The Human Immune system

The human immune is extremely complex. It has evolved over hundreds of millions of years to respond to invasion by the pathogenic microbes that regularly attempt to infect our bodies, and invasion by the microbes that tried to infect our genetic ancestors. There are similarities between the immune system of humans and those of the most primitive of vertebrates, going back five hundred million years on the evolutionary ladder.

The immune system does not rely on one single mechanism to deter invaders, but instead uses many strategies, the most important of which are detailed below. The main division between the strategies is that between Innate immunity, which does not require previous exposure to the invading microbe, and Acquired immunity, whereby the immune system "remembers" how to deal with a microbe that it has dealt with before.

The Phagocytes

Phagocytes are the soldiers of the immune system, and provide innate immunity. They are responsible for swallowing, killing and digesting invading microbes. The process of swallowing microbes is known as phagocytosis. There are two main types of phagocyte

As mentioned above, the process of swallowing of microbes by the phagocytes is known as phagocytosis. After the invading microbe has been ingested, the next task for the phagocyte is to kill the microbe. This is achieved in two main ways.

When these tasks are complete, the Macrophages have one further task to complete. They return to the lymph nodes, displaying the remnants of the destroyed invader on their surface. This has the effect of stimulating the cells of the Acquired immunity system into action.

The Complement System

The complement provides innate immunity. It is comprised of a collection of proteins that "recognise" corresponding proteins on the cell walls of invading microbes. When such invading microbes have been recognised, the following actions are taken

Acquired Immunity

The acquired immunity system comprises B Cells and T Cells. Together, these cells which provide acquired immunity are known as Lymphocytes. The acquired immunity system further divides into two parts, humoral immunity and Cell Mediated Immunity (CMI).

B Cells provide "Humoral Immunity". Each B cell secretes a unique antibody, which acts against a particular antigen. An antigen is a chemical feature (a protein) which is unique to any given type of invading organism. When B cells meet an invading organism for which they have the antibody, they do one of two things.

T Cells provide "Cell Mediated Immunity", often referred to as "CMI". T cells have several functions. They can be:-


Cytokines are the last element of the immune system which we shall discuss here. Cytokines (meaning "cell movers") are the messengers of the immune system. The above mentioned elements of the immune system (Complement, Phagocytes, Lymphocytes) do not work separately, but all work together in co-operative fashion. If they are to work effectively, they need a good system for communicating messages. This system is provided by the cytokines. Among the cytokines are the Interleukins, of which there are known to be at least twelve, Gamma-Interferon, Lymphotoxin, and Tumor Necrosis Factor.