Adaptive immunity is capable of identifying unique foreign microorganisms and molecules (i.e., foreign antigens) and selectively eliminating them. In comparison to innate immune responses, in all members of a species, adaptive immune responses are not the same but are reactions to particular antigenic challenges.

Four characteristic attributes represent adaptive immunity:

a. Antigen specificity

b. Diversity

c. Immunologic memory

d. Recognition of Self/non-self

The immune system’s antigenic specificity enables it to discern subtle distinctions between antigens. Antibodies can differentiate between two molecules of protein that differ by only one amino acid. In its recognition molecules, the immune system can produce immense complexity, enabling it to identify billions of unique structures for foreign antigens. If an antigen has been recognized and reacted to by the immune system, it exhibits immunological memory; that is, a second encounter with the same antigen causes an increased immune reactivity state. Because of this feature, after an initial experience, the immune system will confer life-long immunity to several infectious agents. Finally, the immune system typically only responds to foreign antigens, suggesting that it can recognize itself. The immune system’s ability to differentiate oneself from non-self and react only to non-self molecules is necessary. The result of an inappropriate reaction to self-molecules may be fatal.

Adaptive immunity is not autonomous from innate immunity. In initiating the particular immune response, the phagocytic cells critical to non-specific immune responses are closely involved. Conversely, it has been shown that various soluble factors released by a specific immune reaction increase these phagocytic cells’ activity. For instance, as an inflammatory response grows, soluble mediators are created that attract immune system cells. In exchange, the immune reaction may help to monitor the strength of the inflammatory response. The two mechanisms function together to remove a foreign invader through the carefully controlled inter-play of adaptive and innate immunity.

Lymphocytes and Antigen-presenting cells Co-operate in Adaptive immunity :

Two main classes of cells are involved in a successful immune response: T lymphocytes and antigen-presenting cells. Lymphocytes are formed by the hematopoiesis process in the bone marrow. Lymphocytes leave the bone marrow, circulate and remain in various lymphoid organs in the blood and lymphatic systems. Lymphocytes mediate the distinguishing immunological characteristics of specificity, diversity, memory, and self/non-self recognition since they generate and exhibit antigen-binding cell-surface receptors.


1. B lymphocytes mature within the bone marrow. Each expresses a specific antigen-binding receptor on its membrane when they leave it.

2. A membrane-bound antibody molecule is this antigen-binding or B-cell receptor. 3. Glycoproteins consisting of two identical heavy polypeptide chains and two similar light polypeptide chains are antibodies.

4. Disulfide bonds join each heavy chain with a light chain, and additional disulfide bonds hold the two pairs together.

5. A cleft within which antigen binds forms the amino-terminal ends of the pairs of heavy and light chains.

6. The antigen-binding to the antibody causes the cell to divide rapidly when a naive B cell (one that has not previously experienced antigen) first encounters the antigen that matches its membrane-bound antibody. Its progeny differentiates into memory B cells, and plasma cells called effector B cells.

7. Compared to naive cells, memory B cells have a longer life span, and parent B cells release the same membrane-bound antibody.

8. The antibody is formed by plasma cells in a form that can be secreted and has little to no membrane-bound antibodies. While plasma cells live for only a few days, they secrete enormous amounts of antibodies during this period.

9. It has been calculated that more than 2000 antibody molecules per second can be secreted by a single plasma cell. The main effector molecules of humoral immunity are secreted antibodies.


1. T cells move to the thymus gland to mature, unlike B cells, which mature within the bone marrow.

2. The T cell expresses a unique antigen-binding molecule, called the T-cell receptor, on its membrane during its maturation within the thymus.

3. T-cell receptors can recognize only antigens connected to cell-membrane proteins called major histocompatibility complex (MHC) molecules, unlike membrane-bound antibodies on B cells that can recognize antigen alone.

4. The polymorphic (genetically diverse) glycoproteins found on cell membranes are MHC molecules that act in this recognition case called “antigen presentation.”

MHC molecules are of two types:

  1. Class I MHC Molecules
  2. Class II MHC Molecules

Class I MHC molecules consist of a heavy chain linked to a small invariant protein called 2-microglobulin, expressed by almost all vertebrate organisms’ nucleated cells. Only antigen-presenting cells release Class II MHC molecules, consisting of an alpha and a beta glycoprotein chain. T cells proliferate and differentiate into memory T cells and multiple effector T cells when a naive T cell encounters an antigen mixed with an MHC molecule on a cell.

5. Two well-defined subpopulations of T cells exist T helper cells (Th) and T cytotoxic cells (Tc). While the third type of T cell has been postulated, called a T suppressor (Ts) cell

6. The presence of CD4 or CD8 membrane glycoproteins on their surfaces will differentiate T helper and T cytotoxic cells from one another.

7. T cells that display CD4 typically act as Th cells, whereas those that express CD8 usually operate as Tc cells.

8. The cell is activated after a Th cell recognizes and interacts with an antigen-MHC class II molecule complex, which becomes an effector cell that collectively secretes different growth factors known as cytokines.

9. In activating Tc cells, B cells, macrophages, and numerous other cells involved in the immune response, the secreted cytokines play an essential role.

10. Various types of immune reactions arise from variations in the pattern of cytokines generated by activated Th cells.

11. Tc cell recognizes an antigen-MHC class I molecule complex proliferates and differentiates into an effector cell called a cytotoxic T lymphocyte (CTL), under the influence of Th-derived cytokines.

12. The CTL does not typically secrete many cytokines, unlike the Tc cell, and exhibits cell-killing or cytotoxic activity instead.

13. The CTL plays a crucial role in tracking the body’s cells and removing any antigen-showing cells, such as virus-infected cells, tumor cells, and foreign tissue graft cells.

14. Cells exhibiting foreign antigen complexed with an MHC molecule of class I are referred to as altered self-cells; these are CTL targets.

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