Humoral and Cellular Immunity

Humoral or Antibody Mediated Immunity

The term Humoral is derived from the Latin word “humor”, meaning body fluid. The humoral immunity refers to the interaction of B cells with antigen and their subsequent proliferation and differentiation into antibody secreting plasma cells and Memory cells (which secret cells antibodies on the second or further response or exposer to the same antigen). These produced and secrets antibodies, which binds with the antigens to get them neutralize and eliminate them from biological system.

Antibodies can eliminate antigens by:

• By neutralizing the bound antigen through electrical charge neutralization present over antigen molecule.
• Forming clusters with antigens, these clusters are more readily ingested by phagocytic cells.
• Binding with antigens, activate complement system that facilitates lysis of antigen.
• • Binding of antibodies to antigen, prevent binding of antigen to the host cells.

Process for Development of Humoral Immunity

(1) The humoral immunity is generated or mediated by B Cells.

(2) The antibody present on B Cells can recognize different antigenic epitopes present on antigen (epitopes are the discreet sites of antigens or antigenic determinants that binds with antibodies).

(3) Appropriate interaction between antigen and B cells membrane bound antibody occurs.

(4) The interaction activates B cells and induces clonal selection of B cells. Due to which, B Cells divides repeatedly and differentiate to generate a population of activated B cells, are called Plasma cells.

(5) This activation takes 3-4 days to generate plasma cells.

Plasma cells further differentiate into:

• Antibody-secreting cells — Antibody secreting cells have no membrane bound antibody, can secret one of the five types of antibodies with the affinity to antigen from which they gets activated.
• Memory cells — Memory cells are smaller in size as compared to plasma cells and carries antibody on their surface. Memroy cells has longer life span (even for years) and high sensitivity towards that specific antigen.

During the second or subsequent exposure of the same antigen, the memory cells proliferate and rapidly differentiates into plasma cells and produce high affinity antibodies, which is called the secondary response.

Cell Mediated Immune Response (Cellular Immunity)

Cellular immunity is mediated by T cells. T cells are formed in the bone marrow but mature in the thymus. The process involves antigen presentation to T cells by APCs (antigen-presenting cells).

T Cells can recognize the antigen only in the condition in which the antigen is bind with MHC molecules.

Steps:

• Extracellular antigens are internalized by APCs via endocytosis, fragmented, and presented on the APC surface bound to MHC II molecules.
• Intracellular proteins are degraded via proteasome, bound to MHC I molecules, and presented on the APC surface.
• T cells recognize antigens only when bound to MHC molecules:
  • CD4 T cells recognize antigen with MHC II — function as helper T cells (TH cells), secrete cytokines to activate B cells, Tc cells, and other immune cells.
  • CD8 T cells recognize antigen with MHC I — become cytotoxic T lymphocytes (CTLs) that kill infected cells.