Human Physiology, 14th edition (2016)

The Immune System 513

Figure 15.17 Interactions between antigen-presenting cells, helper T cells, and killer T cells. (1) An antigen-
presenting cell activates a helper T cell by presenting it with a viral antigen in association with a class-2 MHC molecule. (2) The helper
T cell secretes interleukin-2, which promotes the proliferation of activated killer T cells. (3) The killer T cells destroy the cells infected
with the viruses.

Antigen-presenting cell

Interleukin-2

Virus

Viral antigen

Class-2 MHC

molecule

Helper T cell

Infected cell

Proliferation

Killer T cell

Viral antigen

Class-1 MHC

molecule

Infected cell

destroyed

1

2

3

Helper T cell

Antigen-presenting cell

Class-2 MHC

molecule

Activated

helper T cell

Class-2 MHC

molecule

Foreign antigen

B cell

Foreign antigen

B cell proliferation

and differentiation

Specific

antibody

Plasma cell

Memory cell

1

2

3

Figure 15.18 Interactions between antigen-

presenting cells, helper T cells, and B cells. (1) An antigen-

presenting cell activates a helper T cell by presenting it with a

foreign antigen in association with a class-2 MHC molecule.

(2) The activated helper T cell then interacts with a B cell specific

for that antigen. (3) This stimulates the B cell to divide and form

a clone, some members of which become antibody-secreting

plasma cells and some of which become memory cells.