What is Humoral Immunity?

Immunity provided by humus (body fluid), through antibodies is known as “humoral immunity”.

It is also referred as “antibody-mediated immunity” because of the major role played by antibodies in this immune response. In humoral immunity, binding of antibodies to microorganism (foreign agents) results in the formation of immune complex (Ag-Ab complex).

Immune complex initiates activation of complement system and other factors such as attraction of other immune cells near to it, inhibition of their migration from the site of Ag-Ab complex etc., to complete the reaction (removal of foreign agent from the body).

Binding of Antibodies to soluble toxins neutralizes the toxic nature of the toxin. Antibody binding to viral particles stops subsequent binding of virus to host cells.

Immune system executes humoral response in three steps.

1. Activation of B Cells:

Activation of B cells is the first step in humoral response to produce antibodies necessary for the humoral immune response. Activation of B cells may be direct or indirect.

In direct B cell activation, B cells interact with the T cell independent antigens and start multiplying to produce plasma cells and memory cells.

In indirect activation, B cells get stimulation from T helper cells. T helper cells induce B cells differentiation and colony formation after interacting with the antigen, processed and presented by the antigen presenting cell.

2. Effector Phase

Communication between helper-T cells and B-cells through lymphokines released by activated T cells initiate differentiation and multiplication of B cells to produce a colony.

Some cells from the colony become plasma cells to secrete antibodies that can react with the antigens responsible for their production.

The remaining cells become memory cells to retain “memory” of the foreign invader (antigen) and remain in the body in G₀ phase till they are used in future. If the immune system encounters with the same antigen for the second time, memory cells react very quickly and remove the pathogen very efficiently.

Life span of memory cells is much longer than life of plasma cells and it depends upon the strength and nature of antigen that has induced its production.

Memory of certain antigens such as polio virus, small pox virus etc. remains for the whole life to give life long protection.

3. Antibody Binding With Antigens:

Antibodies secreted by plasma cells interact with their respective antigens resulting in the formation of Ag-Ab complex.

The Ag-Ab complex initiates subsequent immune reactions for its removal from the body.

Characteristic features of Ag-Ab complex and the subsequent reactions depend upon the type of antigen (Refer Chapter 8 for information on antigen antibody interaction).