2. Specific immunity to infection
If the initial innate response to a foreign invader (such as a disease -causing pathogen) fails, the acquired immune system takes over.
The acquired system is extremely complex and consists of many interrelated components. Two key elements are
- B-cells which produce specific proteins called antibodies that neutralise the invader
- T-cells that attack the invader or regulate responses of other immune cells
There are many different T-cells, but important groups are:
- Killer cells destroy pathogen-infected cells and other ‘foreign’ cells
- Helper T-cells (Th) are regulators of cellular immunity. There are many types of Th cells, but the types important to this learning material are Th1, Th2 and Th17 cells
When pathogenic bacteria, viruses or fungi (each consisting of many specific antigens) enter the body, B cells respond by producing antibodies (immunoglobulins) specific for each antigen.
There are many different types of immunoglobulins, but in response to infection two important types are IgG and IgA
Th1 helper cells ‘assist’ some of the B cells to become plasma cells. Plasma cells rapidly divide and secrete more antibodies that neutralise invading pathogens, until the infection is controlled.
After recovery from an infection, the body retains some of the B and T lymphocytes (known as memory cells) which persist in the blood and lymph systems. If re-exposed to the same antigen threat, the memory cells mount a strong rapid immune response
This is the basis of vaccination e.g. flu virus, treated to destroy their disease-causing properties but not their antigenic properties, are injected to induce permanent resistance to the strain of flu