Immunity and Immunodeficiency Disease

This explains in details how group of cells, molecules and organs resist almost all forms of invasion by organisms, tumour cells and foreign substances.

• Immune  system consists  of group of cells, molecules  and  organs that  collectively defend the  body against  invasion  by organisms, tumour cells  and  foreign substances.

• The health  of the body  is  dependent  on the immune system’s ability  to  recognize, neutralise or destroy  the invaders.

• The ability  of the body  to  resist almost  all forms  of invasion  is  known as  immunity.

Components  of Immune  System

• Organs:  thymus,  bone marrow,  lymph  nodes and spleen

• Cells: White blood  cells, antigen-presenting cells

• Molecules:  immune  modulators;  cytokines,

• Antibodies/immunoglobulins:  IgA,  IgE,IgG,IgD &  IgM

• Complement  system

Classification  of  Immunity

• Body  immunity  is  classified  into  two;

1. Innate /natural  immunity:  a non-specific type  of immunity;  in-born ability  of the body to resist invasion  by organisms  or foreign substances.

2. Acquired/adaptive immunity-  as specific type  of immunity  developed  in response to  a specific  antigenic challenge


1) In the first line  of Defense: the elements of Immunity  found includes; Surface Barriers; skin and mucosa, secretions of GIT, respiratory and genitourinary tracts Normal flora-microbes that live in or on the body  that mutually coexist with  the body  preventing occupation of such areas by pathogenic organisms. This immunity is classified as innate immunity.

2) In the second line of Defense: the element of immunity includes Internal defensive mechanisms; phagocytosis, inflammation, fever, natural killer(NK) cells, antimicrobial substances, complement system. This immunity is also classified as innate immunity.

3) In the third  line of Defense: Elements of immunity includes Specialized B and T lymphocytes  this immunity is classified as Specific immunity


• Antigen-substance  that  elicits an immune response. Most  potent antigens  are  complex proteins or polypeptides  but  occasionally can  also be  carbohydrate  moieties.

• Antibody(immunoglobulins):  γ-globulins produced  by plasma cells(differentiated  B cells) in response  to  antigenic  challenge

• Memory: ability of  the  immune  system  to remember  a  previous encounter  with antigen  so that  a second  exposure to same  antigen  produces a more rapid and powerful response.


• Lymphocytes constitute  25-40% of white blood cells.

• Depending  on the modes of development, lymphocytes are  classified  into  B or  T lymphocytes.

• In  utero, lymphocytes  develop from their precursors  in bone marrow  and  when released into  circulation differentiate  into  two categories;

• The lymphocytes that  are  destined  for cellular immunity-  migrate to thymus  and are transformed to  T lymphocytes.

• The  lymphocytes  that  are destined  for humoral immunity are  transformed in  bursal equivalents(fetal  liver  in  utero  and  bone  marrow after  birth) to  B lymphocytes.

• N.B. B  lymphocytes  are first  discovered  in  bursa  of Fabricius,  a lymphoid  organ  located close  to cloaca  of  birds;  in mammals,  there  is  no  bursa  but the  B lymphocytes  are  processed  in the  bursa equivalents.

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