11.1 Essential ideas

11.1.1 Antibody production in mammals

  • Every organism has unique molecules on the surface of its cells, called antigens, which, when recognised by the immune system, provoke an immune response.
  • Pathogens are agents of disease. Pathogens may be living organisms, such as bacteria or fungi, and they may also be non-living viruses or proteins.
  • Pathogens are normally species-specific although some can cross species barriers. All pathogens display antigens.
  • When an antigen is encountered, lymphocytes respond by making specific antibodies that bind to antigens, blocking their function and aiding in their destruction. This is the basis of adaptive immunity.

Antibodies aid in the destruction of pathogens

1. Pathogen neutralisation.  Antibodies block antigens from adhering to target cells. They may also block toxins produced by pathogens from entering target cells. Antibodies cause agglutination, or clumping, of pathogens.

Figure 11.1.1a – Antibodies block and neutralise antigens present on pathogens.Figure 11.1.1a – Antibodies block and neutralise antigens present on pathogens.

2. Opsonization.  Antibodies interact with receptors on the surface of phagocytes, making it easier for the pathogen to be engulfed by phagocytosis.

Figure 11.1.1b – OpsonizationFigure 11.1.1b – Opsonization

3. Complement activation.  Antibodies interact with complement proteins, initiating a series of metabolic events that eventually lead to the lysis of the pathogen.

Figure 11.1.1c – Complement activation and cascade Figure 11.1.1c – Complement activation and cascade

Application: ABO blood groups in humans

  • ABO blood types are defined by the type of antigen present on erythrocytes (red blood cells).

Figure 11.1.1d – ABO blood groups Figure 11.1.1d – ABO blood groups

  • It is important to know the ABO blood group for blood transfusions. Receiving blood with different surface antigens will trigger an immune response.
  • Prior to a blood transfusion, a sample is tested with serum to determine blood type. The sera contain anti-A or anti-B antibodies. Agglutination in the sample indicates the presence of the corresponding antigen. 

Blood type



Donates safely to

Receives safely from

Reaction to added antibodies







A, O

Figure 11.1.1e – ClumpFigure 11.1.1e








Both A and B

A, B, AB

All blood types


Neither A nor B

All blood types


The specific immune response produces antibodies

Figure 11.1.1f details the roles of different white blood cells in the immune response when attacked for the first time by a pathogen – in this case a virus.

Figure 11.1.1f – The primary immune responseFigure 11.1.1f – The primary immune response

  • B lymphocytes are activated by helper T lymphocytes. This begins the humoral response.
  • Activated B cells multiply to form clones of plasma cells and memory cells.
  • Plasma cells secrete antibodies.
  • In cell-mediated immunity, cytotoxic T cells continue to destroy infected cells. Humoral immunity and cell-mediated immunity occur simultaneously.

Long-term immunity depends on memory cells

  • It can take up to two weeks for antibodies to be effective after a primary infection.
  • When the body is attacked for the second time by the same pathogen, the immune response takes less time and the peak concentration of antibody in the blood is much higher.
  • This is due to the persistence of memory cells, which are stored in the lymph nodes and spleen. Memory cells keep a genetic record of how to produce the correct antibody, so they are able to mobilise and clone plasma cells quickly during a repeat infection.

Figure 11.1.1g – Blood concentrations of antibody after first and second exposure to the same antigenFigure 11.1.1g – Blood concentrations of antibody after first and second exposure to the same antigen

Key concept

Immunity is based on recognition of self and destruction of foreign material.

Figure 11.1.1h – Mad cowFigure 11.1.1h – Mad cow
Bovine spongiform encephalitis (BSE), commonly known as ‘mad cow disease’, is caused by protein pathogens, called prions.

Figure 11.1.1i – RabiesFigure 11.1.1i – Rabies
Rabies is caused by a virus that is able to cross species boundaries.

Language help

  • Opsonization is from the Greek for ‘prepare for eating’.
  • Zoonosis is the process through which an infectious disease is transmitted from one species to another.

Concept help

  • There are many different surface molecules on erythrocytes, but only a few are checked before a blood transfusion. Another important antigen, the D antigen, is often called the ‘Rh factor’. It is either present or absent on all red blood cells. This is why blood types can be + or – (i.e. A+, B–, etc.).
  • Blood type O– is often called ‘the universal donor’, and AB+ ‘the universal recipient’.
  • Active immunity is said to be adaptive, since antigens from the environment direct the types of plasma cells that are made and maintained in the body.
  • Immunity can be passed from mother to child in the form of antibodies through breast milk – though this type of immunity is passive, since the infant’s immune system has not ‘learned’ how to produce the antibodies on it own.

Figure 11.1.1j – Blood typesFigure 11.1.1j – Blood types
Four types of blood serum: Anti-A, Anti-B, Anti-AB and Anti-D

Figure 11.1.1k – AgglutinationFigure 11.1.1k – Agglutination
Antibodies cause agglutination of the blood (left).

Course link

  • HIV attacks a specific kind of helper T cell, called a CD4 helper T cell. Review the biology of HIV and AIDS in 6.2.3.