Immunity and vaccination: phagocytosis, the specific response and antibody questions

Non-specific first: phagocytosis

• The phagocyte is attracted by chemicals released by the pathogen (chemotaxis) and binds to antigens on its surface.
• The cell membrane engulfs the pathogen, enclosing it in a phagosome (a vesicle).
• A lysosome fuses with the phagosome and releases lysozymes, which hydrolyse the pathogen.
• Soluble products are absorbed; antigens are displayed on the phagocyte's surface, making it an antigen-presenting cell.

The cellular response

A helper T cell with a complementary receptor binds to the presented antigen. This activates it to divide by mitosis, producing clones that stimulate phagocytes, activate cytotoxic T cells, stimulate B cells, and form memory T cells. Cytotoxic T cells kill infected body cells by releasing perforin, which makes holes in the cell membrane so it is freely permeable and the cell dies.

The humoral response

A B cell with a complementary antibody on its surface takes in the antigen and presents it. Helper T cells bind and activate it — this is clonal selection. The activated B cell divides by mitosis to form clones: plasma cells, which secrete large quantities of the specific antibody, and memory B cells, which remain in the blood for years.

The secondary response is faster and larger because memory cells are already present in higher numbers and divide rapidly into plasma cells, so antibody concentration rises sooner and the pathogen is destroyed before symptoms appear.

Antibody structure and action

• Four polypeptide chains — two heavy, two light — held by disulfide bridges, with variable regions forming two antigen-binding sites.
• The variable region's tertiary structure is complementary to one antigen, so binding is specific and forms an antigen–antibody complex.
• Agglutination: each antibody binds two pathogens, clumping them so phagocytes engulf many at once.

Vaccines, herd immunity and antigenic variability

A vaccine introduces a dead or attenuated pathogen, or an antigen from it, so memory cells form without the disease. Herd immunity means that when a large enough proportion of the population is vaccinated, transmission is reduced so much that unvaccinated people — babies, the immunosuppressed — are also protected.

Antigenic variability explains why the flu vaccine changes yearly: mutation changes the antigen's shape, so existing memory cells and antibodies are no longer complementary and the secondary response does not occur.

Monoclonal antibodies and the ELISA test

• Monoclonal antibodies are identical antibodies from one clone of plasma cells, used to target a specific antigen — for example to deliver a cytotoxic drug only to cancer cells, reducing side effects.
• In a direct ELISA, an antigen is attached to a well and a specific antibody with an attached enzyme binds to it; a substrate is added and a colour change shows the antigen was present.
• Washing between steps is essential — it removes unbound antibody, which would otherwise give a false positive.

Ethical evaluation marks

Questions often close with 'discuss'. Award-worthy points include: testing on animals and human volunteers carries risk; vaccine side effects must be weighed against disease risk; expensive monoclonal antibody treatments raise questions about who receives them; and consent must be informed. Write one point for and one against, then a conclusion referring back to the data given.