General Properties of Adaptive Immune Responses

🔍 Recognition of Self vs Non-Self

• Clonal Selection Hypothesis:

  • The embryo produces a wide variety of lymphocytes.

  • Each lymphocyte is genetically programmed to recognize a specific antigen.

  • When exposed to its specific antigen after development, it clonally expands—produces identical cells (a clone) that make the same antibody.

• Clonal Deletion (Self-Tolerance):

  • During development in the bone marrow (B cells) and thymus (T cells), lymphocytes are tested.

  • If a lymphocyte binds to a self-antigen, it is destroyed or inactivated.

  • This prevents autoimmunity by creating tolerance to self.

🎯 Specificity

• The immune system reacts differently to each foreign antigen.

• Each immune response targets a specific antigen.

• Cross-reactions can occur:

  • Some antigens share similar haptens (small molecules that trigger immune responses).

  • Example: The bacterium that causes syphilis has haptens similar to those on human heart cells, causing potential misdirected immune attack.

🌈 Diversity

• The immune system can recognize and respond to millions of different antigens.

• This is due to the production of:

  • A wide range of antibodies (by B cells).

  • A variety of T cell receptors (by T cells).

• One pathogen may have multiple epitopes (antigenic determinants), and the immune system can make a different antibody for each.

🧠 Memory

• After the first exposure to an antigen:

  • The immune system produces both antibodies and memory cells.

• These memory cells remain for years or decades.

• On re-exposure to the same antigen:

  • The immune response is faster and stronger, often preventing illness (basis of immunity and vaccination).