T cells are needed to control intracellular pathogens and to activate B-cell responses to most antigens Autoantibodies against extracellular antigens cause inflammatory injury by mechanisms akin to type II and type III hypersensitivity reactions

Sophisticated means of host defense were hard-wired in the genome by the time organisms diverged into plants and animals Memory T cells are increased in frequency and have distinct activation requirements and cell-surface proteins that distinguish them from armed effector T cells The induction of a tissue-specific response requires the presentation of antigen by antigen-presenting cells with co-stimulatory activityStages in B-cell development are distinguished by the expression of immunoglobulin chains and particular cell-surface proteins

Antigenic peptides bound to self MHC class II molecules trigger armed helper T cells to make membrane-bound and secreted molecules that can activate a B cellThe variable chains of lymphocyte antigen receptors are associated with invariant accessory chains that carry out the signaling function of the receptor Lymphocyte survival is maintained by a balance between death-promoting and death-inhibiting members of the Bcl-2 family of proteins The DNA sequence encoding a complete V region is generated by the somatic recombination of separate gene segments Malignant lymphocyte tumors frequently carry chromosomal translocations that join immunoglobulin loci to genes regulating cell growth