The Immune Response

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Scanning electron micrograph of HIV virion (smaller structure) on a lymphocyte (large structure.).
Scanning electron micrograph (SEM) of HIV-1 virions budding from a cultured lymphocyte.
Credit: CDC/C. Goldsmith, P. Feorino, E.L. Palmer, W.R. McManus

The human body has several different lines of defense against infections and disease. The immune system is made up of different cells, tissues, and organs working together to fight infection.


By the time you finish this lesson, you should be able to:

  • Describe the body’s complex set of immune responses to an antigen.
  • Explain the role of different types of disease-fighting cells of the immune system.
  • Describe two key features of the immune response that allow for protection against disease.


antibody—chemicals made by B lymphocytes that can bind to antigens.

antigen—any type of molecular structure that lymphocytes recognize as foreign to the body and that can trigger an immune response.

B cells—a lymphocyte that produces antibodies that bind to an antigen.

cytotoxic T cells—lymphocytes that produce chemicals that kill infected cells or tumor cells.

effector cell—a cell that actively engages a pathogen to engulf and destroy it.

helper T cell—a lymphocyte that stimulates mitotic divisions that result in the production of increased numbers of T cells after contact with an antigen.

immunological memory—the characteristic of the immune response that involves lymphocytes’ abilities to recognize an antigen in the future that it encountered in a previous immune response.

immunological specificity—the characteristic of the immune response that involves lymphocytes’ ability to engage with a particular type of antigen.

immunity—resistance to disease.

lymphocytes—a type of white blood cell.

macrophage—a large cell that engulfs cells foreign to the body.

memory cell—a cell that recognizes a pathogen from previous contact and is capable of destroying it at a later exposure.

self-markers—particular molecular structures on every cell that immune response cells recognize as “self”.

The immune response is basically defined by and can be understood by two key features: specificity and memory.

  • Immunological specificity involves lymphocytes
    Scanning electron micrograph of a human T cell.
    Scanning electron microscope image of a human T cell. Credit: NIAID

    (white blood cells) being able to zero in on a particular type of pathogen. The lymphocytes (B and T cells) can specifically fight one type of pathogen when it appears in the body.

  • Immunological memory involves the formation of B and T lymphocytes that do not participate in the pathogen-fighting process, but rather remain in the blood as future disease-fighters that “remember” the specific pathogen in future encounters in the body.

Each kind of cell, virus, or chemical particle has a unique molecular structure. An individual’s cells have a distinctive structure that other body cells recognize as self-markers. Pathogen-fighting lymphocytes (blood cells) recognize the body’s own “self-markers” and ignore them.  When they encounter foreign agents or particles, however, they begin a series of mitotic divisions which greatly increase the numbers of pathogen-fighting cells.

These new huge populations of cells (effector cells) actively engage pathogens to engulf and destroy them while other cells (memory cells) do not actively engage the pathogens. Memory cells stay in the blood and “remember” the specific pathogenic structure that stimulated their formation. When these cells encounter the same pathogen in the future, they are capable of destroying it and the individual does not get the disease. (Effector cells are T lymphocytes; memory cells are B lymphocytes.)

Any type of molecular structure that can trigger lymphocytes to multiply and destroy foreign particles are called antigens. The antigen is a foreign substance that the lymphocytes recognize as not being “self”.

What happens when an injury allows bacteria to enter your body or when you encounter a disease-causing organism?

bacteria or viruses enter the body


lymph vessels carry infected interstitial fluid to lymph glands

macrophages in lymph glands engulf the foreign cells and enzymes break antigen molecules into pieces

CC BY-SA 2.5,

antigen fragments bind to MHC molecules on the plasma membrane of nucleated body cells

antigen-MHC complexes are formed

lymphocytes recognize that these complexes are foreign and activate:

1) Helper T cells-Stimulates mitotic divisions; produce T lymphocytes (effector cells)

2) Cytotoxic T cells-Produce chemicals that kill infected cells or tumor cells.

3) B lymphocytes (memory cells)-Produce antibodies that bind to the antigen

Three cells: red blood cell (left), a platelet (center), and a white blood cell (right).
Scanning electron micrograph of a T lymphocyte (at right), a platelet (center), and a red blood cell (at left). Credit: Electron Microscopy Facility at the National Cancer Institute at Frederick.

As the antigens are destroyed, fewer T and B cells are produced and the immune response stops. Also, chemical signals from cells suppress the immune response.

The lymphatic system has an important role in the immune process. Glands that make up the lymphatic system (the lymph nodes) are packed with cells that initiate the disease-fighting immune response.



Did you know that you had cells in your body called natural killer cells? They patrol your body for tumor cells and virus-infected cells to destroy them.

Check Your Understanding:

  1. List the function of each of the following cells:
    a. B cells

    b. helper T cells

    c. effector cells 

     d. memory cells

     e. cytotoxic T cells


  1. What is immunological specificity?



  1. What is immunological memory?



  1. List steps in the immunological process upon introduction of a pathogen.




 5. List four places were pathogens could be found in the body.



  1. Why don’t lymphocytes attack body cells?



Thinking critically

  1. If the lymph vessels pick up pathogen-containing fluid from an injury site or an infected area, and then carry the pathogen around the body via the blood and lymph fluid, why doesn’t this always spread the pathogen throughout the body and result in infection and disease?




  1. Under what situation would the recognition of a “non-self” cell not be advantageous?