Lymphocyte Development

During lymphocyte development, as the cells differentiate from the hematopoietic stem cells in the bone marrow into either B cells or T cells, they acquire their ability to recognize distinct epitopes. Then, once they have committed to that specificity, they pass through rigorous checkpoints intended to ensure that their antigen-specific receptor is both functional and that it will not evoke a response against "self" molecules. Most developing lymphocytes fail these tests and, as a consequence, are induced to undergo apoptosis. ■ hematopoietic stem cell, p. 376

B cells undergo the developmental stages described in this section in the bone marrow; T cells go through this maturation process in the thymus. The events involved in the adaptive immune response, from the maturation of lymphocytes to the development of their effector functions, are summarized in figure 16.21.

Primary lymphoid organ

(bone marrow) Immature B cells

Mature (naive) B cells

Primary lymphoid organ

(thymus) Immature T cells

Mature (naive) T cells

Tissues

Tissues

Travel to secondary lymphoid tissue, developing into antigen-presenting cells; co-stimulatory molecules are expressed if antigen represents microbial invasion or tissue damage.

Dendritic cells gather antigen

Free antigen

Free antigen

Plasma cells produce antibody; depending on site of infection and duration of exposure, the class may be IgM, IgG, IgA, or IgE

Travel to secondary lymphoid tissue, developing into antigen-presenting cells; co-stimulatory molecules are expressed if antigen represents microbial invasion or tissue damage.

Antigen presented by MHC Class I molecules in the presence of co-stimulatory molecules activates naive T-cytotoxic cells.

Dendritic cells gather antigen

Antigen presented by MHC Class I molecules in the presence of co-stimulatory molecules activates naive T-cytotoxic cells.

Antigen presented by MHC Class II molecules in the presence of co-stimulatory molecules activates naive T-helper cells.

Antigen presented by MHC Class II molecules in the presence of co-stimulatory molecules activates naive T-helper cells.

T-dependent antigens

Plasma cells produce antibody; depending on site of infection and duration of exposure, the class may be IgM, IgG, IgA, or IgE

Th 1 cells activate macrophages that present antigen via MHC class II molecules; also produce cytokines that orchestrate other responses

Secondary lymphoid organs

Activation, proliferation, development of effector T cell functions

Naive T-cytotoxic (CD8) cells

Naive T-cytotoxic (CD8) cells

Naive T-helper (CD4) cells

Activation, proliferation, development of effector T cell functions

Naive T-helper (CD4) cells

Activation, proliferation, development of effector T cell functions

Memory T-cytotoxic cells

Effector

T-cytotoxic cells

Memory T-helper cells Effector T-helper cells

Th2 Th1

Naive B cells

Naive B cells

Activation, proliferation, class switching, affinity maturation

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