Question

9-11

9. See the figure below for 9 i-lii (22 points total). 100 Total Primary response Secondary response Antibody concentration in serum, units per ml Total IgG 0.1 IgM IgG IgM Klag 0.01 1 2° Ag 1° Ag Time after immunization 9 i) During the lag period in the graph above, why are there no antibodies being produced? (8 points)

Answer 1

9. i) When an individual is exposed to an antigen, there is an initial lag phase or latent phase before the antibodies are produced. During this lag phase no antigen specific antibody can be detected in the blood. In the lag phase B cells undergo differentiation into the antibodies producing plasma cells and once the B cells have differentiated into plasma cells, antibody is secreted and can be detected.

9 ii) The primary immune response leads to the formation of memory cells with high specificity for the inducing antigen. The primary response primes the immune system so that it possesses specific immunological memory through its clones of memory B cells, each of which responds to different antigen. Antibodies generated during secondary immune response attain affinity maturation during which a small cluster of B cells undergo changes including class- switching and sequence diversification.

10. V(D)J recombination is mediated by an enzyme complex called V(D)J recombinase. V(D)J recombinase ensures that only appropriate gene segments recombine by recognising conserved DNA sequence elements, term recombination signal sequences (RSS), located adjacent to each V, D, and J, coding sequences.Recombination signal sequences (RSS) consist of nine (nonamer) and seven (heptamer) base pairs flanking each Is gene segment.The region between the nonamer and heptamer has either 12 or 23 nucleotides and signals which segment may be linked.

11. Primary Lymphoid Organ : Thymus
The thymus is the primary lymphoid organ where maturation of the T cells take place. T-cell development is not complete until the cells undergo selection in the thymus. Thymus is bilobed encapsulated organ located behind the sternum and in front of the heart. The thymus is a specialized environment where immature T cells, known as thymocytes, mature into functional T cells by passing through well-defined developmental stages in well-defined microenvironments.

Secondary Lymphoid Organ: Lymph nodes

They are encapsulated, bean-shaped structures that include networks of stromal cells (i.e., support tissue) packed with lymphocytes, macrophages, and dendritic cells. Connected to both blood vessels and lymphatic vessels, lymph nodes are the first organized lymphoid structure to encounter antigens that enter the tissue spaces. The lymph node provides ideal microenvironments for encounters between antigen and lymphocytes and productive, organized cellular and humoral immune responses.

Primary lymphoid organs differ from the secondary lymphoid organs in being the site where lymphocytes undergo maturation whereas secondary lymphoid organs provide the microenvironment  

to the mature lymphocytes to interact with the antigen and become the effector cells.