H2 Biology Extension A - The Immune Response: How B Cells and T Cells Defend the Body

Step 1 - Innate versus adaptive immunity

When a pathogen enters the body, the first line of defence is innate immunity.

Step 1 - Innate versus adaptive immunity

This includes physical barriers like the skin and mucous membranes, chemical defences such as lysozyme in tears, and cellular responses like phagocytosis by neutrophils and macrophages.

Step 1 - Innate versus adaptive immunity

Innate immunity is non-specific - it responds the same way to all pathogens and does not improve with repeated exposure.

Step 1 - Innate versus adaptive immunity

If the pathogen overcomes innate defences, the adaptive immune system is activated, providing a specific and targeted response.

Step 2 - Antigen presentation triggers the adaptive response

The bridge between innate and adaptive immunity is antigen presentation.

Step 2 - Antigen presentation triggers the adaptive response

After a macrophage engulfs and digests a pathogen by phagocytosis, it processes the pathogen's proteins and displays fragments of them as antigens on its surface using M H C class two molecules.

Step 2 - Antigen presentation triggers the adaptive response

This macrophage is now called an antigen presenting cell.

Step 2 - Antigen presentation triggers the adaptive response

Helper T cells with complementary T cell receptors recognise and bind to the antigen M H C complex, which activates the helper T cell and triggers the full adaptive immune response.

Step 3 - The humoral response: B cells and antibody production

Activated helper T cells stimulate B cells that carry surface antibodies matching the same antigen.

Step 3 - The humoral response: B cells and antibody production

These B cells undergo clonal selection, then rapidly divide by mitosis in a process called clonal expansion.

Step 3 - The humoral response: B cells and antibody production

Most daughter cells differentiate into plasma cells, which are antibody factories that secrete large quantities of specific antibodies into the blood and tissue fluid.

Step 3 - The humoral response: B cells and antibody production

Some daughter cells become memory B cells, which persist in the body for years and enable a faster, stronger secondary immune response upon re-infection.

Step 4 - The cell-mediated response: killer T cells

The cell mediated response targets cells that are already infected.

Step 4 - The cell-mediated response: killer T cells

Killer T cells, also called cytotoxic T cells, recognise antigens displayed on M H C class one molecules on the surface of infected body cells.

Step 4 - The cell-mediated response: killer T cells

Once activated by helper T cells, killer T cells release perforin, a protein that creates pores in the membrane of the infected cell, causing it to lyse and die.

Step 4 - The cell-mediated response: killer T cells

This is crucial for dealing with intracellular pathogens like viruses that hide inside host cells, where antibodies cannot reach them.

Step 5 - Primary versus secondary immune response

The first time the body encounters a pathogen, the primary immune response is slow because only a few lymphocytes recognise the antigen.

Step 5 - Primary versus secondary immune response

It takes several days for clonal expansion to produce enough plasma cells and antibodies to clear the infection.

Step 5 - Primary versus secondary immune response

On second exposure to the same antigen, memory cells are rapidly activated.

Step 5 - Primary versus secondary immune response

The secondary immune response is faster, produces a much higher concentration of antibodies, and often clears the pathogen before symptoms develop.

Step 5 - Primary versus secondary immune response

This is the principle behind vaccination - exposing the body to a harmless form of the antigen to generate memory cells without causing disease.