Histology of the Immune System

Lymphoid Cells and their Accessories – primary mediators of immune reactions

subtypes based upon isotypic commitment and T cell dependence/independence status

strategies for distinguishing lymphoid cells (difficult to distinguish based on morphology alone)

recombines activating genes (RAG)
terminal deoxynucleotidyl transferase (tDT) and purine nucleoside phosphorylase (PNP)
surface markers (e.g., surface immunoglobulin (sIg), T cell receptors (TCR), "cluster of differentiation" (CD) and histocompatibility (MHC) antigens)

Lymphoid Ontogeny

two distinct phases:

Phase I: acquisition of immunocompetence (antigen specificity and MHC restriction)

occurs in central lymphoid organs – bone marrow (bursa) or thymus

stem cell in bone marrow Þ pre B Þ immature B Þ small Þ mature B during myelogenous phase (mammals) – B cells mature, then leave bone marrow

stem cell in bone marrow Þ pre-T prothymocyte Þ thymocyte Þ mature T in bone marrow followed by thymus – T cells leave before maturation and go to thymus

Phase II: clonal expansion and terminal differentiation (driven by antigen and accessory cells)

occurs in peripheral lymphoid organs (spleen, lymph nodes, mucosa-associated lymphoid tissue)
follicles in these organs contain B lineage cells and parafollicular zones that contain T cells
follicles may have germinal centers of blastic cells, surrounded by a mantle zone of quiescent cells

accessory cells (macrophages, DRC, IDC) are critical in both phases

Other Lymphoid Cells (non-B, non-T)

include K cells and NK cells

numerically less conspicuous, yet have immunologically significant roles

Thymus Lobule

Collagen Þ cortex (T cells mature – positive selection) Þ medulla (negative selection) Þ Hassel’s corpuscle

Lymph Node – where T and B cells meet

fibrous capsule Þ cortex Þ medulla; mantle zone: contains mature B cells; geminal center: where B and T cells meet

spleen – secondary lymphoid tissue structure

reticular meshwork