Adaptive Immune System
Lymphocyte Histology
- On histology, lymphocytes possess a large nucleus and agranular cytoplasm (they are agranuloctyes).
- Note that agranulocytes can contain granules (as we'll see with natural killer cells) just not to the density that we see in standard granulocytes.
B-cells
- B cells provide humoral-mediated immunity.
- B cells are derived from the bone marrow and generate humoral immunity via antibody production.
- They populate key secondary lymphoid organs (the lymph nodes and spleen), as well as other lymphoid tissues: tonsils, small intestine Peyer’s patches, etc…
- When antigen binds to B cell receptors (BCRs), it triggers B cell clonal expansion and antibody secretion, as well as long-term immunologic memory via memory B and T lymphocytes and antibody-secreting plasma cells.
T-cells
- T cells provide cell-mediated immunity.
- T-cells are produced in the bone marrow but mature in the thymus.
- They express an antibody-shaped antigen-binding T-cell Receptor (TCR).
Show that we subdivide them into two broad categories of T cells:
- CD4+ helper T cells.
- CD8+ cytotoxic T cells.
- Indicate that CD4+ T cells are activated by antigen presentation on MHC II (major histocompatibility complex class II) molecules.
- Show that dendritic cells, macrophages, and B cells, are all professional antigen presenting cells (APCs), which present degraded antigen proteins (peptides) on MHC II molecules.
- CD4+ T-cells produce cytokines and characteristic transcription factors. There are many subtypes of helper T cell. We can divide them into 3 different T helper subsets (Th1, Th2, Th3), regulatory T cells, and follicular T cells which help B cells produce antibody (although science continues to discover new subsets).
- Then, show that CD8+ cytotoxic T cells recognize peptides on MHC I molecules. Note that natural killer cells, which we address next, have a different interaction with MHC I molecules.
- Indicate that all nucleated (and some nonnucleated cells) display degraded protein peptides on MHC I molecules, which activate CD8+ cytotoxic T cells.
- CD8+ T-cells trigger cell-death (apoptosis) of pathogenic cells: eg, viral-infected cells, foreign tissues, and tumor cells.
Natural killer (NK) cells
- Natural killer (NK) cells act both as part of the innate and adaptive immune systems.
- Their innate functions include perforin and granzyme release to kill infected cells and trigger release of inflammatory cytokines.
- NK cells comprise a significant amount of cytoplasmic granules, but the density of this granule population is sparse enough that NK cells are still considered agranulocytes.
- Their adaptive immune functions include their antigen specificity, ability for clonal proliferation, and their immunologic memory, which parallels that of T and B cells.
- Natural killer cells (NK cells) also mediate an important function of antibody-dependent cellular cytotoxicity (ADCC).