Acquired (specific, adaptive) immunity

• If the second line fails to remove the invader, the body uses a third line of defence: lymphocytes that carry out a series of specific responses called the immune response.
• It takes two interconnected forms: humoral (antibody-mediated) and cellular (cell-mediated) immunity.

A) Humoral (antibody-mediated) immunity

• Targets pathogens and toxins in the body fluids (blood plasma and lymph) – bacteria, viruses outside cells, toxins – by making antibodies.
• The steps:

1. A pathogen carrying a specific antigen enters. B lymphocytes recognise it – each B cell is specific and responds to only one antigen. Every B-cell type has one kind of immune receptor, with the same Y-shape as the antibody it will later secrete. On binding, the B cell takes in the pathogen, breaks it into antigens, and these combine inside the B cell with a protein called the major histocompatibility complex (MHC). The antigen–MHC complex moves to the cell surface and is displayed.
2. At the same time macrophages engulf the pathogen, digest it with lysosomal enzymes into small fragments (antigens), bind these to MHC inside the macrophage, and display the antigen–MHC complex on the outer surface of the plasma membrane.
3. Helper T cells (T_H) recognise the antigen–MHC complex on the macrophage and bind to it. This activates the T_H cell to release interleukins, which in turn activate the B cells that carry the same antigen bound to MHC. (Note: helper T cells can recognise an antigen only after it has been processed and displayed on MHC.)
4. The activated B cells divide, multiply and differentiate into memory cells and plasma cells. Plasma cells produce huge quantities of antibodies that travel via lymph and blood to the infection site. Memory cells stay in the blood for a long time (the book says 20–30 years) and recognise the same antigen if it returns; they then divide quickly into new memory and plasma cells and give a faster response than the first.
5. Antibodies bind antigens on the invading pathogens, which stimulates macrophages to engulf them again. This continues for days or weeks.

• Antibodies are fairly large molecules and cannot enter cells, so they are not effective against microbes inside cells (for example virus-infected cells).
• Those must be dealt with by T lymphocytes.

B) Cellular (cell-mediated) immunity

• Carried out by T lymphocytes, each of which has surface immune receptors giving it a specific response to one type of antigen presented by macrophages.
• Steps:

1. A pathogen (bacterium or virus) enters; macrophages engulf it, break it into fragments and display the antigen–MHC complex.
2. A T_H cell with the matching receptor binds the complex and becomes active. It releases interleukins, which induce the bound T_H cells to divide, forming a clone of activated T_H cells and memory T_H cells that last long in the blood.
3. Activated T_H cells secrete cytokines, which: (a) attract macrophages to the infection site in large numbers; (b) stimulate macrophages, cytotoxic T cells and B cells – thus activating both cellular and humoral immunity; (c) activate natural killer cells to attack abnormal cells such as cancer cells or infected cells.
4. Cytotoxic T cells (T_C) use their specific receptors to recognise foreign cells – transplanted tissue, cells carrying microbial antigens, or cancer cells – and destroy them, either by secreting perforin, which makes pores in the target membrane, or by secreting lymphotoxins that switch on certain genes in the target cell's nucleus, leading to nuclear destruction and cell death.

C) Switching the response off

• After the antigen has been destroyed, suppressor T cells (T_S) bind (using their own specific receptors) to plasma cells, helper T cells and cytotoxic T cells and secrete lymphokines.
• These suppress or halt the immune response: plasma cells stop making antibodies and many T_H and T_C cells die.
• Some are kept in the lymphoid organs, ready to respond quickly to a similar infection.