Specific immunity

What is an antigen? Give two examples of molecules or structures that can act as antigens.

State what lymphocytes are, in which organ all of them are formed and in which organs B lymphocytes and T lymphocytes mature.

Observe the diagram and state which of the two cells (A or B) is a B lymphocyte and which is a T lymphocyte. Explain what each is doing and to which type of specific immunity its action corresponds.

Describe the basic structure of an antibody (immunoglobulin): how many chains form it, what shape it has, and which part of the molecule recognises and binds to the antigen.

Distinguish between antigen and antibody, and explain what it means that the binding between them is specific, comparing it to a key and its lock.

Antibodies do not destroy pathogens directly: they mark them and neutralise them. Explain two ways in which an antibody helps to eliminate a pathogen (neutralisation, opsonisation, agglutination or complement activation).

When a B lymphocyte encounters the antigen it recognises, it proliferates forming a clone. Explain what clonal selection consists of and what two types of cells are generated from that activated lymphocyte, stating the function of each one.

Each lymphocyte has receptors for a single antigen, but the organism can respond to millions of different antigens. Explain how that enormous diversity of receptors is generated during the maturation of lymphocytes (rearrangement of the immunoglobulin genes) and why it is not necessary to have a different complete gene for each antibody.

A laboratory wants to obtain monoclonal antibodies, that is, identical antibodies directed against a single antigenic determinant. Explain why a single clone of B lymphocytes produces monoclonal antibodies and cite two applications of these antibodies in medicine (diagnosis or treatment).

Explain what cytotoxic T lymphocytes (CD8) do and why they are particularly important against virus-infected cells and tumour cells.

MHC class I molecules present antigens produced inside the cell (for example, from a virus), and class II molecules present antigens that have been phagocytosed. State which type of T lymphocyte recognises each class of MHC and reason why this distribution makes biological sense.

In an in vitro experiment, mature B lymphocytes and helper T lymphocytes (CD4+) from a mouse previously immunized against an extracellular bacterial toxin are incubated separately. The purified toxin in its native conformation is added to both cell-free cultures.

1. a) After 48 hours, it is observed that B lymphocytes proliferate and secrete specific antibodies, whereas CD4+ T lymphocytes remain completely inactive. Justify this result based on the structural nature of BCR and TCR receptors and the activation requirements of each cell lineage.
2. b) The T lymphocyte assay is repeated by adding macrophages from the same mouse that have previously phagocytosed and processed the toxin. Predict what will happen in this case and what would happen if the added macrophages came from a mouse strain with incompatible MHC alleles. Justify your answer considering the phenomenon of MHC restriction.

What are immunological memory cells? Explain why, after suffering certain diseases such as measles, one does not usually fall ill with the same disease again.

In a clinical trial, the serum concentration of antibodies (IgM and IgG) is quantified in a volunteer after injecting them with antigen X on day 0 and a mixture of antigens X and Y on day 28. The following results are obtained:

• Day 7: IgM against X is detected (titer 1:64) along with trace amounts of IgG against X.

• Day 35 (7 days after the 2nd injection): the titers against X are IgM (1:32) and IgG (1:2048), whereas against Y only IgM (1:64) is detected.

1. a) Interpret these results, justifying what type of immune response (primary or secondary) has occurred against each antigen after day 28 and which cells are responsible for the dramatic increase in anti-X IgG.
2. b) Predict what would happen to the IgG and IgM titers against Y if a new dose of antigen Y is administered on day 56. Justify your answer in terms of immunological memory and explain its implication in the design of vaccination schedules with booster doses.

The laboratory of a field hospital has red blood cell concentrates and plasma units from donors of groups A, B, AB, and 0 (all Rh+). Two polytrauma patients arrive requiring urgent support:

• Patient 1 (group AB+): requires an urgent transfusion of red blood cell concentrate, but the blood bank has run out of group AB+ red blood cells.

• Patient 2 (group 0+): requires an urgent plasma transfusion (not red blood cells).

1. a) Justify with reasoning which type of red blood cells (A, B, or 0) you could safely transfuse to Patient 1 and what immunological mechanisms would cause a hemolytic shock if an incompatibility occurred.
2. b) A novice technician proposes transfusing group 0 plasma to Patient 2, stating that "group 0 is the universal donor and never causes problems." Refute their statement by analyzing what the plasma of a group 0 donor contains, and deduce which type of plasma (A, B, AB, or 0) would be suitable for Patient 2 and whether, in this specific case, the technician's proposal is acceptable.