KEY POINTS

T Cells (T Lymphocytes)

1. Mature T cells develop from lymphoid stem cells that have migrated to the thymus.

2. T cells are a heterogeneous population of cytotoxic T cells and T-helper cells.

Interactions OfAntigen-Presenting Cells and T Cells

1. Major histocompatability complex (MHC) proteins are considered the central regulators of the immune system.

2. MHC class I antigens of infected nucleated cells play a major role in activating cytotoxic T cells.

3. MHC class Il antigens on antigen-presenting cells (APC) play a major role in selective activation of T-helper cells.

Antibodies

1. Initial exposure to foreign antigen induces slow onset of antibody appearance, whereas subsequent exposure induces faster, longer-lasting antibody appearance.

2. Antibodies, or immunoglobulins, are glycoprotein molecules that can be divided into five isotypes, or classes.

3. The B-cell population produces antibodies to millions of different antigens, yet the antibody-antigen interaction is specific.

4. Expansion of an antigen-specific B-memory cell population on initial antigen exposure results in a faster, more pervasive, secondary immune response.

Regulation of Immune Responses

1. The actions, secretions, and surface molecule expression of immune cells play an important role in regulation of the body's immune response.