Lecture 2: Acquired Immunity

Key Points:

Movie of B and T cell Interaction
Top frames shown Ca2+ levels in T-cells, red means high Ca2+. Bottom frames show GFP labeled B Cell.

2. Molecules and Cells of the Acquired Immune System.

Humoral Immunity: soluble factors in the serum (e.g. antibodies)

Cellular Immunity: associated with cells

2.1 B-cells:

2.2 T-cells:

T-helper cells (TH):

Cytotoxic T cells (TC)


2.3 MHC Complexes: Heterodimeric and membrane bound.


Class I - MHC

Class II - MHC


a 1-a 2-a 3, b 2 microglobulin

a 1-a 2, b 1-b 2

Type of cell

All cells

Antigen Presenting Cells (APC):


-dendritic cells

-B lymphocytes

Recognized by:

TC cells

TH cells

Associated with:

-viral infection

-tumor cells

-transplant rejection

-bacterial infection

-viral infection

-protein allergins

-other pathogens


2.4 Development of Immune Cells:

3. Example of Humoral and Cellular Cooperation In Bacterial Infection (A day in the life of a B-cell)

  1. Progenitor B-cell develops into mature B cell in bone marrow, developing a unique antibody on its surface.
  2. If the antibody does not recognize self, then the mature B-cell is allowed to leave the bone marrow (107 new B-cells are produced/day).
  3. B-cells that encounter a foreign particle that can bind to the immunoglobulin on the cell surface become stimulated. If the B-cell does not have a successful encounter it dies within a few days.
  4. In addition to B-cells, macrophages and dendritic cells may also ingest foreign particle.
  5. Bound bacteria are internalized by endocytosis (B-cells), or phagocytosis.
  6. Bacterial peptides, complexed to class II MHC molecules, are presented on the surface of B-cells, macrophages, and dendritic cells.
  7. Specific TH-cells recognize foreign peptide bound to class II MHC, via CD4/T-cell receptor-MHC/peptide interactions, leading to activation of the TH-cell, cell division of that particular TH-cell, and subsequent formation of memory TH-cells.
  8. Activated TH-cells secretes cytokines that activate B-cells.
  9. Cytokines induce the differentiation & cell division of B-cells into antibody secreting plasma cells and memory B-cells.
  10. Plasma cells can secrete 103 molecules of antibody/sec! Fortunately, plasma cells live only a few days, but can produce about 1010 antibodies in that time.
  11. Antibody produced by plasma cells coats (opsinizes) bacteria, leading to more efficient phagocytosis of the foreign particles as well as cell destruction by complement.
  12. Subsequent infection by the same bacteria will lead to a more rapid response because of the presence of memory T- and B-cells. The primary response occurs within 14 days. The secondary response occurs within 3 or 4 days and produces 10-100 fold more antibody.
  1. Note that in four days a single E. coli will double about 200 times, giving rise to more bacterial cells than the number of cells in the human host. Clearly, innate responses play an important role.
  2. The activation of B- and T-cells by foreign antigen leads to an increase in the number of both types of cells. This antigen-dependent amplification of cells is referred to as clonal selection, since a specific sub-population, or clone, of B and T cells is involved.

3. Organs of the Immune System:

3.1 Primary organs:

3.2 Secondary organs:


Lymphatic system:

Mucosal-Associated Lymphoid Tissue:

3.3 Circulation Though the Lymphatic System.