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

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).