Relies on lymphocytes which respond to foreign antigens on the surface of bacteria and viruses

Can take two forms – Humoral response (B cells) and cell mediated response (T cells)


Humoral response

Has two main stages

  • Pathogen with antigens on surface is engulfed by macrophage
  • Macrophage presents antigens via MHC’s on its surface and becomes an APC
  • APC binds to T helper cells with complementary CD4 receptors
  • T helper cell is activated and divides into T memory and T helper cells. T memory cells remain in the body and become activated if the pathogen is encountered again
  • Effector stage: B cell binds to the complementary pathogen and engulfs it by endocytosis
  • B cell becomes an APC
  • Activated T helper cell with a complementary receptor binds to APC
  • Cytokines are released which stimulate the B cell to divide to form clones of B effector cells and B memory cells
  • B effector cells differentiate into plasma cells which secrete antibodies and only last for a few days
  • B Memory cells are long lasting and are involved in the secondary immune response


Cell mediated response

  • Pathogen infects host cells and presents the antigens on MHC to become an APC
  • T killer cell with complementary receptor binds to APC
  • Cytokines from T helper cells stimulate the differentiation
  • T killer cell divides to form clones of active and memory T killer cells
  • Active T killer cells bind to infected cells presenting antigens on MHCs
  • T killer cell releases chemicals causing pores in the infected cell, causing lysis
  • Cell becomes permeable so water and ions enter the cell, it swells, bursts and dies


B cells

Lymphocytes involved in the humoral response. They make specific antibodies against antigens, perform the role of APCs and develop into B memory cells. Each has a unique receptor protein on its surface. They originate in bone marrow.

  • B effector cells – differentiate to produce plasma cells which release antibodies into the blood and lymph – immediate response – involved in the primary immune response
  • B memory cells – remain in the body for months or years, enabling an individual to respond quickly to the same antigen. They are specific to the antigen encountered during the primary immune response – involved in the secondary immune response


T cells

T cells are involved in cell mediated immunity. They have T cell receptors on their surface. T cells originate and mature in the thymus.

  • T helper cell – when activated, these stimulate B cells and aid T killer cells to divide and become cells to produce antibodies
  • T killer cell – Destroy cells with foreign antigens on their surface
  • T memory cell – A long lived T cell that has receptors for an antigen due to its encounter with a prior infection of vaccination

T cells

Killer cells Helper cells
Search for infected cells Undergo clonal selection
Attach to cells Clonal expansion
Secrete toxic substances into them Secrete cytokines
Kill pathogen inside B cells divide and fire antibodies



B cells T cells
Produced and mature in bone marrow Produced in bone marrow but mature in thymus gland
Involved in humoral response (involving antibodies) Involved in cell mediated response (involving body cells)
Produces antibodies Does not produce antibodies
Responds to foreign cells outside body cells Responds to foreign material inside body cells
Responds to bacteria and viruses Responds to own cells affected by virus, cancer or response to a transplanted tissue



  • Cytokines – chemicals that T helper cells release that stimulate division and differentiation of B cells
  • Antibodies – Y shaped protein molecules that belong to a class known as immunoglobulins. Antibodies bind to antigens and act as labels, allowing phagocytosis to recognise and destroy the cell
  • Antigen – any substance foreign to the body recognised as non-self that causes an immune response and is capable of binding with an antibody or T cell. They produce antibodies
  • Lysis – disintegration of cell membrane
  • APC (Antigen Presenting Cell) – displays foreign antigens with MHC on their surfaces for T cells to recognise


The primary immune response is slow because there are few cells which can make the antibody needed to fight the pathogen. There is time needed for macrophages to present antigens, B cells to attach and divide, and plasma cells to differentiate and produce antibodies.


Secondary immune response

Involves memory cells

If infected by the same pathogen again, the immune system responds faster.

There is a greater production of antibodies and it lasts longer.

B memory cells differentiate immediately to produce plasma cells to release antibodies.

T memory cells remember the specific antigen and will recognise it second time round.

Faster because b cells already have complementary antigen receptors, memory cells can divide rapidly into plasma cells and so there is quicker clonal expansion.

The invading pathogens are often destroyed so rapidly that the person is unaware of any symptoms – they are said to be immune


Ways pathogens enter the body:

  • Vector (organism that transmits infection) – prevent by blood clotting, skin and sebum
  • Formites (objects carrying pathogens) – prevent by skin, skin flora and vomiting
  • Direct contact – prevent by lysozymes, defensive secretions, mucus and skin flora
  • Inhalation (e.g. TB) – prevent by mucus, vomiting lysozymes and cilia
  • Ingestion (contaminated food) – prevent by vomiting, mucus and saliva
  • Inoculation (break in the skin e.g. HIV) – prevent by skin, antibodies, immune response and blood clotting