The brachial plexus is formed by the anterior primary divisions of the four lower cervical nerves and the greater part of that of the first thoracic nerve. It is usually joined by small twigs from the fourth cervical and second thoracic nerves.

The immune system comprises both innate and adaptive immune responses. Innate immunity occurs naturally because of genetic factors or physiology; it is not induced by infection or vaccination but works to reduce the workload for the adaptive immune response. Both the innate and adaptive levels of the immune response involve secreted proteins, receptor-mediated signaling, and intricate cell-to-cell communication. The innate immune system developed early in animal evolution, roughly a billion years ago, as an essential response to infection. Innate immunity has a limited number of specific targets: any pathogenic threat triggers a consistent sequence of events that can identify the type of pathogen and either clear the infection independently or mobilize a highly specialized adaptive immune response. For example, tears and mucus secretions contain microbicidal factors.

Contemporary psychology is a diverse field that is influenced by all of the historical perspectives described in the preceding section. Reflective of the discipline’s diversity is the diversity seen within the American Psychological Association (APA).

A cell is the smallest unit of a living thing. A living thing, whether made of one cell (like bacteria) or many cells (like a human), is called an organism. Thus, cells are the basic building blocks of all organisms.

There are many research methods available to psychologists in their efforts to understand, describe, and explain behavior and the cognitive and biological processes that underlie it.

Scientific research is a critical tool for successfully navigating our complex world. Without it, we would be forced to rely solely on intuition, other people’s authority, and blind luck.

Adaptive immune response takes days or even weeks to become established—much longer than the innate response—and is more specific to pathogens and involves molecular memory.

The serratus posticus superior muscle (lat: Serratus posticus superior) - named from its saw-like edge and its relation to the other serrati - is a quadrilateral sheet with a toothed outer margin.

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