cells are the effector cells in the cell-mediated immune response. Cytotoxic T cells recognize fragments of foreign proteins produced by infected cells and possess an accessory protein that binds to class I MHC molecules. The activated cytotoxic T cell secretes proteins that disrupt the membranes of target cells and trigger apoptosis. 1 6

7. Secreted antibodies Secreted antibodies
Helper T cell
Antigen fragments
Pathogen
Antigen- presenting cell
Antigen receptor
MHC 1
B cell
Cytokines
Activated helper T cell 2
Plasma cells
Secreted antibodies
Memory B cells
The humoral response is characterized by secretion of antibodies by clonally selected B cells. Activation of B cells involves helper T cells and proteins on the surface of pathogens. In response to cytokines from helper T cells and an antigen, a B cell proliferates and differentiates into memory B cells and antibody-secreting effector cells called plasma cells.
Plasma cells
Secreted antibodies 7

8. Antibodies don’t kill pathogens directly.
Antibodies do not kill pathogens; instead, they mark pathogens for destruction. In neutralization, antibodies bind to viral surface proteins, preventing infection of a host cell. Antibodies may also bind to toxins in body fluids and prevent them from entering body cells.

9. HIV infects helper T cells!

10. Figure 35.16
Humoral (antibody-mediated) immune response
Cell-mediated immune response
Key
Antigen (1st exposure)
Stimulates
Engulfed by
Gives rise to
Antigen- presenting cell
Cytotoxic T cell
B cell
Helper T cell
Memory helper T cell
Both the humoral and cell-mediated responses can include primary and secondary immune responses. Memory cells enable the secondary response.
Antigen (2nd exposure)
Plasma cells
Memory B cells
Memory cytotoxic T cells
Active cytotoxic T cells
Secreted antibodies
Defend against extracellular pathogens
Defend against intracellular pathogens and cancer 10