the CD8+ T cell TCR to the peptide-class I MHC complex on professional antigen-presenting cells (e.g., dendritic cells) activates the CD8+ T cells which then respond against cytosolic infections such as viruses, intracytoplasmic bacteria, and protozoa (Harty et al., 2000). Activated CD8+ T cells induce death of infected cells through mechanisms that include (1) release of granules containing the pore-forming molecular perforin or (2) engagement of Fas receptors on target cells (Harty et al., 2000). Both mechanisms induce apoptosis, or programmed cell death, in the target cell. In addition, activated CD8+ T cells secrete cytokines, molecules critical to intercellular communication, that recruit and activate macrophages and neutrophils (Harty et al., 2000).
In contrast to CD8+ T cells, CD4+ T cells are predominantly activated in response to extracellular antigens that are endocytosed or phagocytosed, broken down into peptides, and bound to class II MHC molecules on the surface of professional antigen-presenting cells (Guermonprez et al., 2002). Class II MHC molecules are expressed on dendritic cells, macrophages, B cells, and activated T cells. The CD4+ T cells express TCRs that bind peptide-class II MHC complexes. Recognition of peptide antigen-MHC complexes activates CD4+ T cells against a variety of antigens including, but not limited to, bacteria, parasites, and proteins. Activated CD4+ T cells direct aspects of the immune response via the secretion of immunoregulatory cytokines and other soluble mediators. These inflammatory mediators can induce B cells to undergo immunoglobulin (Ig) class switching (e.g., IgM to IgE); to support the activity of CD8+ T cells; to recruit and activate eosinophils, basophils, neutrophils, mast cells, and macrophages; and to down-regulate immune responses (Koretzky, 2008; Wan and Flavell, 2009). Several lineages of CD4+ T cells, with overlapping and competing effects based on those described above, have been identified (Wan and Flavell, 2009). One CD4+ T cell lineage, referred to as regulatory T cells, functions to maintain self tolerance and immune homeostasis (Wan and Flavell, 2009). In addition, some CD4+ T cells can induce cytolysis via the mechanisms described for CD8+ T cells (Soghoian and Streeck, 2010).
In summary, T cells contribute to the establishment and maintenance of immune responses, the clearance of pathogens, and the maintenance of self-tolerance. T cells play roles in many disease processes including, but not limited to, rheumatoid arthritis, type 1 diabetes, and asthma (Wan and Flavell, 2009).
Effector Functions of Antibodies and Autoantibodies
Antibodies are antigen-binding proteins produced by terminally differentiated effector B cells called plasma cells. Antibodies that bind antigens